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Does the genetic material the sperm carries affect its physical properties

Does the genetic material the sperm carries affect its physical properties



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Basically, what I'm asking is, is the actual sperm cell built from the blueprint in the DNA of the man or is it itself also a consequence of the DNA it carries?

I'd like to know a few more things related to this. For example, which DNA is in the nucleus of a sperm? Is it the same as the DNA of other bodily cells in the man's body, or is it just the DNA it's delivering? Can the sperm's performance be affected by any specific genetic traits that can be observed in a potential human the sperm could make? For example, a sperm swims faster if it's carrying the genetic material that would result in a tall person (probably a silly question).

The original question I had is if the statement that female sperm live longer is true. If it turns out that the man's body makes all the sperm more or less the same and they're just boxes that can swim, then I would guess the answer is no.


I think there is a strong driving force for sperm to be free living haploid versions of human beings. Since they are a product of meiosis sperm are the product of recombination of both the male chromosome sets.

While I'm sure that if we look closely enough, the germline cells that produce the sperm are contributing some protein and structure to the sperm, the life cycle of the sperm is mostly the result of the genome that it contains within itself. Sperm are prone to defects. On the average in human beings about half of sperm are non-motile: simply unable to form the flagellum or power its motion adequately for motility. That's a pretty strong selective force in the life of our other, 'half genome' selves.

Sperm selection - the sorting of which sperm from a single male achieves fertilization is pretty rigorous. It can take 30 minutes to days. Once in contact with the ovum, the sperm executes a critical set of transformations and biochemical activations to get past the various layers of the cell.

Sperm are from the man, but they are each living creatures on their own which we hope will deliver themselves up to the ovum to create another human being. They vary in their genetic quality and effectiveness and this is highly related to their genetic make up.

Because each sperm is the product of two stages of Meiosis, they can be full of genetic defects, most of which would be fatal in a diploid human being. The functions of the sperm as it goes through its short but competitive life weed out genetic abberations and help assure that the next generation of diploid human beings is as healthy as they can.

So they are not DNA boxes that swim. I'd be hard pressed to think of a passive role for a cell in biology - they are all working pretty hard… I'd be interested in a comment with suggestions of something that is not under selective pressure in biology.

It sort of reminds me of the old argument about passive mating in females. Behavioral biologists used to think that female animals had little or no active role in mating. That myth was retired about 30 years ago now. That link points to an introduction to an entire volume reviewing that whole discussion of active female roles in reproduction in case anyone reading is interested.

BTW I'm not an expert on gametes, but I'd be surprised if the ova also didn't have a complimentary but quite different selective process they had to get through to become a fully presented candidate for fertilization.

Just to add a note about how the functioning sperm genes affect the diploid organism: This is one of the most important things that sperm does. The genes that function in the living spermatazoan are the same genes the diploid organism uses, or at least a substantial number of them. Further, these genes are scattered across all the chromosomes and so major chromosomal defects and important genes are functional if fertilization is completed. As an example here's a cool blog post about how mosquito sperm have been shown to use odorant receptors to orient themselves in the fertilziation process. Those same receptors and the signalling genes that link that signal to the sperm behavior are probably quite useful to the adult mosquito. I'd be surprised if human sperm also did not use similar pathways to function.

Not to beat a dead horse, but the recent nature has a report of small RNAs in mouse sperm as being an epigenetic carrier for stress.


Shigeta, you have some good points there. I wanted to clarify some things.

Sperm and ova are considered tissues, not individual living creatures. They are not individually capable of cell division nor production of offspring. They are specialized cells with specific functions that are created by a multicellular organism as a means of transferring DNA.

The sperm carries 1/2 of a chromosome set from the male's own DNA, after cross-overs during meiosis (meaning that the DNA in a sperm has some different combinations than the parent DNA).

Metabolism primarily occurs at the mitochondria. Protein synthesis does occur in the sperm, using nucleus-encoded mRNA. What I was unable to find an answer to was whether this mRNA was transcribed from the nucleus of that same sperm, or whether it was from the primary spermatocyte DNA (diploid) or from the sperm nucleus (haploid). The chromatin in the sperm nucleus is several times more tightly wound than in a somatic cell, so I am not sure how it would be accessible to transcription, nor am I sure that the full set of enzymes required for transcription, modification, and translation is present in the spermatozoa. I'll be very interested in hearing if anyone is able to find this information!


For example, a sperm swims faster if it's carrying the genetic material that would result in a tall person

and in the comments

It seems to me that there would have to be some defined standard of a normal DNA that the sperm would check against to determine how "broken" it is, and I doubt there's something like that

If some aspects of sperm cells indirectly connected with some parts of phenotype, so we can expect that result, but not because of sperm cell carries that noble phenotype gene.

DNA in sperm cell is packed and not expressing (not transcribed).
Because DNA is packed and because test checks consuming energy, which sperm needs for another tasks - pretty wise to shut down checks, repairs.
Also DNA is not like ZIP archive, where cell can check the check sum, to make shure it works witch good DNA. Cell have difficulties working with information, if it needs to connect information results from distance sources. (Let say as example: if gene A have mutation A* and gene B have mutation B* then do X).

Also there is big big difficulties with that mater: some defined standard of a normal DNA - all DNA that leads to organism which is able to procreate, to survive - is good enough for nature. Thats Evolution spirit). And who can, who not - will be sorted out, later and with 100% accuracy.

For example, humans with mental retardation often have above average strength.

Our DNA actually consists from imperfections, which showed up long ago and became standard de facto, some are compensated at the moment and are RFC(Request for Comments). This or another set of genes is a balanced system. Balance can be shifted and that will have some consequences, actually thats how another system is prepared for testing by Evolution factors. By shifting balance far enough, for making some things at extreme, we may expect that another things have to break. But strength is not because of mental retardation, or vice verse. (It may be connected, may be not; also I would not trow away, some ontogenesis matter in that case) They both are results, not cause. But there are some reasons to be connected, if you take look on apes behavior as example.

just boxes that can swim

Man it's a Rocket


Does the genetic material the sperm carries affect its physical properties - Biology

Gymnosperms are seed plants that have evolved cones to carry their reproductive structures.

Learning Objectives

Discuss the type of seeds produced by gymnosperms

Key Takeaways

Key Points

  • Gymnosperms produce both male and female cones, each making the gametes needed for fertilization this makes them heterosporous.
  • Megaspores made in cones develop into the female gametophytes inside the ovules of gymnosperms, while pollen grains develop from cones that produce microspores.
  • Conifer sperm do not have flagella but rather move by way of a pollen tube once in contact with the ovule.

Key Terms

  • ovule: the structure in a plant that develops into a seed after fertilization the megasporangium of a seed plant with its enclosing integuments
  • sporophyll: the equivalent to a leaf in ferns and mosses that bears the sporangia
  • heterosporous: producing both male and female gametophytes

Characteristics of Gymnosperms

Gymnosperms are seed plants adapted to life on land thus, they are autotrophic, photosynthetic organisms that tend to conserve water. They have a vascular system (used for the transportation of water and nutrients) that includes roots, xylem, and phloem. The name gymnosperm means “naked seed,” which is the major distinguishing factor between gymnosperms and angiosperms, the two distinct subgroups of seed plants. This term comes from the fact that the ovules and seeds of gymnosperms develop on the scales of cones rather than in enclosed chambers called ovaries.

