Question

What is co-dominance? Explain it through determination of blood groups is human. Draw a table showing the genetic basis of blood groups in Human population.

Answer 1

In $F_1$ hybrid, both alleles express themselves equally and there no mixing of the effect of both alleles, therefore hybrid progeny ($F_1$) resembles both parents. The alleles which do not show the dominance-recessive relationship and are able to express themselves independently when present together are called co-dominant alleles. This phenomenon is known as co-dominance. The symbols used for co-dominant genes are different. One method is to show by their own capital alphabets, e.g. R (for red hair in cattle) and W (for white hair in cattle). In another method capital base symbols are employed for both the alleles with different superscripts, e.g. $I^A$, $I^B$.

ABO blood group is also a good example of co-dominance. For ABO system of blood groups, allele$I_A$ produces N-acetylgalactosamine transferase enzyme which recognises H- antigen present in RBC membrane and adds N-acetylgalactosamine to sugar parts of H antigens to form A antigen. The allele $I^B$ produces galactosyl transferase enzyme which recognized H antigen to form B antigens. Allele i does not produce any sugar or antigen. $I_A$ and $I_B$ are completely dominant over i, in other word antigens A and B are produced. This is because of co-dominance. These antigens determine the type of blood group. Blood group A has antigens B have antigen, AB has both antigens while blood group. Blood group A have antigen A, group B have antigen B, AB has both antigens while blood group O do not carry any antigens. Thus, six genotypes and four phenotypes are possible.