Co-domination is an unusual type of interaction of allelic genes

To understand what is codomination in genetics, we will analyze the possible types of interaction of allelic genes. According to the hypothesis of gamete purity, proposed by Gregor Mendel, with the formation of gametes, only one of the two allelic genes of each parental organism is responsible for it, responsible for this feature. So in a gamete a normal diploid set of allelic genes is formed. Further, full domination may occur in the interaction, when a dominant trait will suppress recessive, incomplete dominance and codomination.

Incomplete dominance

In this case, the dominant allele does not completely suppress the recessive, resulting in a new, intermediate sign. A well-known example of incomplete dominance is the coloration of flowers of certain colors, for example, cosmeia. Suppose there is a homozygous red flower with a genotype (AA) (clean line) and a white flower (aa), also a clean line. When they cross, flowers with a pink color appear - an example of codomination. Their genotype is Aa, but both the dominant and recessive alleles are manifested. When crossing, an intermediate - pink color was obtained.

Co-operation

Another type of gene expression is codomination. This phenomenon is similar to incomplete dominance, but still has one significant difference. Co-domination is the interaction of genes, in which opposite signs manifest simultaneously, but do not mix and produce an intermediate sign.

When crossing a white petunia flower with red it can turn out red, pink, white or two-color. A flower with red and white stripes appears as a result of a process such as codomination. This is the most common example of such interaction.

Co-domination is also characteristic of other plants.

Interaction of non-allelic genes

It is worth saying that only the allelic genes are applicable to such concepts as full dominance, incomplete dominance and codomination. Examples and numerous experiments confirm that in the case of non-allelic genes, other types of interaction are called - cooperation, epistasis, complementarity, polymorphism. An example of precisely a polymer, and not an incomplete dominance, is the inheritance of human skin color.

Human cohomination

Another simple but striking example of codomination is the inheritance of blood groups. As you know, there are four blood groups. The first group of O (I) is manifested when there are two homozygous recessive genes in the genotype. The second group of A (II) may also appear in the genotype AO or AA. In the phenotype, only the dominant A gene will be present, which completely suppresses the recessive gene. A similar situation will be for the third blood group B (III), which is formed when the genotype is BB or VO. Dominant gene B will suppress the recessive gene O and manifest as a result of complete dominance. But what will happen when crossing homozygotes with AA and BB genotypes? Both gene A and gene B are dominant, therefore none of them can completely suppress the other and will manifest itself. In this case, with a 100% probability, the fourth blood group will be obtained - AB, there is a codomination. The same occurs when the heterozygotes AO and BO are crossed, when any result is possible:

R: AOHWO;

F1: AO (II), AB (IV), BO (III), GS (I).

That is why the blood type of the child may not coincide with the blood group of the parents. From the example it can be seen that codomination is manifested not only in the coloring of plants.

Codemaking and mutations

It is worth mentioning that the manifestation of both signs is not always codomination. This is proved by a rare genetic feature peculiar to humans and some animals - heterochromia (color mismatch in the iris of the eyes). Heterochromia is complete, for example, when one eye is brown, and the second is blue, or partial, for example when there is a gray segment on the green envelope. Heterochromia, despite the apparent analogy with the coloration of flowers, is an example of not a codomination, but a genomic mutation. Breaking skin pigmentation is also not a codomination, as indicated by genetics. In this case, the codomination is confused with the diseases.

The codification and the first law of Mendel

The phenomena of codomination and incomplete dominance, at first glance, suggest that Mendel 's first law on the uniformity of hybrids does not hold. Gregor Mendel in his experiments dealt with peas, for which neither codomination nor partial dominance is typical, but only complete domination. In those cases, if a mixed sign or their simultaneous manifestation is impossible, its formulation was absolutely correct. Nearly a century later, when both codomination and incomplete dominance were investigated, the first law was amended to say that when hybridizing homozygous first-generation hybrids with opposite traits in the second generation, hybrids appear that are identical in this feature. There is a dominant feature in case of complete dominance or a mixed sign - in case of incomplete dominance.

You can use an example with the inheritance of a blood group to visually demonstrate the correctness of the amended first law of Mendel:

P: AA × BB;

F1: AB, AB, AB, AB.

The result of crossing two pure lines will be a heterozygous individual, in the phenotype of which a mixed sign is manifested, since there is a codomination. This corresponds to the amendment made.