Complete dominance and other types of gene interaction

Many of us have heard of dominant and recessive genes - some nucleotide chains hidden in our genome, which are responsible for hereditary traits. How do they interact with each other? What is domination and how does it happen? Why recessive alleles are not always dominant? These issues are occupied by scientists since the discovery of genes.

History of research

Interaction of alleles has always been of great interest for geneticists. During the research it was found that there are different types of gene interaction - complete dominance, overdomination, multiple allelism, incomplete dominance and codomination.

Rightfully called the father of modern genetics, Gregor Mendel was the first to become interested in the laws of the transmission of hereditary traits. In the course of his famous experiments on the hybridization of pea plants, Mendel noticed that crossing yellow and green peas does not result in an intermediate feature. In the first generation, all the peas were yellow. Mendel himself then could not explain the results of his brilliant experiment. The theoretical basis appeared much later, after the revival of interest in genetics and the discovery of an elementary unit of heredity - the gene. It depends on him the color of the pea, the shape of the nose, the color of the eyes, growth, the presence of hereditary diseases in humans.

Let us return to Mendel's experiment. The yellow color of the peas is answered by gene A, for green - a. When crossing two different pure lines, the splitting will be as follows:

P: AA x aa

F1: Aa Aa Aa Aa

Despite the fact that in the genotype of all the resultant crosses of plants there was a gene of both yellow and green color, only yellow appeared in the end. In other words, the dominant sign completely drowned the recessive one. Similarly, the shape of the peas was inherited - the smooth predominated over the wrinkled. It is this example that shows the complete dominance of genes - suppression by the dominant trait of recessive if both of them are present in the genotype.

Examples of complete dominance

Crossing of different plants is not the only area where full domination is manifested. Examples of this type of interaction can also be cited from the field of human genetics: if one of the parents has brown eyes, the second has blue eyes, and both are homozygous for these signs, then all children will have brown eyes.

Similarly, the presence of Rh factor, polydactyly, freckles, dark hair color is inherited. All these signs are dominant and will not show up in a phenotype recessive.

Complete dominance is of great importance in the inheritance of genetic diseases. Most of them (Teya-Sachs disease, Urbach-Vite disease, Gunther's disease), are inherited in an autosomal recessive type, that is, if a normal gene (dominant) is present in the genotype, the mutant allele will not manifest itself.

On incomplete dominance

Incomplete domination is one of the types of gene interaction that often occurs in nature. With it, a recessive allele is suppressed not completely dominant, and a new, intermediate sign is manifested in the phenotype. A vivid example of incomplete dominance is the coloration of the colors of the cosmos. If the red plant is crossed with white, then in the first generation the phenotype splitting will be as follows: 1 (AA): 2 (Aa): 1 (aa). That is, one flower will be red, one white, and two pink. The latter represent an example of incomplete dominance, since the dominant trait, red, suppressed the recessive not completely. As a result, the body shows the influence of both genes.

Incomplete dominance is characteristic not only of cosmece, but also of many other colors: lion's pharynx, tulips, carnations.

Overdomination

Overdominance is an interesting and somewhat paradoxical type of gene interaction, in which the dominant gene manifests itself more intensely in the phenotype of the heterozygous organism (Bv) than in the phenotype of homozygote (BB). Overdomination is not found in nature as often as complete domination. An example is the mutation of the HBV gene, which reduces the risk of getting malaria.

Co-operation

There are several interesting types of gene interaction, and one of them is codomination. In this case, the dominant allele does not mask and does not suppress the recessive, and in the phenotype both signs manifest themselves to a certain extent.

The easiest way to understand the phenomenon of codomination is the example of red-white flowers of rhododendron, or a night beauty. This color is obtained by crossing red and white flowers, and although the red pigment is dominant, it does not drown out the allele responsible for the white color. So we get unusual two-colored flowers with Aa genotype.

As an example of codomination is the mechanism of inheritance of blood groups. Let one of the parents have a second blood group (I ^A I ^A ), and the second third (I ^B I ^B ), then the child will have a fourth group that is not intermediate between the second and third, with the genotype I ^A I B.