Gymnosperms are older than angiosperms on the evolutionary scale. They are found far earlier in the fossil record than angiosperms. As will be discussed in subsequent sections, the various environmental adaptations gymnosperms have represent a step on the path to the most successful (diversity-wise) clade (monophyletic branch).

Gymnosperm Reproduction and Seeds

Gymnosperms are sporophytes (a plant with two copies of its genetic material, capable of producing spores ). Their sporangia (receptacle in which sexual spores are formed) are found on sporophylls, plated scale-like structures that together make up cones. The female gametophyte develops from the haploid (meaning one set of genetic material) spores that are contained within the sporangia. Like all seed plants, gymnosperms are heterosporous: both sexes of gametophytes develop from different types of spores produced by separate cones. One type of cone is the small pollen cone, which produces microspores that subsequently develop into pollen grains. The other type of cones, the larger “ovulate” cones, make megaspores that develop into female gametophytes called ovules. Incredibly, this whole sexual process can take three years: from the production of the two sexes of gametophytes, to bringing the gametophytes together in the process of pollination, and finally to forming mature seeds from fertilized ovules. After this process is completed, the individual sporophylls separate (the cone breaks apart) and float in the wind to a habitable place. This is concluded with germination and the formation of a seedling. Conifers have sperm that do not have flagella, but instead are conveyed to the egg via a pollen tube. It is important to note that the seeds of gymnosperms are not enclosed in their final state upon the cone.

Female cone of Tamarack pine: The female cone of Pinus tontorta, the Tamarack Pine, showing the rough scales. This is the cone that produces ovules.

Male cone of Tamarack pine: The male cone of Pinus tontorta, the Tamarack pine, showing the close proximity of the scales. This is the cone that produces pollen.


Most cases of trisomy 13 result from having three copies of chromosome 13 in each cell in the body instead of the usual two copies. The extra genetic material disrupts the normal course of development, causing the characteristic features of trisomy 13.

Trisomy 13 can also occur when part of chromosome 13 becomes attached (translocated) to another chromosome during the formation of reproductive cells (eggs and sperm) or very early in fetal development. Affected people have two normal copies of chromosome 13, plus an extra copy of chromosome 13 attached to another chromosome. In rare cases, only part of chromosome 13 is present in three copies. The physical signs and symptoms in these cases may be different than those found in full trisomy 13.

A small percentage of people with trisomy 13 have an extra copy of chromosome 13 in only some of the body's cells. In these people, the condition is called mosaic trisomy 13. The severity of mosaic trisomy 13 depends on the type and number of cells that have the extra chromosome. The physical features of mosaic trisomy 13 are often milder than those of full trisomy 13.

Learn more about the chromosome associated with Trisomy 13


Conclusion

Studies reveal that the exposure to cell phones, microwave ovens, laptops, or Wi-Fi produces deleterious effects on the testes, which may affect sperm count, morphology, motility, an increased DNA damage, causing micronuclei formation and genomic instability, as well as disruptions in protein kinases, hormones and antioxidative enzymes. Such effects were found to be responsible for infertility due to an over-production of ROS in exposed cells. Studies suggest that the abnormalities reported due to RF-EMF-exposure depend on physical parameters such as duration of the exposure, distance to the source of radiation, power density, and depth of the penetration. Unfortunately, current studies are unable to suggest a true mechanism of how RF-EMF radiation affects the male reproductive system. Therefore, more studies are necessary to provide better evidence of RF-EMF radiations emitted from cell phones, microwaves, Wi-Fi and Wi-Fi-connected laptops, which can be provided by in vitro and in vivo studies in combination with physical bio-modeling. Moreover, very limited research is available on protective measures, which actually worsens the problem as the electro-smog pollution is constantly increasing and one could then expect even more health problems including increased rates of male infertility due to such kind of radiation. On the other hand, possible protective effects of various antioxidants should be elucidated. Yet, this would only address the problem at symptomatic level.


Does the genetic material the sperm carries affect its physical properties - Biology

PART VI. PHYSIOLOGICAL PROCESSES

27. Human Reproduction, Sex, and Sexuality

In humans and some other organisms, the sex of an offspring is determined by the chromosomes they inherit from their parents.

Chromosomal Determination of Sex

Recall from chapter 10 that two of the 46 chromosomes are involved in determining sex and are called sex-determining chromosomes. The other 44 chromosomes are known as autosomes. There are two kinds of sex-determining chromosomes: the X chromosome and the Y chromosome that do not carry equivalent amounts of information, nor do they have equal functions (figure 27.4).

FIGURE 27.4. Human Male and Female Chromosomes

The chromosomes have been arranged into homologous pairs: (a) a male karyotype, with an X and a Y chromosome, and (b) a female karyotype, with two X chromosomes.

X chromosomes carry genetic information about the production of a variety of proteins, in addition to their function in determining sex. For example, the X chromosome carries information on blood clotting, color vision, and other characteristics. The Y chromosome, with about 80 genes, however, appears to be primarily concerned with determining male sexual differentiation.

When a human sperm, a haploid sex cell produced by sexually mature males, is produced, it carries 22 autosomes and a sex-determining chromosome. Unlike eggs, which always carry an X chromosome, half the sperm cells carry an X chromosome and the other half carry a Y chromosome. If an X-carrying sperm cell fertilizes an X-containing egg cell, the resultant embryo will develop into a female. If a Y-carrying sperm cell fertilizes the egg, a male embryo will develop. It is the presence or absence of the sex-determining region Y (SRY) gene located on the short arm of the Y chromosome that determines the sex of the developing individual. The SRY gene produces a chemical, called testes determining factor (TDF), which acts as a master switch that triggers the events that converts the embryo into a male. Without this gene, the embryo would become female.

The early embryo resulting from fertilization and cell division is not recognizable as either male or female. Sexual development begins when certain cells become specialized, forming the embryonic gonads known as the female ovaries and the male testes. This specialization of embryonic cells is called differentiation. If the SRY gene is present and functioning, the embryonic gonads begin to differentiate into testes 5 to 7 weeks after conception (fertilization).

Chromosomal Abnormalities and Sexual Development

Evidence that the Y chromosome and its SRY gene control male development comes from many kinds of studies, including research on individuals who have an abnormal number of chromosomes. An abnormal meiotic division that results in sex cells with too many or too few chromosomes is a form of nondisjunction (see chapter 9). If nondisjunction affects the X and Y chromosomes, a gamete might be produced that has only 22 chromosomes and lacks a sex-determining chromosome. On the other hand, it might have 24, with 2 sex-determining chromosomes. If a cell with too few or too many sex chromosomes is fertilized, sexual development is usually affected. If a normal egg cell is fertilized by a sperm cell with no sex chromosome, the offspring will have only 1 X chromosome. These people, always women, are designated as XO. They develop a collection of characteristics known as Turner’s syndrome (figure 27.5).

FIGURE 27.5. Turner's Syndrome

Individuals with Turner’s syndrome have 45 chromosomes. They have only 1 of the sex chromosomes, and it is an X chromosome. Individuals with this condition are female, have delayed growth, and fail to develop sexually. This woman is less than 150 cm (5 ft) tall and lacks typical secondary sexual development for her age. She also has the “webbed neck” that is common among individuals with Turner’s syndrome.

About 1 in 2,000 girls born has Turner’s syndrome. A female with this condition is short for her age and fails to mature sexually, resulting in sterility. In addition, she may have a thickened neck (termed webbing), hearing impairment, and some abnormalities in her cardiovascular system. When the condition is diagnosed, some of the physical conditions can be modified with treatment. Treatment involves the use of growth-stimulating hormone to increase her growth rate and female sex hormones to stimulate sexual development, although sterility is not corrected.

