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Non-allele gene interactions


The analysis of the proportions between the phenotypic classes of the progeny of a cross can inform us about the number of genes involved in a given character.

For example, when it comes to inheritance controlled by a single pair of alleles with complete dominance, segregation leads to the classic 3: 1 ratio, ie at the cross between heterozygotes, ¾ offspring have the dominant feature and ¼ theme the recessive feature . This indicates that only one gene is involved in inheritance.

When we simultaneously analyze two characteristics, each condition by a pair of alleles with complete dominance and independent segregation, the ratio 9: 3: 3: 1 arises. The offspring of the double heterozygous cross are 9/16 with both dominant characteristics, 6/16 with one dominant and recessive characteristics and 1/16 with both recessive characteristics.

There are cases where two or more genes, whether or not located on the same chromosome, interact to produce a particular character. When this happens, the analysis of phenotypic proportions among descendants can tell us how many genes are involved in the formation of the trait and what type of interaction exists between them.

Gene interaction in the form of chicken crest

In 1905, English geneticist William Bateson and his collaborators concluded, after a series of experimental crosses, that the shape of the crest in chickens is conditioned by the interaction of two independently secreting pairs of alleles. Combinations between different alleles can produce four types of crest: rose, pea, nut and simple.

Simple Pea X Crossing

When pure pea crested strains are crossed with pure single crested strains, a generation F is obtained.1 consisting only of pea crested birds. In the experiment of the English researchers, when the birds of F1 were cross-bred, the offspring were 332 pea-crested and 110 single-crested birds, a very close ratio of 3: 1

PEE X ee
F1 Pea Crest Ee X Ee Pea Crest
F2

AND IS

Pea Crest

And is

Pea Crest

And is

Pea Crest

and is

Simple Crest

Pink Crossing X Simple

When pure strains of pink crested birds are crossed with pure strains of single crest, a generation F is obtained.1 consisting only of pink crested birds. In Bateson's experiment, when the birds of F1 were crossed with each other, a generation F2 consisting of 221 pink-crested and 83 single-crested birds, also very close to 3: 1.

PRR X rr
F1 Rose Crest Rr X Rr Rose Crest
F2

RR

Rose Crest

Rr

Rose Crest

Rr

Rose Crest

rr

Simple Crest

Pink Crossing Pea

When pure strains of pink-crested birds are crossed with pure strains of pea crest, all offspring have a single type of crest, called a “nut,” unlike their parent. In the experiment by Bateson, when the F-nut crested birds1 were crossed with each other, generation F2 presented 99 walnut crest, 26 pink crest, 38 pea crest and 16 single crested birds, a very close ratio of 9: 3: 3: 1. This is the expected ratio at the double heterozygous crossing for two independently segregated allele pairs.

PeeRR X EErr
F1 Nut Crest EeRr X EeRr Nut Crest
EREreRer
ER

EERR

Nut

Erer

Nut

EeRR

Nut

EeRr

Nut

Er

Erer

Nut

EErr

Pea

EeRr

Nut

Erer

Pea

eR

EeRR

Nut

EeRr

Nut

eeRR

rose

eeRr

rose

er

EeRr

Nut

Erer

Pea

eeRr

rose

eerr

Simple

F2

Simple cross walnut X test

When Bateson's team crossed, as a test, some generation F nutcrested birds1 with single-crested birds of a supposedly double-recessive genotype rree, 139 walnut-crested, 142 pink-crested, 112 pea-crested and 141 single-crested offspring were obtained, a ratio very close to 1: 1: 1: 1. These results confirm that the individuals of F1 they are double heterozygous and produce four gametes in equal proportions, as expected by the independent segregation law.

Bateson and his collaborators then concluded that the type of crest in chickens is conditioned by two allele genes, R / R and And is, which interact and segregate independently. The interaction between alleles R and AND results in a nut crest; between the recessive allele r and the dominant AND results in a pea crest, and between the recessive alleles r and and results in simple crest.