Many of us have heard about dominant and recessive genes - some chains of nucleotides hidden in our genome that are responsible for hereditary traits. How do they interact with each other? What is dominance and how does it happen? Why are recessive alleles not always suppressed dominant? Scientists have been interested in these issues since the discovery of genes.
Research history
The interaction of alleles has always been of great interest to geneticists. In the course of studies, it was found that there are various types of gene interaction - complete dominance, overdomination, multiple allelism, incomplete dominance, and coding.
Rightfully nicknamed the father of modern genetics, Gregor Mendel was the first to be interested in the laws of transmission of hereditary traits. During his famous experiments on the hybridization of pea plants, Mendel noted that the cross of yellow and green peas does not result in an intermediate sign. In the first generation, all the peas were yellow. Mendel himself then could not explain the results of his brilliant experiment. The theoretical foundation appeared much later, after the revival of interest in genetics and the discovery of the elementary unit of heredity - the gene. It depends on it the color of a pea, the shape of the nose, eye color, growth, the presence of hereditary diseases in humans.
Back to the Mendel experiment. For yellow color of peas gene A is responsible, for green - a. When two different clean lines are crossed, the splitting will be as follows:
R: AA x aa
F1: Aa Aa Aa Aa
Despite the fact that the genotype of all the resulting plants was a gene of both yellow and green color, as a result, only yellow appeared. In other words, the dominant trait completely drowned out the recessive one. In the same way, the shape of the peas was inherited - the smooth one prevailed over the wrinkled one. It is this example that reflects the complete dominance of genes - suppression by the dominant trait of a recessive in the presence of both of them in the genotype.
Examples of total dominance
Crossbreeding of plants of different colors is not the only area where complete dominance 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 the Rhesus factor, polydactyly, freckles, dark hair color is inherited. All these signs are dominant and will not allow the recessive to appear in the phenotype.
Complete dominance is of great importance in the inheritance of genetic diseases. Most of them (Tay-Sachs disease, Urbach-Vite disease, Gunther disease) are inherited in an autosomal recessive manner, that is, if a normal gene (dominant) is present in the genotype, the mutant allele will not manifest itself.
About incomplete dominance
Incomplete dominance is a type of gene interaction that is often found in nature. With it, the recessive allele is not completely suppressed by the dominant one, and a new, intermediate trait appears in the phenotype. A striking example of incomplete dominance is the coloring of cosmea flowers. If you cross a red plant with a white one, then in the first generation the phenotype cleavage will be as follows: 1 (AA): 2 (Aa): 1 (aa). That is, one flower will be red, one white, and two pink. The latter are an example of incomplete dominance, since the dominant trait, the red color, did not completely suppress the recessive. As a result, the influence of both genes is manifested in the body.
Incomplete dominance is characteristic not only for cosmea, but also for many other flowers: snapdragons, tulips, carnations.
Overdomination
Overdomination is an interesting and somewhat paradoxical type of gene interaction in which the dominant gene is more intense in the heterozygous organism (BB) phenotype than in the homozygous (BB) phenotype. Overdomination is not found in nature as often as complete dominance. An example is the HBV gene mutation, which reduces the risk of developing malaria.
Coding
There are several more interesting types of gene interaction, and one of them is coding. In this case, the dominant allele does not mask and does not suppress the recessive one, and in the phenotype both characters manifest themselves to a certain extent.
The easiest way to understand the coding phenomenon is the example of red-white flowers of rhododendron, or nocturnal 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 white color. This is how unusual two-color flowers with the Aa genotype are obtained.
An example of coding 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 a third (I B I B ), then the child will have a fourth group, which is not intermediate between the second and third, with the genotype I A I B.