Describe in detail The pattern of inheritance in incomplete dominance & co – dominance.

Incomplete dominance : All traits in most organisms did not follow Mendel’s Laws of dominance. Eg.Flower colour in Snapdragon (Antirrhinum sp.): When true-breeding red-flowered (RR) and truebreeding white-flowered plants (rr), the resultant F1 generation showed a mixture of the two parental types ie. Were all Pink.(Rr).

 

When the F1 was self-pollinated the F2 resulted in the following ratio 1 (RR) Red : 2 (Rr) Pink: 1 (rr) White. Here the genotype ratios were exactly as we would expect in any Mendelian monohybrid cross, but the phenotype ratios had changed from the 3:1 dominant : recessive ratio.

What happened was that R was not completely dominant over r and this made it possible to

distinguish Rr as pink from RR (red) and rr (white)

Co – dominance: Exhibited by Human ABO Blood group system

The RBC carry surface polysaccharides A & B. Depending upon the type of this sugar there are different types of Blood groups in human. The gene responsible for production of this (I) has 3 different kinds of alleles –

When alleles I^A & i are present I^A expresses sugar type A , but i does not produce any sugar thus, Allele ‘ I^A’  is dominant over allele ‘i’

When alleles I^B & i are present I^B express sugar type B , but i does not produce any sugar thus Allele ‘I^B’ is dominant over allele ‘i”

Allele I^A & I^B are  are together, both express sugars type A & type B respectively ie. type AB blood group. Thus, I^A & I^B are co dominant.