An individual who has XXY chromosomes is basically male (figure 27.6). This genetic condition is termed Klinefelter’s syndrome. It is one of the most common examples of abnormal chromosome number in humans. This condition is present in about 1 in 500 to 1,000 men. Most of these men lead healthy, normal lives and it is impossible to tell them apart from normal males. However, those with Klinefelter’s syndrome may be sterile and show breast enlargement, incomplete masculine body form, lack of facial hair, and some minor learning problems. These traits vary greatly in degree, and many men are diagnosed only after they undergo testing to determine why they are infertile. Treatments include breast-reduction surgery and testosterone therapy.

FIGURE 27.6. Klinefelter's Syndrome

Individuals with two X chromosomes and a Y chromosome are male, are sterile, and often show some degree of breast development and female body form. They are typically tall. The two photos show an individual with Klinefelter’s syndrome before and after receiving testosterone hormone therapy.

The development of embryonic gonads begins very early during fetal growth. First, a group of cells begins to differentiate into primitive gonads at about week 5 (figure 27.7). By week 5 to 7, if a Y chromosome is present, the gene product (testes determining factor) from the chromosome begins the differentiation of these embryonic gonads into testes. They will develop into ovaries beginning about week 12 if 2 X chromosomes are present (and the Y chromosome is absent).

FIGURE 27.7. Differentiation of Sexual Characteristics

The early embryo grows without showing any sexual characteristics. The male and female sexual organs eventually develop from a common basic structure. (a) Shows the development of the internal anatomy. (b) Shows the development of the external anatomy.

As soon as the gonad has differentiated into an embryonic testis at about week 8, it begins to produce testosterone. The presence of testosterone results in the differentiation of male sexual anatomy, and the absence of testosterone results in the differentiation into female sexual anatomy in the developing embryo (Outlooks 27.1).

At about the seventh month of pregnancy (gestation), in normal males each testis moves from a position in the abdominal cavity to an external sac, called the scrotum. The testes pass through an opening called the inguinal canal. This canal closes off but continues to be a weak area in the abdominal wall, and it may rupture later in life. This can happen when strain (for example, from improperly lifting heavy objects) causes a portion of the intestine to push through the inguinal canal into the scrotum, a condition known as an inguinal hernia.

Occasionally, the testes do not descend, resulting in a condition known as cryptorchidism (crypt = hidden orchidos = testes). Sometimes, the descent occurs normally during puberty if not, there is a 25 to 50 times increased risk for testicular cancer. Because of this increased risk, surgery can be done to allow the undescended testes to be moved into the scrotum. Sterility will result if the testes remain in the abdomen. This happens because normal sperm cell development cannot occur in a very warm environment. The temperature in the abdomen is higher than the temperature in the scrotum. Normally, the temperature of the testes is very carefully regulated by muscles that control their distance from the body. Physicians have even diagnosed cases of male infertility as being caused by tight-fitting pants that hold the testes so close to the body that the temperature increase interferes with normal sperm development. Recent evidence has also suggested that teenage boys and young men working with computers in the laptop position for extended periods may also be at risk for lowered sperm counts.

7. Describe the processes that cause about 50% of babies to be born male and 50% to be born female.

8. Name two developmental abnormalities associated with nondisjunction of chromosomes.


For more information about structural changes to chromosomes:

The National Human Genome Research Institute provides a list of questions and answers about chromosome abnormalities, including a glossary of related terms.

Chromosome Disorder Outreach offers a fact sheet on this topic titled Introduction to Chromosomes. This resource includes illustrated explanations of several chromosome abnormalities.

The Centre for Genetics Education provides a fact sheet about chromosome changes.

The University of Leicester's Virtual Genetics Education Center provides an explanation of structural chromosome aberrations.

Your Genome from the Wellcome Genome Campus discusses chromosome disorders, including types of structural abnormalities in chromosomes that are involved in genetic diseases.


Diet easily affects quality of sperm, find researchers

The study published in 'PLOS Biology' was found by researchers at Linkoping University who fed healthy young men a high sugar diet.

Sweden: Researchers have found a study that sperm is affected by diet and its results appear rapidly. The research also gave new insights into the sperm process which, in the long term, can continue to assess sperm values by modern testing methods.

The study published in 'PLOS Biology' was found by researchers at Linkoping University who fed healthy young men a high sugar diet. Anita Ost, a senior lecturer in the Department of Clinical and Experimental Medicine at the University, and also the head of the study said: "We see that diet influences the motility of the sperm, and we can link the changes to specific molecules in them. Our study has revealed rapid effects that are noticeable after one to two weeks."

Many environmental and lifestyle factors, including obesity and related diseases, may affect sperm quality, such as type 2 diabetes, are well-known risk factors for poor sperm quality.

Epigenetic anomalies, which influence physical properties or rates of gene expression, are of concern to the research group which conducted the new study, even if the genetic material, the DNA code, is not altered. In some cases, such epigenetic changes will contribute to the transmission of properties through the sperm or egg from parent to parent.

In an earlier study, researchers have shown that male fruit flies that had eaten excess sugar shortly before the patient became overweight more often produced offspring.

Scientists have hypothesized that RNA sperm fragments may engage in epigenetic processes, but it is too early to tell whether they do in human beings. The new study was initiated by the researchers to investigate whether high consumption of sugar affects the RNA fragments in human sperm.

The study examined 15 normal, non-smoking young men, who followed a diet in which they were given all food from the scientists for two weeks. The diet was based on the Nordic Nutrition Recommendations for healthy eating with one exception: during the second week, the researchers added sugar, corresponding to around 3.5 litres of fizzy drinks, or 450 grammes of confectionery, every day. The sperm quality and other indicators of the participants` health were investigated at the start of the study, after the first week (during which they ate a healthy diet), and after the second week (when the participants had additionally consumed large amounts of sugar).

One third had poor sperm motility at the beginning of the test. Motility is one of several factors influencing the quality of sperms, and that of the general population was the proportion of people with low sperm motility studied. The authors were surprised to learn that the motility of sperm of all participants during the research had become natural.

"The study shows that sperm motility can be changed in a short period, and seems to be closely coupled to diet. This has important clinical implications. But we can`t say whether it was the sugar that caused the effect, since it may be a component of the basic healthy diet that has a positive effect on the sperm," said Anita Ost.

The scientists have also observed that the tiny RNA fragments associated with sperm motility also changed.

They now plan to carry on the research and explore whether there is a connection between male fertility and RNA sperm fragments.


Can We Really Inherit Trauma?

Headlines suggest that the epigenetic marks of trauma can be passed from one generation to the next. But the evidence, at least in humans, is circumstantial at best.

In mid-October, researchers in California published a study of Civil War prisoners that came to a remarkable conclusion. Male children of abused war prisoners were about 10 percent more likely to die than their peers were in any given year after middle age, the study reported.

The findings, the authors concluded, supported an “epigenetic explanation.” The idea is that trauma can leave a chemical mark on a person’s genes, which then is passed down to subsequent generations. The mark doesn’t directly damage the gene there’s no mutation. Instead it alters the mechanism by which the gene is converted into functioning proteins, or expressed. The alteration isn’t genetic. It’s epigenetic.

