Explain with examples that Mendel’s dominant factor fails to govern phenomenon of inheritance in many cases.

By incomplete dominance Mendel fails to govern the phenomenon of inheritance. In case of flower colour, Mendel found that red colour of flowers was dominant over white colour of flowers. The white coloured flowers reappeared in F₂.

Later a deviation to this rule was observed in plant species commonly called four o clock (Mirabilis jalapa). The plant produces flowers with red and white colours. When pure breeding red (R) flowered plants were crossed with pure breeding white (r) flowered plants, the F₁ plants were pink (Rr) flowered. Appearance of pink flowers, an intermediate shade between red and white, provided relief to those who believe in blending inheritance when F₂ hybrids were crossed (Rr × Rr), the F₂ generation showed a phenotypic ratio of 1 red : 2 pink : 1 white, instead of typical 3: 1 ratio. This ratio contradicted Mendel’s principles. Appearance of red and white factors (genes) in F₂indicates that these have not been altered or blended while present together in the pink flowered individuals. F₂ reds and whites are same as the parental red and white.

This was declared as case of incomplete dominance. In this case neither gene is dominant to the other. Each expresses itself in the presence of its allele to produce an intermediate effect.

Another important aspect of incomplete dominance in that phenotypic ratio (1: 2: 1) is the same as genotypic ratio (1 homozygous dominant: 2 heterozygous dominant: 1 homozygous recessive) resulting from cross when two monohybrids are crossed. When dominance is incomplete, a cross of two monohybrids (Rr × Rr) gives a phenotypic ratio of 1: 2: 1 which is identical to the genotypic ratio. The heterozygous individual shows incomplete dominance.

Incomplete dominance in four o clock.

CODOMINANCE

In case of blood types in human, both genes (A & B) produce an effect in a heterozygous individual. This is called codominance. The genes which govern A and B blood types are alleles. Each control the formation of a different red blood cell Protein or antigen. Antigen a in case of person having blood group A and antigen b in individuals with blood group B. Neither gene is dominant to the other. The heterozygous individuals with blood group AB contains both antigen a and b. Both proteins are detected in equal amounts in the red cells.

The case of incomplete dominance and codominance suggest that the dominance is not universal and absolute.