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

Incomplete domination is a special type of interaction of gene alleles, in which a weaker recessive trait can not be completely suppressed by the dominant one. In accordance with the regularities discovered by G. Mendel, the dominant sign 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 encountered 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 traits that did not give the parent gene in the homozygous form. Incomplete domination was not in the conceptual apparatus of genetics until the beginning of the XX century, when the laws of Mendel were rediscovered. At the same time, many natural scientists conducted genetic experiments with plant and animal objects (tomatoes, legumes, hamsters, mice, fruit flies).

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

In the same year, 1902, Chermak Correns described the case when, after crossing plants with white and red corollas, the offspring had pink-colored flowers - incomplete dominance. This is a manifestation in hybrids (genotype Aa ) of the trait, which is intermediate with respect to homozygous dominant ( AA ) and recessive ( aa ) phenotypes. A similar effect is described for many species of flowering plants: lion's pharynx, hyacinth, night beauty, strawberry berries.

Incomplete dominance is the reason for the change in the work of enzymes?

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

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

Characteristics inherited by an intermediate type

Such incomplete inheritance is well monitored on signs with a variable manifestation:

1. The color intensity. U. Batson, having crossed black and white hens of Andalusian breed, got offspring with silver plumage. In determining the color of the iris of the human eye, this mechanism is also present.
2. Degree of expression of the trait. The structure of human hair is also determined by incomplete inheritance of the trait. The genotype AA gives curly hair, aa - straight lines, and people with both alleles have undulating 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 crossing to determine hybrids is not required, since they are outwardly different from the dominant pure line.

Splitting of features when crossing

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

In strawberry coloring, I year is inherited by the principle of incomplete dominance. If to cross a plant with red berries ( AA ) and plant with white berries - genotype aa , then in the first generation all the plants obtained will give fruits with a pink color ( Aa ).

Crossed the hybrids from F1, in the second generation of F2 we get the ratio of descendants that coincides with that of the genotypes: 1AA + 2Aa + 1aa . 25% of plants from the second generation will give red and unpainted fruits, 50% of plants - pink.

A similar picture will be observed in two generations when crossing clean lines of flowers of the night beauty with purple and white flowers.

Features of inheritance in case of lethality of genes

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

In the Karakul sheep of gray coloring, newborn lambs homozygous for the dominant color allele die due to the fact that such a genotype causes disorders of the development of the stomach.

In humans, an example of the lethality of the dominant form of the gene is brachidactyly (short-haired). The symptom is revealed in the case of a heterozygous genotype, while the dominant homozygotes die in the early stages of intrauterine development.

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

Splitting of signs in the presence of lethal genes

In the first generation, when crossing AA x aa, lethality does not appear, since all descendants will have the genotype Aa. Here are examples of splitting features in the second generation for cases with lethal genes:

It is important to understand that both alleles operate under incomplete dominance, and the effect of partial inhibition of the trait is the result of the interaction of gene products.