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As we all know, allele genes are those that act in determining the same character and are present in the same loci (plural of locus, from Latin, local) on homologous chromosomes. So far, we have only studied cases where there were only two types of alleles for a given trait (single alleles), but there are cases where more than two types of alleles are present in determining a particular character in the population. This type of inheritance is known as multiple alleles (or polyalelia).
Although there may be more than two alleles for determining a particular character, a diploid individual has only one pair of alleles for determining this trait, that is, one allele on each locus of the chromosome that constitutes the homologous pair.
Cases of multiple alleles in both animals and vegetables are quite common, but examples of polyalelia in the determination of coat color in rabbits and the determination of blood groups of the ABO system in humans are classic.
A very interesting and easy to understand example is the determination of coat in rabbits, where we can observe the genetic manifestation of a series with four allele genes: the first Çexpressing color Aguti or Wild; the second Çchtransmitting color Chinchilla; the third ÇHrepresenting the color Himalayas; and the fourth allele ÇThe, responsible for color Albina.
Being the relationship of dominance → C> Cch > CH > CThe
The C gene is dominant over all three others, the Çch dominant over the Himalayas and albino, but recessive towards the aguti, and so on.
The table below represents the combinations between the alleles and the resulting phenotypes.
|CC, C Cch, C CH and C CThe||Wild or aguti|
|ÇchÇch, ÇchÇH and CchÇThe||Chinchilla|
|ÇHÇH and Ch CThe||Himalayas|
The difference in rabbit coat color from pea seed color is that we now have more different genes at work (4) than the two classic genes. However, it is crucial to know that Mendel's first law is still obeyed, that is, for the determination of coat color, the rabbit will have two of the four genes. The novelty is that the number of genotypes and phenotypes is higher when compared, for example, with the color of pea seed.
The emergence of multiple alleles (polyalelia) is due to one of the properties of genetic material, which is to mutate. Thus, it is believed that from the gene C (aguti), due to an accidental error in DNA duplication, originated the gene Çch (chinchilla). The existence of multiple alleles is interesting for the species, as there will be greater genetic variability, allowing more opportunity for adaptation to the environment (natural selection).