Question

Explain the following terms with example
A. Co-dominance
B. Incomplete dominance

Answer 1

Hint:- Mendel observed monohybrid crosses and proposed his first law - Law of Dominance. According to this law, if two dissimilar parents (true breeds) are crossed, the one who is dominant always shows its characters in the F1 generation, and the other whose characters are hidden in the F1 generation is called recessive.

Complete step-by-step solution:-
The law of dominance has two major exceptions: Codominance and Incomplete dominance.
A. Co-dominance - Mendel proposed that F1 generation always resembles one of the parents (dominant). But sometimes, the F1 generation resembles both the parents and this phenomenon is known as Codominance. The most common example of codominance in ABO blood grouping, in which the blood groups are controlled by the gene I. The gene ‘I’ has three alleles namely IA, IB and i. Both IA and IB are dominant over i, but are co-dominant to each other. Thus when any one of the alleles IA or IB is present with i, the blood group would be A or B respectively. But if both IA and IB are together, the blood group would be AB.
B. Incomplete dominance - The second exception to the Law of Dominance is Incomplete Dominance in which the F1 progeny do not resemble any one of the parents. The traits of the F1 generation are in between the traits of the two parents. Example of Incomplete Dominance is seen in the flower colour of the plant Antirrhinum (snapdragon). In this plant, when the true breeding dominant red flowered plant (RR) is crossed with true breeding recessive white flowered plant (rr), the F1 progeny has pink flowers (Rr).

Note:-
We know that the alleles are present in pairs. For dominant character, either one or both the alleles present will show dominant traits, for example - TT or Tt both signify tall plants. Whereas in case of recessive trait, both the alleles should be present in recessive form (homozygous condition), such as tt for dwarf plants.