Incomplete dominance is the result of the interaction of alleles of one gene

Incomplete dominance is a special type of gene allele interaction in which a weaker recessive trait cannot be completely suppressed by a dominant one. In accordance with the laws discovered by G. Mendel, the dominant trait completely suppresses the recessive manifestation. The researcher studied pronounced contrasting signs in plants with the manifestation of either dominant or recessive alleles. In some cases, Mendel was faced with a failure to comply with this pattern, but did not give her an explanation.

A new form of inheritance

Sometimes, as a result of crossing, the descendants inherited intermediate characters that the parent gene did not give in a homozygous form. Incomplete dominance was not in the conceptual apparatus of genetics until the beginning of the 20th century, when Mendel's laws were rediscovered. At the same time, many natural scientists conducted genetic experiments with plant and animal objects (tomatoes, legumes, hamsters, mice, Drosophila).

After cytological confirmation in 1902 by Walter Setton of the Mendelian laws, the principles of the transmission and interaction of characters began to be explained from the perspective of the behavior of chromosomes in the cell.

In the same 1902, Chermak Correns described a case when, after crossing plants with white and red corollas, the offspring had pink flowers - incomplete dominance. This is a manifestation in hybrids (genotype Aa ) of a trait that is intermediate in relation to homozygous dominant ( AA ) and recessive ( aa ) phenotypes. A similar effect is described for many types of flowering plants: snapdragon, hyacinth, nocturnal beauty, strawberries.

Incomplete dominance - is this the reason for the change in the functioning of enzymes?

The mechanism of the appearance of the third variant of the trait can be explained from the standpoint of the activity of enzymes, which are proteins by nature, and genes determine the structure of the protein. A plant with a homozygous dominant genotype ( AA ) will have enough enzymes, and the amount of pigment will be in order to stain the cell juice intensively.

In homozygotes with recessive alleles of the gene ( aa ), pigment synthesis is impaired, the corolla remains unpainted. In the case of the intermediate heterozygous genotype ( Aa ), the dominant gene still gives a certain amount of the enzyme responsible for pigmentation, but it is not enough for a bright saturated color. It turns out the color is "half".

Traits inherited by an intermediate type

Such incomplete inheritance is well tracked on traits with variable manifestation:

1. Color intensity. W. Betson, crossing black and white chickens of the Andalusian breed, received offspring with silver plumage. This mechanism is also present in determining the color of the iris of the human eye.
2. The severity of the sign. The structure of human hair is also determined by the incomplete inheritance of the trait. The AA genotype gives curly hair, aa gives straight hair, and people with both alleles have wavy hair.
3. Measurable indicators. The length of the ear of wheat is inherited by the principle of incomplete dominance.

In the F2 generation, the number of phenotypes coincides with the number of genotypes, which characterizes incomplete dominance. Analyzing crossbreeding is not required to identify hybrids, as they are externally different from the dominant net line.

Crossbreeding of characters

Complete and incomplete dominance as gene interaction occurs in accordance with the arithmetic of the laws of G. Mendel. In the first case, the ratio in F2 of phenotypes (3: 1) does not coincide with the ratio of offspring genotypes (1: 2: 1), since phenotypically the combinations of AA and Aa alleles appear identically. Then incomplete dominance is a coincidence in F2 of a proportion of differing genotypes and phenotypes (1: 2: 1).

In strawberries, the color is inherited for a year according to the principle of incomplete dominance. If you cross a plant with red berries ( AA ) and a plant with white berries - genotype aa , then in the first generation all the resulting plants will give fruits with a pink color ( Aa ).

Crossing hybrids from F1, in the second generation F2 we get the offspring ratio that matches the genotypes: 1AA + 2Aa + 1aa . 25% of plants from the second generation will produce red and unpainted fruits, 50% of plants - pink.

We will observe a similar picture in two generations when crossing pure lines of flowers of a night beauty with corollas of purple and white.

Features of inheritance in case of gene mortality

In some cases, it is difficult to determine how genes interact by the ratio of offspring phenotypes. In the second generation, splitting with incomplete dominance differs from the expected 1: 2: 1, and from 3: 1 - with full. This happens when a dominant or recessive trait in a homozygous state gives a phenotype that is not compatible with life (lethal genes).

In gray Karakul sheep, newborn lambs homozygous for the dominant color allele die due to the fact that this genotype causes disturbances in the development of the stomach.

In humans, an example of the lethality of the dominant form of the gene is brachidactyly (short-toed). The trait is detected in the case of a heterozygous genotype, while dominant homozygotes die in the early stages of fetal development.

Recessive alleles of genes can also be lethal. Sickle cell anemia leads, in the case of two recessive alleles in the genotype, to a change in the shape of red blood cells. Blood cells cannot effectively attach oxygen, and 95% of children with this abnormality die from oxygen starvation. In heterozygotes, an altered form of red blood cells does not affect viability to such an extent.

Cleavage of traits in the presence of lethal genes

In the first generation, when crossing AA x aa, mortality will not occur, since all descendants will have the genotype Aa. Here are examples of second-generation traits for cases with lethal genes:

It is important to understand that both aleles act with incomplete dominance, and the effect of partial suppression of the trait is the result of the interaction of gene products.