Question

The ABO blood type of humans exhibit condominance. What is condominance, and how does this effect what blood type a human could have?

Answer 1

Father of genetics, Gregor Mendel, proposed three fundamental laws of genetics. One of those three laws is “Law of Dominance” which states that recessive alleles will always be masked by dominant alleles. But these laws do not hold true in all the inheritance cases and those inheritance patterns are known as “Non- Mendelian Inheritance”

One of those inheritance patterns holds true in case of Codominance. This is a type of inheritance pattern in which two alleles at a locus in an animal who is heterozygous for two alleles display both the phenotypes observed in animals homozygous for one allele. In more simpler terms, both the alleles are expressed simultaneously in the heterozygote.

One of the commonly studied examples of codominance is the ABO blood group system in humans. This system is used for the determination of blood groups in humans. It recognizes the presence of one, both, or neither of A and B antigens on RBC (erythrocytes).

The blood groups are inherited by both the parents and controlled by a single gene which have three types of alleles i.e. i, I^A and I^B. I^A allele gives type A blood group, I^B gives type B blood group and i gives type O. I^A and I^B both are dominant over i and only ii genotype people can have O blood group. People having I^AI^A or I^Ai express type A blood group and people having I^BI^B or I^Bi express type B blood group. Interestingly, I^AI^B people have both the phenotypes, this is because A and B express themselves in a codominant manner which means the resulting blood group will be AB.