The field of epigenetics gained momentum about a decade ago , when scientists reported that children who were exposed in the womb to the Dutch Hunger Winter, a period of famine toward the end of World War II, carried a particular chemical mark, or epigenetic signature, on one of their genes . The researchers later linked that finding to differences in the children’s health later in life, including higher-than-average body mass.

The excitement since then has only intensified, generating more studies — of the descendants of Holocaust survivors, of victims of poverty — that hint at the heritability of trauma . If these studies hold up, they would suggest that we inherit some trace of our parents’ and even grandparents’ experience, particularly their suffering, which in turn modifies our own day-to-day health — and perhaps our children’s, too.

But behind the scenes, the work has touched off a bitter dispute among researchers that could stunt the enterprise in its infancy. Critics contend that the biology implied by such studies simply is not plausible. Epigenetics researchers counter that their evidence is solid, even if the biology is not worked out.

“These are, in fact, extraordinary claims, and they are being advanced on less than ordinary evidence,” said Kevin Mitchell, an associate professor of genetics and neurology at Trinity College, Dublin. “This is a malady in modern science: the more extraordinary and sensational and apparently revolutionary the claim, the lower the bar for the evidence on which it is based, when the opposite should be true.”

Investigators in the field say the critique is premature: the science is still young and feeling its way forward. Studies in mice, in particular, have been offered as evidence of such trauma-transmission, and as a model for studying the mechanisms. “The effects we’ve found have been small but remarkably consistent, and significant,” said Moshe Szyf, a professor of pharmacology at McGill University . “This is the way science works. It’s imperfect at first and gets stronger the more research you do.”

The debate centers on genetics and biology. Direct effects are one thing: when a pregnant woman drinks heavily, it can cause fetal alcohol syndrome. This happens because stress on a pregnant mother’s body is shared to some extent with the fetus, in this case interfering directly with the normal developmental program in utero.

But no one can explain exactly how, say, changes in brain cells caused by abuse could be communicated to fully formed sperm or egg cells before conception. And that’s just the first challenge. After conception, when sperm meets egg, a natural process of cleansing, or “rebooting,” occurs, stripping away most chemical marks on the genes. Finally, as the fertilized egg grows and develops, a symphony of genetic reshuffling occurs, as cells specialize into brain cells, skin cells, and the rest. How does a signature of trauma survive all of that?

One working theory is based on animal research. In a series of recent studies, scientists at the University of Maryland School of Medicine , led by Tracy Bale, have raised male mice in difficult environments, by periodically tilting their cages, or by leaving the lights on at night. This kind of upbringing, effectively a traumatic childhood, changes the subsequent behavior of those mice’s genes in a way that alters how they manage surges of stress hormones.

And that change, in turn, is strongly associated with alterations in how their offspring handle stress: namely, the young mice are numbed, or less reactive, to the hormones compared to control animals, said Dr. Bale, director of the university’s Center for Epigenetic Research in Child Health and Brain Development. “These are clear, consistent findings,” she said. “The field has advanced dramatically in just the past five years.”

Perhaps the best explanation for how such trauma marks could be attached to a father’s sperm cells comes from Oliver Rando at the University of Massachusetts. His studies, also in mice, have zeroed in on the epididymis, a tube near the testicles where sperm cells load before ejaculation. There, they learn to swim over a period of days, and their genes can be marked, said Dr. Rando.

The molecules that affect the changes appear to be “small RNAs,” fragments of genetic material that scientists are still learning about, Dr. Rando said.

“This tube produces small RNAs and ships them to the sperm as they develop, suggesting that there exists a place that senses the dad’s environmental conditions and can change the package RNAs delivered through the sperm to the baby,” Dr. Rando said. He makes no broad claims beyond that.

Other researchers have attempted to fill out the picture. Once those RNA packages arrive at the epididymis, the hypothesis goes, they prompt a of cascade of changes at conception that evade the stripping, or rebooting, process and the subsequent reshuffling during early development.

The critics are far from persuaded. “It’s all very nice work , and yes, there are changes in the testicular cells,” said John M. Greally, a professor of genetics, medicine, and pediatrics at the Albert Einstein College of Medicine. “But as usual, the story that’s often told is overblown relative to the results, and too much causality is claimed.”

And this debate concerns only the animal research. The human studies thus far are much less persuasive, most experts agree, and have identified no plausible mechanism for epigenetic transmission. Some of the studies have focused not on small RNAs but on an altogether different chemical signature, called cytosine methylation, that could very well be added after conception, not before, Dr. Rando said.

The idea that we carry some biological trace of our ancestors’ pain has a strong emotional appeal. It resonates with the feelings that arise when one views images of famine, war or slavery. And it seems to buttress psychodynamic narratives about trauma, and how its legacy can reverberate through families and down the ages. But for now, and for many scientists, the research in epigenetics falls well short of demonstrating that past human cruelties affect our physiology today, in any predictable or consistent way.


17.1 Biotechnology

In this section, you will explore the following questions:

  • What are examples of basic techniques used to manipulate genetic material (DNA and RNA)?
  • What is the difference between molecular and reproductive cloning?
  • What are examples of uses of biotechnology in medicine and agriculture?

Connection for AP ® Courses

Did you eat cereal for breakfast or tomatoes in your dinner salad? Do you know someone who has received gene therapy to treat a disease such as cancer? Should your school, health insurance provider, or employer have access to your genetic profile? Understanding how DNA works has allowed scientists to recombine DNA molecules, clone organisms, and produce mice that glow in the dark. We likely have eaten genetically modified foods and are familiar with how DNA analysis is used to solve crimes. Manipulation of DNA by humans has resulted in bacteria that can protect plants from insect pests and restore ecosystems. Biotechnologies also have been used to produce insulin, hormones, antibiotics, and medicine that dissolve blood clots. Comparative genomics yields new insights into relationships among species, and DNA sequences reveal our personal genetic make-up. However, manipulation of DNA comes with social and ethical responsibilities, raising questions about its appropriate uses.

Nucleic acids can be isolated from cells for analysis by lysing cell membranes and enzymatically destroying all other macromolecules. Fragmented or whole chromosomes can be separated on the basis of size (base pair length) by gel electrophoresis. Short sequences of DNA or RNA can be amplified using the polymerase chain reaction (PCR). Recombinant DNA technology can combine DNA from different sources using bacterial plasmids or viruses as vectors to carry foreign genes into host cells, resulting in genetically modified organisms (GMOs). Transgenic bacteria, agricultural plants such as corn and rice, and farm animals produce protein products such as hormones and vaccines that benefit humans. (It is important to remind ourselves that recombinant technology is possible because the genetic code is universal, and the processes of transcription and translation are fundamentally the same in all organisms.) Cloning produces genetically identical copies of DNA, cells, or even entire organisms (reproductive cloning). Genetic testing identifies disease-causing genes, and gene therapy can be used to treat or cure an inheritable disease. However, questions emerge from these technologies including the safety of GMOs and privacy issues.

Information presented and the examples highlighted in the section support concepts outlined in Big Idea 3 of the AP ® Biology Curriculum Framework. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP ® Biology course, an inquiry-based laboratory experience, instructional activities, and AP ® exam questions. A Learning Objective merges required content with one or more of the seven Science Practices.

