In the experiments of G. Mendel, the dominant trait completely suppressed the manifestation of the recessive one.
But in some cases, Mendel was faced with contradictions that he could not explain.
Heterozygous organisms do not always have a dominant trait. Often, heterozygotes show an intermediate phenotype.
In 1902, C. Correns described a case when, after crossing plants with white and red corollas, the offspring had pink flowers.
In such cases, one speaks of incomplete dominance.
Incomplete dominance is a special type of allele interaction in which a weaker recessive trait cannot be completely suppressed by a dominant one.
Example: When crossing a white-flowered night beauty plant (aa) with a plant with red flowers (AA), all F1 hybrids have pink flowers (Aa).
When hybrids with a pink color are crossed among themselves in F1, splitting occurs in the ratio: 1 (red): 2 (pink): 1 (white).
In case of incomplete dominance, the heterozygote has a phenotype intermediate between the phenotypes of homozygotes. At the same time, Mendel’s first law of uniformity for first-generation hybrids is fulfilled.
In the second generation (F2), the phenotype and genotype are split in a ratio of 1: 2: 1.
Traits inherited by intermediate type
Incomplete dominance has been studied in many flowering plants (snapdragon, hyacinth, strawberry, wheat, etc.).
Example: in strawberries, incomplete dominance is observed with the inheritance of fruit color (red, pink, white).
The phenomenon of incomplete dominance is also common in animals.
Example: when crossing pure lines of Andalusian chickens with black and white color, offspring with silver plumage are obtained.
The phenomenon of incomplete dominance is also observed in humans.
Example: this is how brachydactyly (shortening of the phalanges of the fingers) is inherited. In a person with genotype AA, development of fingers and skeleton is normal. In heterozygotes Aa, shortening of the phalanges of the fingers is observed, while in recessive homozygotes aa, skeletal development occurs with multiple disorders, and they turn out to be unviable.