Question

In incomplete dominance and codominance, genotypic and phenotypic ratios are identical. Explain how co-dominance differs from incomplete dominance in phenotypic nature of their hybrids?

Answer 1

In incomplete dominance, the hybrid progeny (${{F}_{1}}$phenotype) does not resemble either of the two parents or in codominance, the hybrid progeny (${{F}_{1}}$phenotype) resembles both of the parents.

Complete answer:
In complete dominance ${{F}_{1}}$ phenotype is intermediate between dominant and recessive phenotype. A good example of incomplete dominance is the flower colour in snapdragon (dog flower or Antirrhinum majus). In this red flowered plant crossed with white flowered plant and the resulting ${{F}_{1}}$ offspring had pink flowers. Here one allele is incompletely dominant over another. If the ${{F}_{1}}$is selfed, the plants of ${{F}_{2}}$ generation are of three types red, pink and white flowered in the ratio of 1 : 2 : 1.
Codominance: In the ${{F}_{1}}$hybrid, both alleles express themselves equally and there is no mixing of the effect of the alleles, therefore hybrid progeny (${{F}_{1}}$) resembles both parents. The alleles which do not show dominance-recessive relationships are able to express themselves independently when present together. ABO blood group is a good example of codominance.
In incomplete dominance, the phenotypic effect allele is more prominent than the other allele and in codominance the phenotypic effect of both the alleles is equally prominent and both processes of dominance produce a third type of phenotype.

Note: The genes on chromosomes regulating the characters are known as genotype. The genes on chromosomes regulating the physical expression of the characters are called phenotype.