Question

What is codominance? Explain it through determination of blood groups in humans. Draw a table showing the genetic basis of blood groups in the human population.

Answer 1

Hint: In F1 hybrid, both alleles expressed themselves equally and there no mixing on the effect of both the alleles, therefore hybrid progeny that is everyone resembles both the parents

Complete answer:
The alleles which do not show the dominance recessive relationship and are able to express themselves independently when present together are called codominant alleles. This phenomenon is known as codominance. The symbols used for codominant genes are different. One method is to show by there on capital alphabet example R for red hair in cattle and W for white hair in cattle. In another method capital base symbols are employed for both alleles with different subscript's example capital Ia,Ib.

ABO blood group is also a good example of codominance. For ABO blood groups , allele Ia produces N-acetylglucosamine transferase enzyme which recognizes H antigen present in the RBC membrane and adds N acetylglucosamine to sugar parts of H antigens to form A antigen.

The allele B produces galactosyltransferase enzyme which recognizes H antigen to form B antigens. Allele I does not produce any sugar or antigen. IA&B are completely dominant over one another , in other words antigens A and we are produced to stop this is because of codominance. These antigens determine the type of blood group. Blood Group A has antigen A and blood Group B has antigen B , AB has both the antigens while O do not carry any antigens. Thus, six genotypes and four phenotypes are possible.

The genetic basis of the blood group can be

Allele from Parent 1	Allele from Parent 2	Genotype of offspring	Blood-types of offspring
IA	IA	IA IA	A
IA	IB	IA IB	AB
IA	I	IA i	A
IB	IA	IA IB	AB
IB	IB	IB IB	B
IB	i	IB i	B
i	i	ii	O

Note: 1) Blood group AB is universal recipient
2) Blood group O is a universal donor.