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

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Answer:

YES. This information is enough to assume that neither of the blood groups dominates over the other one.

Explanation:  

Available data:

  • A man with blood group B ----> Genotype IBIB or IBi
  • A woman with blood group AB ---> Genotype IAIB
  • Their daughter has blood group AB --> Genotype IAIB

The fact that the daughter´s genotype is IAIB tells us that she got the IA allele from the mother, who is IAIB, and the IB allele from his father, who is IB-.

  • If blood type B was dominant over A or AB, then the daughter should have Blood type B.
  • If blood type A was dominant over B or AB, then the daughter should have Blood type A.
  • Blood type AB tells us that both types are being expressed, A and B.

The fact that the daughter inherited one type of allele from each parent, but she is expressing the blood type AB, suggests that neither of the alleles dominates the other one. This is an example of co-dominance, where the heterozygote phenotype is an additive expression of both of the parents' genes.

This is enough information to assume that neither of the blood groups dominates over the other one.

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