To understand what coding is in genetics, we will analyze the possible types of interaction of allelic genes. According to the hypothesis of gamete purity proposed by Gregor Mendel, when a gamete is formed, only one of the two allelic genes of each parent organism responsible for this trait gets into it. So in the gamete a normal diploid set of allelic genes is formed. Further, in the interaction, complete dominance may appear when the dominant trait suppresses recessive, incomplete dominance and coding.
Incomplete dominance
In this case, the dominant allele does not completely suppress the recessive one; as a result, a new, intermediate trait is obtained. A well-known example of incomplete dominance is the coloring of flowers of some flowers, for example cosmei. Suppose there is a homozygous red flower with genotype (AA) (clean line) and a white flower (aa), also a clean line. When they cross, pink flowers appear - an example of coding. Their genotype has the form Aa, but both the dominant and recessive alleles are manifested. When crossing, an intermediate was obtained - pink color.
Coding
Another type of gene expression is coding. This phenomenon is similar to incomplete dominance, but still has one significant difference. Codomination is the interaction of genes in which opposite traits appear simultaneously, but do not mix and do not produce an intermediate trait.
When a white petunia flower is crossed with red, it can turn out red, pink, white or two-tone. A flower with red and white stripes appears as a result of a process such as coding. This is the most common example of such an interaction.
Codomination is also characteristic of other plants.
Non-allelic gene interaction
It is worth saying that only concepts such as complete dominance, incomplete dominance and coding are applicable only to allelic genes.
Examples and numerous experiments confirm that in the case of non-allelic genes, other types of interaction are called - cooperation, epistasis, complementarity, polymerization. An example of precisely polymer, rather than incomplete dominance, is the inheritance of human skin color.
Human coding
Another simple but striking example of coding is the inheritance of blood groups. As you know, there are four blood types. The first group O (I) manifests itself in the presence of two homozygous recessive O genes in the genotype. The second group A (II) can also occur with the genotype AO or AA. In this case, only the dominant gene A, which completely suppresses the recessive gene, will appear in the phenotype. A similar situation will be for the third blood group B (III), which is formed with the genotype BB or VO. The dominant gene B suppresses the recessive O gene and appears as a result of complete dominance. But what will happen when crossing homozygotes with genotypes AA and BB? Both gene A and gene B are dominant, which means that none of them can completely suppress the other and will manifest itself. In this case, with a probability of 100%, a fourth blood group will be obtained - AB, coding takes place. The same thing happens when the heterozygotes of AO and VO are crossed, when any result is possible:
P: AOhVO;
F1: AO (II), AB (IV), VO (III), OO (I).
That is why the blood type of the child may not coincide with the blood type of the parents. It can be seen from the example that coding is manifested not only in the color of plants.
Codemining and mutations
It should be noted that the manifestation of both signs is not always coding. This is proved by a rare genetic feature common to people and some animals - heterochromia (mismatch in the color of the iris). 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 shell. Heterochromia, despite the apparent analogy with the color of flowers, is not an example of coding, but a genomic mutation. A violation of skin pigmentation is also not coding, as genetics speaks of. Coding in this case is confused with diseases.
Coding and Mendel’s first law
At first glance, the phenomena of coding and incomplete dominance suggest that Mendel ’s first law on the uniformity of hybrids is not fulfilled. Gregor Mendel, in his experiments, dealt with peas, for which neither coding nor partial dominance is characteristic, but only complete dominance. In those cases where a mixed sign or their simultaneous manifestation is impossible, its wording was absolutely correct. After almost a century, when coding and incomplete dominance were investigated, the first law was amended to read that when crossing homozygous hybrids of the first generation with opposite characteristics, hybrids appear in the second generation that are identical on this basis. A dominant sign appears in the case of complete dominance or a mixed sign in the case of incomplete dominance.
You can use the blood group inheritance example to demonstrate the correctness of the augmented first Mendel law:
P: AA × BB;
F1: AB, AB, AB, AB.
The result of crossing two clean lines will be a heterozygous individual, in the phenotype of which a mixed trait appears, since coding takes place. This is consistent with the amendment.