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Question 3

A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits – blood group A or O – is dominant? Why or why not?

Answer

This information is not enough to tell which of the traits A or O dominant. In human hereditary characters, A, B, ABO blood group is dominant or O is recessive. But in the above case we cannot detect which blood group is dominant. If father’s blood group is A, genotypes will be homozygous AA or heterozygous AO. And women having the blood group O, then the genotype will be homozygous OO and heterozygous AO. 



If A is dominant, the cross of father genotype IAIA with mother have IoIo and the progeny will have 50- 50 percent of the genetic material, where the father blood group is A and mother’s is O.

                       IA                IA

IAIO IAIO
IAIO IAIO

                      Io                        Io

 If the blood group O is dominant, when we cross the father genotype IAIO with the mother genotype IoIo, the child will contain blood group O.

                       IA                Io

IAIO IOIO
IAIO IOIO

                      Io                        Io

So, in both cases the child will have O. So, this is not clear which trait is dominant.

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