Big Idea 3 Living systems store, retrieve, transmit and respond to information essential to life processes.
Enduring Understanding 3.A Heritable information provides for continuity of life.
Essential Knowledge 3.A.1 DNA, and in some cases RNA, is the primary source of heritable information.
Science Practice 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models.
Learning Objective 3.5 The student can justify the claim that humans can manipulate heritable information by identifying an example of a commonly used technology.
Big Idea 3 Living systems store, retrieve, transmit and respond to information essential to life processes.
Enduring Understanding 3.C The processing of genetic information is imperfect and is a source of genetic variation.
Essential Knowledge 3.C.1 Changes in genotype can result in changes in phenotype.
Science Practice 7.2 The student can connect concepts in and across domain(s) to generalize or extrapolate in and/or across enduring understandings and/or big ideas.
Learning Objective 3.24 The student is able to predict how a change in genotype, when expressed as a phenotype, provides a variation that can be subject to natural selection.

The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards:
[APLO 3.13][APLO 3.23][APLO 3.28][APLO 3.24][APLO 1.11][APLO 3.5][APLO 4.2][APLO 4.8]

Teacher Support

Begin the discussion with the ethical considerations, such as genetic modified foods, the availability of a genome to the government or insurance provider, or modifying a genome for therapy or the sex selection with embryos. These topics will be in the minds of students, so get them out in front and then get into the mechanics of the topic.

Basic Techniques to Manipulate Genetic Material (DNA and RNA)

  • Go through the process of DNA extraction in class as a demonstration. This would probably be the first time the students would have an opportunity to actually see DNA. Bring in a gel from gel electrophoresis and the results of Southern Blotting as illustrations of the techniques. This will help the discussion be a little more concrete.
  • Be sure that students understand the different uses of the word clone, such as molecular cloning, cellular cloning, reproductive cloning. Emphasize that the word is neutral and does not automatically infer a negative process. Earlier discussions of the ethics of the subject should help to put it into context.

Biotechnology is the use of biological agents for technological advancement. Biotechnology was used for breeding livestock and crops long before the scientific basis of these techniques was understood. Since the discovery of the structure of DNA in 1953, the field of biotechnology has grown rapidly through both academic research and private companies. The primary applications of this technology are in medicine (production of vaccines and antibiotics) and agriculture (genetic modification of crops, such as to increase yields). Biotechnology also has many industrial applications, such as fermentation, the treatment of oil spills, and the production of biofuels.

Basic Techniques to Manipulate Genetic Material (DNA and RNA)

To understand the basic techniques used to work with nucleic acids, remember that nucleic acids are macromolecules made of nucleotides (a sugar, a phosphate, and a nitrogenous base) linked by phosphodiester bonds. The phosphate groups on these molecules each have a net negative charge. An entire set of DNA molecules in the nucleus is called the genome. DNA has two complementary strands linked by hydrogen bonds between the paired bases. The two strands can be separated by exposure to high temperatures (DNA denaturation) and can be reannealed by cooling. The DNA can be replicated by the DNA polymerase enzyme. Unlike DNA, which is located in the nucleus of eukaryotic cells, RNA molecules leave the nucleus. The most common type of RNA that is analyzed is the messenger RNA (mRNA) because it represents the protein-coding genes that are actively expressed. However, RNA molecules present some other challenges to analysis, as they are often less stable than DNA.

DNA and RNA Extraction

To study or manipulate nucleic acids, the DNA or RNA must first be isolated or extracted from the cells. Various techniques are used to extract different types of DNA (Figure 17.2). Most nucleic acid extraction techniques involve steps to break open the cell and use enzymatic reactions to destroy all macromolecules that are not desired (such as degradation of unwanted molecules and separation from the DNA sample). Cells are broken using a lysis buffer (a solution which is mostly a detergent) lysis means “to split.” These enzymes break apart lipid molecules in the cell membranes and nuclear membranes. Macromolecules are inactivated using enzymes such as proteases that break down proteins, and ribonucleases (RNAses) that break down RNA. The DNA is then precipitated using alcohol. Human genomic DNA is usually visible as a gelatinous, white mass. The DNA samples can be stored frozen at –80°C for several years.

RNA analysis is performed to study gene expression patterns in cells. RNA is naturally very unstable because RNAses are commonly present in nature and very difficult to inactivate. Similar to DNA, RNA extraction involves the use of various buffers and enzymes to inactivate macromolecules and preserve the RNA.

Gel Electrophoresis

Because nucleic acids are negatively charged ions at neutral or basic pH in an aqueous environment, they can be mobilized by an electric field. Gel electrophoresis is a technique used to separate molecules on the basis of size, using this charge. The nucleic acids can be separated as whole chromosomes or fragments. The nucleic acids are loaded into a slot near the negative electrode of a semisolid, porous gel matrix and pulled toward the positive electrode at the opposite end of the gel. Smaller molecules move through the pores in the gel faster than larger molecules this difference in the rate of migration separates the fragments on the basis of size. There are molecular weight standard samples that can be run alongside the molecules to provide a size comparison. Nucleic acids in a gel matrix can be observed using various fluorescent or colored dyes. Distinct nucleic acid fragments appear as bands at specific distances from the top of the gel (the negative electrode end) on the basis of their size (Figure 17.3). A mixture of genomic DNA fragments of varying sizes appear as a long smear, whereas uncut genomic DNA is usually too large to run through the gel and forms a single large band at the top of the gel.

Amplification of Nucleic Acid Fragments by Polymerase Chain Reaction

Although genomic DNA is visible to the naked eye when it is extracted in bulk, DNA analysis often requires focusing on one or more specific regions of the genome. Polymerase chain reaction (PCR) is a technique used to amplify specific regions of DNA for further analysis (Figure 17.4). PCR is used for many purposes in laboratories, such as the cloning of gene fragments to analyze genetic diseases, identification of contaminant foreign DNA in a sample, and the amplification of DNA for sequencing. More practical applications include the detection of genetic diseases.

DNA fragments can also be amplified from an RNA template in a process called reverse transcriptase PCR (RT-PCR) . The first step is to recreate the original DNA template strand (called cDNA) by applying DNA nucleotides to the mRNA. This process is called reverse transcription. This requires the presence of an enzyme called reverse transcriptase. After the cDNA is made, regular PCR can be used to amplify it.

Link to Learning

Deepen your understanding of the polymerase chain reaction by clicking through this interactive exercise.

  1. The process of PCR can isolate a particular piece of DNA for copying, which allows scientists to copy millions of strands of DNA in a short amount of time.
  2. The process of PCR can purify a particular piece of DNA, and very small amounts of DNA can be used for purification.
  3. The process of PCR separates and analyzes DNA and its fragments, which requires very little DNA.
  4. The process of PCR anneals DNA molecules to complementary DNA strands, which maintains the same amount of DNA.

Hybridization, Southern Blotting, and Northern Blotting

Nucleic acid samples, such as fragmented genomic DNA and RNA extracts, can be probed for the presence of certain sequences. Short DNA fragments called probes are designed and labeled with radioactive or fluorescent dyes to aid detection. Gel electrophoresis separates the nucleic acid fragments according to their size. The fragments in the gel are then transferred onto a nylon membrane in a procedure called blotting (Figure 17.5). The nucleic acid fragments that are bound to the surface of the membrane can then be probed with specific radioactively or fluorescently labeled probe sequences. When DNA is transferred to a nylon membrane, the technique is called Southern blotting , and when RNA is transferred to a nylon membrane, it is called northern blotting . Southern blots are used to detect the presence of certain DNA sequences in a given genome, and northern blots are used to detect gene expression.

Molecular Cloning

In general, the word “cloning” means the creation of a perfect replica however, in biology, the re-creation of a whole organism is referred to as “reproductive cloning.” Long before attempts were made to clone an entire organism, researchers learned how to reproduce desired regions or fragments of the genome, a process that is referred to as molecular cloning.

