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 by dominant ones? These questions have occupied scientists since the discovery of genes.
Research history
The interaction of alleles has always been of great interest to geneticists. In the course of research, it was found that there are various types of gene interaction - complete dominance, overdominance, multiple allelism, incomplete dominance and codominance.
Rightly called the father of modern genetics, Gregor Mendel was the first to become interested in the laws of 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 did not result in an intermediate trait. In the firstgeneration, 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 the 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, height, the presence of hereditary diseases in humans.
Let's get back to Mendel's experiment. The A gene is responsible for the yellow color of the peas, and the a gene for the green color. When crossing two different pure lines, the splitting will be as follows:
R: AA x aa
F1: Aa Aa Aa Aa
Despite the fact that in the genotype of all the resulting plants there was a gene for both yellow and green, in the end only yellow appeared. In other words, the dominant trait completely drowned out the recessive one. In the same way, the shape of peas was inherited - smooth prevailed over wrinkled. It is this example that displays the complete dominance of genes - the suppression of the recessive trait by the dominant trait in the presence of both of them in the genotype.
Examples of complete dominance
Crossing plants of different colors is not the only area where complete dominance appears. 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 traits, then all children will have brown eyes.
Similarly, the presence of the Rh factor, polydactyly,freckles, dark hair color. All these traits are dominant and will not allow a recessive phenotype to appear.
Complete dominance is of great importance in the inheritance of genetic diseases. Most of them (Tay-Sachs disease, Urbach-Wite disease, Gunther's disease) are inherited in an autosomal recessive manner, that is, if the genotype contains a normal (dominant) gene, the mutant allele will not manifest itself.
About incomplete dominance
Incomplete dominance is one of the types of gene interaction, 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 cosmos flowers. If you cross a red plant with a white one, 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 are an example of incomplete dominance, since the dominant trait, red, did not completely suppress the recessive one. As a result, the influence of both genes is manifested in the body.
Incomplete dominance is typical not only for cosmea, but also for many other flowers: snapdragons, tulips, carnations.
Overdomination
Overdominance is an interesting and somewhat paradoxical type of gene interaction, in which the dominant gene in the phenotype of a heterozygous organism (BB) manifests itself more intensely than in the phenotype of a homozygote (BB). Overdominance does not occur in natureoften as complete dominance. An example is a mutation in the HBB gene, which reduces the risk of getting malaria.
Co-dominance
There are several other interesting types of gene interaction, and one of them is co-dominance. In this case, the dominant allele does not mask or suppress the recessive one, and both traits manifest themselves to a certain extent in the phenotype.
The easiest way to understand the phenomenon of co-dominance is the example of red-white flowers of rhododendron, or night beauty. This coloration 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. This is how unusual two-color flowers with the Aa genotype are obtained.
An example of codominance is the mechanism of inheritance of blood groups. Let one of the parents have the second blood type (IAIA), and the second one has the third (IВ IB), then the child will have a fourth group, which is not intermediate between the second and third, with the genotype IAIB.
