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

Incomplete domination is a special typeinteraction 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 attribute 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 offspringinherited intermediate signs 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 twentieth 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, fruit flies).

After cytologic 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 the plants with white and red corollas the descendants had flowers of pink coloration - incomplete dominance. This is a manifestation in hybrids (genotype Aa) of the trait, which is intermediate with respect to the 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 cause of the change in the work of enzymes?

The mechanism of the appearance of the third variant of the featurecan 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) Enzymes will be enough, and the amount of pigment will correspond to the norm 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. Color intensity. U. Batson, crossing the 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. Genotype AA gives curly hair, aa - Straight, 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 numbergenotypes, which characterizes incomplete dominance. Analyzing crossing to determine hybrids is not required, since they are outwardly different from the dominant pure line.

Splitting features when crossing

Complete and incomplete dominance as a geneThe interaction takes place in accordance with the arithmetic of G. Mendel's laws. In the first case, the ratio of F2 phenotypes (3: 1) does not coincide with the ratio of genotypes of offspring (1: 2: 1), since the phenotypic combination of alleles AA and Aa 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 you cross a plant with red berries (AA) and a plant with white berries - genotype aa, then in the first generation all the plants obtained will give fruits with a pink color (Aa).

Crossed hybrids from F1, in the second generation of F2 we get the ratio of descendants that coincides with that of 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 phenotypesoffspring it is difficult to establish how 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, incompatible 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 abnormalities in the development of the stomach.

In humans, an example of the lethality of the dominant formgene - 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 form of red blood cells. Blood cells can not effectively attach oxygen, and 95% of children with this anomaly die from oxygen starvation. In heterozygotes, however, the altered form of red blood cells does not affect viability to such an extent.

Splitting of features in the presence of lethal genes

In the first generation with crossing AA x aa The mortality will not be manifested, since all descendants will have a 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 feature is the result of the interaction of gene products.