Cloning small fragments of the genome allows for the manipulation and study of specific genes (and their protein products), or noncoding regions in isolation. A plasmid (also called a vector) is a small circular DNA molecule that replicates independently of the chromosomal DNA. In cloning, the plasmid molecules can be used to provide a "folder" in which to insert a desired DNA fragment. Plasmids are usually introduced into a bacterial host for proliferation. In the bacterial context, the fragment of DNA from the human genome (or the genome of another organism that is being studied) is referred to as foreign DNA , or a transgene, to differentiate it from the DNA of the bacterium, which is called the host DNA .

Plasmids occur naturally in bacterial populations (such as Escherichia coli) and have genes that can contribute favorable traits to the organism, such as antibiotic resistance (the ability to be unaffected by antibiotics). Plasmids have been repurposed and engineered as vectors for molecular cloning and the large-scale production of important reagents, such as insulin and human growth hormone. An important feature of plasmid vectors is the ease with which a foreign DNA fragment can be introduced via the multiple cloning site (MCS) . The MCS is a short DNA sequence containing multiple sites that can be cut with different commonly available restriction endonucleases. Restriction endonucleases recognize specific DNA sequences and cut them in a predictable manner they are naturally produced by bacteria as a defense mechanism against foreign DNA. Many restriction endonucleases make staggered cuts in the two strands of DNA, such that the cut ends have a 2- or 4-base single-stranded overhang. Because these overhangs are capable of annealing with complementary overhangs, these are called “sticky ends.” Addition of an enzyme called DNA ligase permanently joins the DNA fragments via phosphodiester bonds. In this way, any DNA fragment generated by restriction endonuclease cleavage can be spliced between the two ends of a plasmid DNA that has been cut with the same restriction endonuclease (Figure 17.6).

Recombinant DNA Molecules

Plasmids with foreign DNA inserted into them are called recombinant DNA molecules because they are created artificially and do not occur in nature. They are also called chimeric molecules because the origin of different parts of the molecules can be traced back to different species of biological organisms or even to chemical synthesis. Proteins that are expressed from recombinant DNA molecules are called recombinant proteins . Not all recombinant plasmids are capable of expressing genes. The recombinant DNA may need to be moved into a different vector (or host) that is better designed for gene expression. Plasmids may also be engineered to express proteins only when stimulated by certain environmental factors, so that scientists can control the expression of the recombinant proteins.


The Genetics of Down's Syndrome

This article explains what is known about the genetics of Down's syndrome.

Anna Kessling (UK) and Mary Sawtell (UK)

Introduction
Genes are present in every cell of the body. They are inherited from our parents and are responsible for our development from fertilized egg to fully grown adult. They provide the chemical information that is needed to maintain our body and keep it working well. People with Down's syndrome have the same genes as anyone else they just have 1% extra. This is enough to change the finely-tuned balance of the body and produce the physical and intellectual characteristics found in people with Down's syndrome.

Cells
The human body is made up of cells. Each cell is like a tiny factory which makes the materials needed for growth and maintenance of the body. Different parts of the body have specialised cells with special tasks, for example muscle cells are different from nerve cells, heart cells are different from brain cells.

The production and output of each cell is controlled by genes. The genes are the same in every cell of the body. Not all the genes are active in any one cell at any one time, however. The active genes in a cell are only those appropriate to the cell type and its functions.

The body grows by making new cells. It does this by making an exact copy of the genes in a cell. The cell then divides in two with one copy of the genes in each new cell.

Genes
Genes contain the body's instructions for making its parts and for its day to day running. Genes therefore control or influence such things as:

  • Our physical appearance
  • The way babies and children grow and develop, even before they are born
  • The timing of the milestones of growth and development
  • The body's supply and use of building and maintenance materials
  • The way the body ages
  • A person's resistance to illness

Genes are passed from parents to children. Most people have two copies of every gene - in general one copy comes from the mother and the other from the father.

The genes are made of DNA (deoxyribonucleic acid). The arrangement of chemicals in the DNA is different for every gene. These chemical instructions tell the body what components to make, and how, when and in what quantity to make them, in order to ensure normal working of the body. There are also long stretches of DNA, the precise role of which is still a mystery.

Chromosomes
The genes are grouped together in long, thin, thread-like structures called chromosomes. Thus chromosomes carry our genetic information.

There are 46 chromosomes (in 23 pairs) in all body cells (except the sperm and egg cells). Each pair of chromosomes has different genes. One chromosome of each pair comes from the mother and one from the father. A person's characteristics are determined by the interaction of the copies of the genes from each parent.

The chromosome pairs are numbered 1-22 according to size. Chromosome 1 is the largest. Each chromosome has a long arm and a short arm.

The 23rd pair are the sex chromosomes which are called the X and Y chromosomes. Girls have two X chromosomes and boys have an X and a Y. The sex chromosomes are different from the others.

In chromosome tests, the chromosomes (usually from blood cells) are stained with dye. Each pair has a different staining pattern. When seen down a microscope the chromosomes lie in a haphazard manner, but when they are photographed, cut out and arranged in pairs, a picture of an individual's chromosomes known as a karyotype is obtained.

The genes, thousands on each chromosome, are spaced along the length of the DNA molecule, in a definite order. Apart from the X and Y chromosomes, the order of the genes on one chromosome exactly matches the order on the other chromosome of the same pair (and on chromosomes of the same number in everyone else). In general, there are only two copies of each gene and in all people they are always on the same chromosome pair.

The two copies of each gene may not be identical. Any changes may be small "misprints" or bigger alterations, with a corresponding effect of the "readability" of the instructions. When this happens, variation occurs between people - this variation is sometimes advantageous, sometimes neutral and sometimes unhelpful. Often a mistake in the one copy of a gene can be "cancelled out" when there is no mistake is the other copy of the same gene. Geneticists believe that everyone carries a number of defective genes which are partnered by a normal copy which "cancels" the defect. They also believe we all carry a number of genes with small variations which are not cancelled out, and which account for some of the differences in appearance or health among people in the general population.

Ordinary cell division
The body grows by making new cells. New cells are made by one existing cell dividing into two. In order to work, each new cell needs a set of 46 chromosomes. Before a cell divides an exact copy of each chromosome is made, creating a complete new set. The two sets of chromosomes move to opposite ends of the cell. The cell then divides across the middle producing two new cells, each with 46 chromosomes. This type of cell division is known as mitosis.

Eggs and Sperm
Eggs and sperm are different from all other cells in that they contain only a single set of 23 chromosomes. This means that when the egg and sperm come together at fertilisation, the usual number of 46 is restored, ready for the baby's development.

There is a special type of cell division used only in the making of eggs and sperm.This special type of cell division is called meiosis. Meiosis involves two rounds of cell division. The first round is very specialised it is called reduction division, because it reduces the number of chromosomes to 23. The second meiotic division is similar to ordinary cell division (mitosis).

Men make millions of fresh sperm, all the time, from puberty onwards. It is quite different in women. Long before birth, the female embryo has already started making the cells which will form her eggs when she is a mature woman. After puberty, a woman's ovaries release one egg (or occasionally more) per month until menopause.

