Explain the following terms with an example:
(i) Codominance
(ii) Incomplete dominance
Codominance (1 : 2 : 1) - It is the phenomenon of two alleles (different forms of a Mendelian factor present on the same gene locus on homologous chromosomes) lacking dominant- recessive relationship and are able to express themselves independently when present together.
Example - AB blood group: Alleles for blood group A(IA) and blood group B(IB) are codominant so that when they come together in an individual, they produce blood group AB. It is characterized by the presence of both antigen A (from IA) and antigen B (from IB) over the surface of erythrocytes.
Incomplete dominance (1 : 2 : 1)- It is the phenomenon where none of the two contrasting alleles being dominant so that expression in the hybrid is intermediate between the expressions of the two alleles in the homozygous state. F2 phenotypic ratio is 1 : 2 : 1, similar to genotypic ratio.
Example- In Mirabilis jalapa (Four o’clock) and Antirrhinum majus (Snapdragon or dog flower), there are two types of flower colour generation are of three types- red, pink and white flowered in the ratio of 1 : 2 : 1. The pink colour apparently appears either due to the mixing of red and white colours (incomplete dominance).