Question

A) Difference between dominance and codominance.
B) Explain co-dominance taking an example of human blood groups in the population.

Answer 1

Hint: The principle of dominance states that, when two genes are found in an organism one of them masks the presence of another one, in other words one gene is dominant and stops the expression of the other gene.

Complete Answer:
A)

Sl.No	Dominance	Codominance
1.	During breeding or pollination if one gene of one gametes gets expressed in every generation and the other gets suppressed then it is said that the expressed gene is dominant over the other.	In codominance both the parents’ genes get equally expressed. Because in codominance both the genes have the dominant property hence both of them are expressed equally.
2.	For- example, dog plants consist of two colours of flower; white or red, while crossing between both of them in the F1 generation most of the progeny shows red colour flower, therefore red colour pigment gene is dominant over the white colour gene.	For example; roan fur in cattle in which we can observe that white and red fur are equally expressed.

B) Co-dominance is the phenomenon in which both the alleles in a heterozygote express themselves at the same time.
- Co-dominance is observed in blood grouping of humans. The ABO blood grouping controlled by gene I, and have three alleles IA, IB, and i. The allele IA and IB form different forms of sugar and allele i does not form sugar.
- The allele IA and IB are dominant over i, therefore when IA and i is present only IA will be expressed and same in case of IB and i. but when IA and IB present together both are expressed at the same time. Because of this there are three different alleles that have six different genotypes and four different phenotypes.

Note: The blood grouping is also an example of multiple alleles as more than two alternative forms of gene found on a particular locus are considered as the multiple alleles. It is formed as a result of repeated mutation of the same gene but in different directions.