All eggs made by the mother will contain one copy each (not a pair) of chromosomes 1-22, and an X chromosome. All sperm made by the father will contain one copy each (not a pair) of chromosomes 1-22 and either an X or a Y chromosome. If an X-carrying sperm fertilises the egg the baby will be a girl, whilst if a Y-carrying sperm fertilises the egg the baby will be a boy. When a 23-chromosome egg is fertilised by a 23-chromosome sperm, the first cell of a 46 chromosome embryo, fetus and baby is made. All that baby's cells will have the same 46 chromosomes which were in the original fertilised egg.

GENETICS OF DOWN'S SYNDROME

Overview
Down's syndrome occurs in babies born with extra chromosome 21 material in their cells. Down's syndrome is also known as trisomy 21. Trisomy 21 means there are 3 (tri) copies of chromosome (somy) 21.

There are three main types of Down's syndrome:

  • Regular trisomy 21 - also known as standard or free trisomy 21 - in which all the cells have an extra chromosome 21. Around 94% of people with Down's syndrome have this type.
  • Translocation - in which extra chromosome 21 material is attached to another chromosome. Around 4% of people with Down's syndrome have this type.
  • Mosaic - in which only some of the cells have an extra chromosome 21. Around 2% of people with Down's syndrome have this type.

Chromosome 21 is thought to contain around 1% of the body's genes. Down's syndrome therefore arises from a change in gene quantity rather than gene quality.

We do no know exactly why Down's syndrome occurs. This makes it different from other genetic conditions, such as cystic fibrosis or sickle cell disease, whose inheritance can be traced through families. Down's syndrome can be traced through families in less than 1% of people with the condition. These people all have rare types of translocation.

It is very unusual for parents to have more than one child with Down's syndrome, or for relatives of these parents to have a child with the condition.

Regular Trisomy 21
Most people with Down's syndrome have this type.

Individuals with regular trisomy 21 have an extra chromosome 21 in every cell. They therefore have 47 chromosomes in each cell instead of the usual 46.

How does regular trisomy 21 occur?
Regular trisomy 21 occurs because of an unusual cell division which has produced either an egg or a sperm with 24 chromosomes instead of 23. When this egg or sperm fuses with an ordinary egg or sperm, the first cell of the developing baby has 47 chromosomes instead of 46, and all that baby's cells will have 47 chromosomes. No one knows why this happens. There is no way or predicting whether a person is more or less likely to make eggs or sperm with 24 chromosomes.

The extra chromosome can come from uneven division of the chromosomes (called non-disjunction) at either the first or the second meiotic division, in either parent.

Where does the extra chromosome come from?
The extra chromosome comes from either the mother or the father. It makes no difference to the person with Down's syndrome which parent the extra chromosome came from.

Why does regular trisomy 21 happen? Despite much research this question remains unanswered. There is no evidence that any nationality, ethnic group, diet, medicines, illnesses or upbringing have any influence on whether or not a parent can or will have a child with Down's syndrome. Also, because Down's syndrome is present from the time of conception, nothing a woman does in pregnancy will influence whether or not her baby has Down's syndrome. Nothing is known which could have stopped the parent giving an extra chromosome. There is nothing "different" about the parents of a child with the condition.

The parents' ages? Any couple can have a baby with Down's syndrome, but it is well known that older women are more likely to have a baby with the condition than younger women.

There is controversy about the influence of the father's age. Most researchers consider that the father's age does not appear to affect the chance of having a child with Down's syndrome.

At the present time about one baby with Down's syndrome is born for every 1,000 total births. More babies with Down's syndrome are conceived than are born, because the chance of miscarriage is higher if the foetus has Down's syndrome.

The reason why older women are more likely to have children with Down's syndrome is unknown. There are two main current theories. One theory suggests that all women have some eggs with an extra chromosome, and that these eggs are more likely to be used last, towards the end of a woman's reproductive life. The other theory suggests that the rate of trisomic conceptions is the same at all maternal ages, but that affected pregnancies are more likely to continue (less likely to end in miscarriage) in older women. The assumption is that the body recognises that this is a late pregnancy, perhaps the last or only one, and thus tries harder to make sure the pregnancy comes to term.

What is the chance of having another baby with the condition?
Parents with one baby with regular trisomy 21 are usually told that the chance of having another baby with Down's syndrome is 1 in 100. Very few families are known who have more than one child with Down's syndrome, so the real chance is probably less than this.

There are differences of opinion as to how the much-quoted figure of 1 in 100 should be interpreted for older mothers who already have one child with Down's syndrome. Some feel that this 1 in 100 chance should be added to a woman's chance for her age, so that for a woman of 43 years for example, her (age-related) chance would be 1 in 49 (approximately 2%), added to 1 in 100 (1%), giving an overall chance of 1 in 33 (approximately 3%). Others feel that, when the age-related chance is more than 1 in 100, this alone gives the more realistic estimate (so for a woman of 43, the chance remains 1 in 49).

Translocation and partial trisomy
Some 4% of people with Down's syndrome do not have an extra whole, separate chromosome 21 (see regular trisomy 21 above), but have an additional part of chromosome 21 attached to another chromosome. This usually arises when the small arms of chromosome 21 and another chromosome break, and the two remaining long arms join together at their exposed ends. This process of chromosomes breaking and rejoining to other chromosomes is known as translocation (because the chromosome material has transferred its location).

People with Down's syndrome which has arisen in this way still have an extra copy of a large part of chromosome 21. Their features of Down's syndrome are no different from those in a child with regular trisomy 21.

The chromosomes that can be involved are numbers 13, 14, 15, 21, 22.

How does translocation occur? In two-thirds of people with Down's syndrome due to a translocation, the translocation was an isolated event during the formation of the individual egg or sperm involved in their conception. As with regular trisomy 21, there is no known reason why this occurs. It cannot be predicted and it is not a result of anything the parents or other family members have done.

Because it is a new event, this is sometimes called a de novo translocation.

The egg or sperm contains the usual number of chromosomes (i.e. 23) but these include the translocated one. Thus there is one free, whole chromosome 21 and most of a second chromosome 21 attached to another chromosome. If this egg or sperm containing 23 chromosomes (+ translocated part) fuses with an ordinary sperm or egg, the fertilised egg, fetus and baby will have 46 single chromosomes, but one of the chromosomes will have an extra copy of most of the chromosome 21 material attached to it. The translocated chromosome acts like a single chromosome in cell division, and hence all the cells produced from this first cell will contain the extra chromosome 21 portion. This baby will therefore have Down's syndrome.

In the other one third of people with the translocation type of Down's syndrome, the translocation is inherited from one of the parents. This parent has two whole number 21 chromosomes in each cell but one of them is attached to another chromosome. As there is no loss or gain of any genetic material this is known as a balanced translocation and the parent is a carrier of the translocation. It is important to realise that because such parents have the usual amount of genetic material, they have no traces of the syndrome themselves and never will have. They cannot be expected to know they are carriers, as the only way of knowing is to study their chromosomes.

When people who carry a translocation produce an egg or sperm, it is possible for them to pass on both the translocated chromosome and the free chromosome 21 in the egg or sperm. This will result in a fertilised egg with two free 21 chromosomes and a translocated chromosome. The baby will therefore have Down's syndrome.

As 4% of people with Down's syndrome have the translocation type, and one third of this group have inherited it, only about 1% of people with Down's syndrome have inherited the condition.

Is age a factor in translocation Down's syndrome? No. unlike regular trisomy 21, translocation occurs equally frequently whatever the age of the parents.

What is the chance of having another child with the condition?
When neither of the parents is a carrier, the translocation was an isolated event with only a small chance of its happening again (geneticists quote less than 1%). Translocation carriers can have children who are carriers, children whose chromosomes show no rearrangement at all, or children with Down's syndrome.

