Incomplete dominance is a special type of interaction of gene alleles 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 manifestation of the recessive one. The researcher studied pronounced contrasting traits in plants with the manifestation of either dominant or recessive alleles. In some cases, Mendel encountered the failure of this pattern, but did not give an explanation for it.
New form of inheritance
Sometimes, as a result of crossing, the offspring inherited intermediate traits that the parental gene did not give in the 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, fruit flies).
After cytological confirmation in 1902 by W alter Setton of Mendelian patterns, the principles of transmission and interactionsigns began to be explained from the standpoint of the behavior of chromosomes in a cell.
In the same 1902, Cermak 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 (Aa genotype) of a trait that is intermediate in relation to the homozygous dominant (AA) and recessive (aa) phenotypes. A similar effect has been described for many types of flowering plants: snapdragon, hyacinth, night beauty, strawberries.
Incomplete dominance - is it the reason for the change in the work of enzymes?
The mechanism for the 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. A plant with a homozygous dominant genotype (AA) will have enough enzymes and the amount of pigment will be normal to intensely color the cell sap.
In homozygotes with recessive alleles of the gene (aa), pigment synthesis is impaired, the corolla remains uncolored. In the case of an intermediate heterozygous genotype (Aa), the dominant gene still produces some pigmentation enzyme, but not enough for a bright, saturated color. It turns out the color is "half".
Features inherited by intermediate type
Such incomplete inheritance is well tracked on traits with variable expression:
- Color intensity. W. Batson, having crossed black and white hens of the Andalusian breed,got offspring with silver plumage. This mechanism is also present in determining the color of the human iris.
- Degree of manifestation of the trait. The structure of human hair is also determined by the incomplete inheritance of the trait. The AA genotype produces curly hair, aa produces straight hair, and people with both alleles have wavy hair.
- 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 crosses are not required to identify hybrids, as they are outwardly different from the dominant pure line.
Splitting traits when crossing
Complete and incomplete dominance as 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 the offspring (1:2:1), since phenotypically, combinations of AA and Aa alleles manifest themselves in the same way. Then incomplete dominance is a coincidence in the F2 proportion of different genotypes and phenotypes (1:2:1).
In strawberries, coloring 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).
Having crossed hybrids from F1, in the secondgeneration F2 we get the ratio of offspring, coinciding with that of the genotypes: 1AA + 2Aa + 1aa. 25% of plants from the second generation will produce red and uncolored fruits, 50% of plants will be pink.
We will observe a similar picture in two generations when crossing the pure lines of flowers of the night beauty with purple and white corollas.
Features of inheritance in case of lethality of genes
In some cases, it is difficult to determine how genes interact based on 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 complete dominance. This happens when a dominant or recessive trait produces a phenotype in the homozygous state 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 such a genotype causes disturbances in the development of the stomach.
In humans, an example of the lethality of the dominant form of the gene is brachydactyly (short-fingered). The trait is detected in the case of a heterozygous genotype, while dominant homozygotes die in the early stages of intrauterine development.
Recessive alleles of genes can also be lethal. 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 cannot effectively take up oxygen, and 95% of children with this anomaly die fromoxygen starvation. In heterozygotes, the altered form of red blood cells does not affect viability to such an extent.
Splitting traits in the presence of lethal genes
In the first generation, when crossing AA x aa, lethality will not appear, since all descendants will have the Aa genotype. Here are examples of trait splitting in the second generation for cases with lethal genes:
|
Crossing option Aa x Aa |
Total domination | Incomplete domination |
| Lethal allele dominant |
F2: 2 Aa, 1aa By genotype - 2:1 By phenotype- 2:1 |
F2: 2 Aa, 1aa By genotype - 2:1 By phenotype- 2:1 |
| Lethal recessive allele |
F2: 1AA, 2Aa By genotype - 1:2 According to phenotype - no splitting |
F2: 1AA, 2Aa By genotype - 1:2 By phenotype- 1:2 |
It is important to understand that both alleles act with incomplete dominance, and the effect of partial suppression of a trait is the result of the interaction of gene products.
