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    Determining Genotypes

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    A pregnant woman is worried about the chances of her child having hemophilia as both she and her husband have paternal grandfathers with hemophilia. Neither their parents nor their maternal grandparents have hemophilia.

    What is the genotype of the paternal grandfathers who have hemophilia?

    What is the genotype of their grandmothers and grandfathers without hemophilia?

    Which of the pregnant couple's parents could carry the gene for hemophilia?

    Does (or can) either of the pregnant couple carry the gene for hemophilia?

    What are the possible genotypes of the couple?

    Which is the likelihood that their child will have hemophilia?

    How would things change for the couple's genotypes if their maternal grandfathers had hemophilia and their paternal ones didn't?

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    Solution Preview

    Hemophilia is X-linked recessive, which means that it is present only on the X-chromosome, and can remain unexpressed if a dominant allele exists on the other X-chromosome for women, but will be expressed if both X's have the recessive allele in women, or if the singular X chromosome in men contains the same allele.

    Since the paternal grandfathers of the couple had hemophilia, and they are male, their singular X-chromosomes must have the recessive allele. It is the only way males can express the disorder.

    The genotype of all grandfathers without hemophilia must be negative for hemophilia - again because if they do, they most certainly have the disorder. Grandmothers on the other hand, have two X-chromosomes. If both these X-chromosomes carry the same recessive allele for hemophilia, then the woman will have the disorder. If only one of them carries it, while the other chromosome carries a dominant allele, then the woman is a carrier of the disorder but does not express it. Finally, the third possibility is that both X-chromosomes had dominant alleles, in which case the genotype is completely negative for hemophilia.

    Since we're concerned about the genotype of grandmothers that did not have hemophilia, we can immediately rule out double recessive X's. This leaves us with heterozygous carrier or homozygous dominant. Unfortunately, there isn't enough information to really say which one of these the ...

    Solution Summary

    The type of genotype of the paternal grandfathers who have hemophillia are examined.