For translocations involving chromosomes 21 and any other chromosome, the chance of another child with Down's syndrome being born is about one in six if the mother is the carrier and about one in twenty if the father is the carrier. A few people are carriers for a translocation between two chromosomes 21 in these people, who are quite ordinary themselves, the only possible outcome is a child with Down's syndrome.

How can we know which type of Down's syndrome a baby has?
There are no differences in the features or ability levels of people with regular trisomy 21 and translocation Down's syndrome. The only way of knowing what type of Down's syndrome a person has, is by taking a blood sample and looking at the chromosomes.

A very few children with translocation have partial trisomy 21 - where only a part of chromosome 21 is present in 3 copies. These children may have fewer characteristics of Down's syndrome. Like the more usual type of translocation described above, this type may arise de novo, or a parent may carry it.

As one third of people with translocation Down's syndrome have inherited the condition, their parents have a high chance of having another affected child and may wish to know whether this is so. To identify these parents, chromosome tests are done on all new babies with Down's syndrome. Blood samples can then be taken from parents of babies with translocations, to find out whether one of the parents carries the translocation.

Genetic counselling should always be available to families with a child with Down's syndrome.

Translocation carriers and other members of the family? Even if parents do not intend to have more children, knowing that one of them is a carrier can be important for all their children or other relatives. Relatives of a person who carries a translocation have an increased chance of being translocation carriers.

Mosaicism
People with mosaic Down's syndrome have an extra chromosome 21 in only some of their cells. They therefore have a mixture of trisomic cells and ordinary cells. The mixture can vary from very few to nearly 100% trisomic cells. Depending on the proportion of trisomic cells, and which parts of the body contain these cells, individuals may be less affected both in their physical features and in their ability level than those with regular trisomy or translocation. As for all types of Down's syndrome, it is not possible to say at birth how affected a person will be, only time will tell as the child develops.

How does mosaic Down's syndrome happen?
Mosaicism arises after the egg and sperm have fused at conception. As the cells divide and multiply by ordinary cell division, a chromosome goes astray and a single cell with an extra chromosome 21 is formed. This cell continues to divide by ordinary cell division together with the non-trisomic cells and a mixture is produced.

As with the other two types of Down's syndrome (apart from when a parent is a carrier) there is no known reason why mosaic Down's syndrome occurs. It happens equally often in parents of all ages.

What are the chances of having another child with mosaic Down's syndrome?
Mosaic Down's syndrome is very rare, therefore there are no accurate figures on this. It is believed that the chance is lower than it would be if the child had regular trisomy 21.

Some questions we have been asked
Can a test-tube baby have the condition?
Yes - full chromosome tests are not carried out before the embryo is implanted.

Is trisomy 21 the only kind of trisomy?
No. some babies are born with trisomy 13 or trisomy 18. Chromosomes 13 and 18 are larger than chromosome 21 and so the genetic imbalance is greater and the effects on the baby are generally more severe than in trisomy 21. Trisomy for the X chromosome or an extra copy of the Y can occur, with relatively few effects for the child. Trisomies of every other chromosome can occur, but usually those pregnancies end in miscarriage. Parents who have had a baby with one kind of trisomy are not thought to be more likely to have a baby with any other kind of trisomy.

Is trisomy much more frequent in miscarriages?
Much more. In about half of all miscarriages occuring in the first 3 months of pregnancy, the developing baby has an altered number of chromosomes.

Will gene therapy make a difference?
As we understand more about how the genes on chromosome 21 interact to cause Down's syndrome, we can imagine a situation in which it might eventually become possible to switch off some of the genes (or maybe even the whole extra chromosome) responsible for Down's syndrome. A great deal more understanding of the basic mechanisms of Down's syndrome and a great deal more development in gene therapy is needed before we can contemplate such treatment, but it remains a long-term possibility.

Are there any differences in the genes or chromosomes of grandparents of children with Down's syndrome?
Unless the grandparent is a carrier of a balanced translocation (a very rare event) there are no known differences in the genes or chromosomes of grandparents of children with Down's syndrome from those of anyone else's grandparents. There is nothing known that the grandparents might have done which would explain their grandchild being born with Down's syndrome.

Can adults with Down's syndrome have children and if so, what is the chance of their children having the condition?
Yes. A woman with Down's syndrome can have children. If her partner does not have Down's syndrome, the theoretical chance of the child having Down's syndrome is 50%. There have been only a few reports of men with Down's syndrome fathering children. Again, if a man's partner did not have Down's syndrome, the chance that the baby would have the condition is 50%. If both partners have Down's syndrome there is a high chance of their children having the condition. As these events are still rare it is difficult to obtain accurate figures.

My grandmother's sister had Down's syndrome. She died 40 years ago when she was 30. Does this mean that I could have an increased chance of having a baby with the condition?
It is unlikely. There is a small chance that she would have had the inherited form of translocation Down's syndrome, in which case you could be a carrier. Chromosome testing was not introduced until 1959 so the family may not know what kind of Down's syndrome she had. If your grandmother had a lot of children and grandchildren and none of them was affected, it is very unlikely that she was a carrier. If you want to be completely sure that you are not a carrier, chromosome test would need to be carried out on your blood.

People with Down's syndrome are all very different from each other, in looks and personality and ability. Why is this, when they all have extra material from chromosome 21?
People with Down's syndrome get the extra chromosome material along with the full set of chromosomes from their parents. All the genes they inherit are ordinary ones, which explains why they resemble their families in the same way as ordinary children. The differences in genes that children with Down's syndrome inherit from their parents, together with differences in their environment, explain the differences between one child with Down's syndrome and another.

Can the tests ever be wrong?
If an experienced health professional has seen features of Down's syndrome in your child, and the blood test result shows regular trisomy 21 or a translocation type, there should be no doubt. In mosaic Down's syndrome, because all the cells in the body do not show trisomy, it is possible for a blood sample not to include any trisomic cells, or to contain only trisomic cells, which may lead to difficulties in arriving at the correct diagnosis. In this situation, while further tests may help, it is not actually possible to disprove mosaicism.

Blood tests are done by humans. Mistakes can, of course, be made at one of the many stages that a blood sample passes through before a result is given to the patient. This, however, is very rare.

  • Down's syndrome is always caused by the presence of extra chromosome 21 material in a person's cells.
  • There are 3 types of Down's syndrome. Most people with the condition have regular trisomy 21. Much smaller numbers have translocation, or mosaic Down's syndrome.
  • The only way of finding out what type of Down's syndrome people have is to do a blood test and examine their chromosomes under a microscope.
  • 4% of people with Down's syndrome have the translocation type. About 1 in 3 of these (about 1% of people with Down's syndrome) have inherited the conditions. One of their parents will be a carrier of the translocation. These parents have an increased chance of having a second child with the condition.
  • There is no known reason why Down's syndrome occurs (except on the rare occasions when it has been inherited).
  • Parents who have one child with regular trisomy 21 are thought to have a slightly increased chance of having another child with the condition.

H. S. Cuckle, N. J. Wald and S. G. Thompson (1987) British Journal of Obstetrics and Gynaecology, vol. 94, pp. 387-402.

E. Alberman et al. (1995) British Journal of Obstetrics and Gynaecology, vol 102, pp. 445-447.

C. Cunningham (1988) Down's Syndrome: An Introduction for Parents. Souvenir Press.


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