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Question
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?
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Solution 1
No. This information is not sufficient to determine which of the traits – blood group A or O – is dominant. This is because we do not know about the blood group of all the progeny.
Blood group A can be genotypically AA or AO. Hence, the information is incomplete to draw any such conclusion.
Solution 2
- If blood group A is dominant: The mother would be homozygous for the recessive allele O. The father, with blood group A, could be either homozygous or heterozygous. If he is homozygous, he would not carry the allele O. The daughter would inherit allele O only from her mother, but since allele O is recessive, it would not be expressed. However, if the father is heterozygous, he would also have an allele for O. In this case, the daughter could inherit allele O from both parents and if homozygous for the recessive allele O, she would have blood group O.
- If blood group O is dominant: The father would lack allele O in order to have blood group A. The mother could be either homozygous or heterozygous for allele O. In both situations, the daughter would inherit one allele for blood group O, and since allele O is dominant, it would always be expressed, resulting in the daughter having blood group O.
Thus, since the daughter could have blood group O in both scenarios, we cannot definitively say which blood group is dominant.
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