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

Incomplete dominance – a special type of gene interaction between alleles in which the weaker recessive trait can not be completely overpowered by the dominant. In accordance with the laws, open G. Mendel, dominant trait completely suppresses the recessive. The researcher studied the pronounced contrasting traits in plants by expression of either dominant or recessive alleles. In some cases, Mendel was faced with the failure of this pattern, but did not give her an explanation.

New form of inheritance

Sometimes as the result of crossing descendants inherit intermediate characteristics that were not given in the form of homozygous parent gene. Incomplete dominance was not in the conceptual device of genetics of the early twentieth century when it was rediscovered Mendel's laws. At the same time, many scientists carried out genetic experiments with animals and objects (tomatoes, beans, hamsters, mice, fruit flies).

After cytological confirmation in 1902 by Walter Seton of Mendel's laws the principles of transmission and interaction of signs began to explain from the position of the behavior of chromosomes in the cell.

In the same 1902, Correns Cermak described the case where, after crossing plants with white and red corollas from the descendants of flowers rose color – incomplete dominance. This is a manifestation of the hybrids (genotype AA) of characteristic that is intermediate relative to homozygous dominant (AA) and recessive (AA) phenotypes. A similar effect is described for many types of flowering plants: snapdragons, hyacinth, night beauties, strawberries.

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

The Mechanism of appearance of the third version of the symptom can be explained from the standpoint of the activity of enzymes, which in nature are proteins, and the genes determine the structure of the protein. Plants with homozygous dominant genotype (AA) of enzymes will be enough, and the amount of pigment will match the rate to intensively stain the cell SAP.

Homozygous recessive alleles of a gene (AA) the synthesis of the pigment is broken, the Corolla remains unpainted. In the case of the intermediate heterozygous genotype (AA), a dominant gene still gives a certain amount of enzyme responsible for the pigmentation, but it is not enough for bright saturated color. It turns the color “half”.

Signs that are inherited by intermediate type

This is an incomplete inheritance of a well-tracked on the signs with variable expression:

1. The Intensity of the color. W. Batson, crossed the black and white chickens Andalusian breed was the offspring with a silver tail. To determine the color of the iris of the human eye is also present in this mechanism.
2. The severity of the symptom. Hair texture also is determined by incomplete inheritance of the trait. The genotype of AA gives curly hair, AA – straight, and people with both alleles have a wavy hair.
3. Measurable performance indicators. The length of the ear of wheat is inherited according to the principle of incomplete dominance.

In the F2 generation the number of phenotypes equal to the number of genotypes that describes incomplete dominance. Analyzing the hybridization to determine the hybrids is not required, since they look different from the dominant clean lines.

Splitting characters when crossing

Complete and incomplete dominance genetic interaction occurs according to the arithmetic laws of Mendel. In the first case, the ratio of F2 phenotypes (3:1) does not coincide with the ratio of genotypes of offspring (1:2:1) as phenotypic combinations of alleles AA and AA Appear the same. Then incomplete dominance – a match in F2 the proportions of different genotypes and phenotypes (1:2:1).

Strawberry coloring I inherited the principle of incomplete dominance. If you cross a plant with red fruit (AA) and a plant with white berries – genotype AA, in the first generation, all the resulting plants will bear fruit with a pink color (AA).

Cross hybrids of F1, the second F2 generation will receive a proportion of descendants matching with such genotypes: 1АА+2Аа+1аа. For 25% of the plants from the second generation will give red fruits and unpainted, 50% of the plants – pink.

A Similar picture will be observed in two generations by crossing pure lines night beauty of flowers with corollas purple and white colors.

Features of the inheritance in the case of mortality genes

In some cases, the ratio of offspring phenotypes is difficult to establish how genes interact. In the second generation cleavage by incomplete dominance is different from the expected 1:2:1 and 3:1 — at full. This happens when dominant or recessive trait gives in the homozygous state the phenotype incompatible with life (lethal genes).

Karakul sheep gray color newborn lambs, homozygous for the dominant allele color, die due to the fact that this genotype causes developmental disorders of the stomach.

An example of human lethality, the dominant form of a gene – brachydactyly (korotkoplodnyh). The symptom is diagnosed in the case of heterozygous genotype, while dominant homozygotes die in the early stages of fetal development.

Can be Lethal and recessive alleles of genes. Sickle cell anemia results, in the case of two recessive alleles in the genotype, to change the shape of red blood cells. Blood cells cannot effectively attach oxygen and 95% of children with this anomaly die from oxygen starvation. Heterozygotes have changed the shape of erythrocytes is not affected to such an extent the viability.

Splitting attributes in the presence of lethal genes

In the first generation when crossing a AA x AA Lethality will not occur, because all descendants will have the genotype AA. examples of fission characteristics in the second generation for the cases with lethal genes:

It is Important to understand that operate under incomplete dominance both were burning, and the effect of partial suppression of the symptom is the result of interaction of gene products.