I dont understand the answer to this question

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Size:  498.5 KBThe answer is in red but I dont understand the answer. Can someone explain this to me please?Attachment 587160587162
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    (Original post by Ann_Onymous)
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Size:  498.5 KBThe answer is in red but I dont understand the answer. Can someone explain this to me please?Attachment 587160587162
    Males have to inherit their X chromosome from their mother, they can't inherit it from their father because they inherit the Y chromosome from him. Because the mother is homozygous recessive, the son is 100% going to inherit the X chromosome with the recessive allele, so there is 0% chance that the son can inherit the antigen G allele. I hope that helps? if not let me know what you don't understand and I'll explain it better!
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    Basically, this is an example of a condition inherited in a sex-linked recessive manner. In such conditions, an abnormal allele on the X chromosome, is transmitted from the grandfather, via his daughter to his grandson, the daughter being a carrier because in her XX sex chromosome, one X is normal and one carries the abnormal allele.

    In the grandfather and the grandson, the Y acts as what is referred to as a "sleeping partner" ie it has no effect on the phenotype; but because the mother and the grandmother in this example had the abnormal G antigen phenotype, they must have been homozygous for the abnormal allele.

    One condition inherited in this way is haemophilia, in which there is a deficiency of Factor VIII in the blood clotting mechanism leading to a bleeding tendency. It ran in the ancestors of the British royal family. Because of the explanation above, haemophilia is 8 times more common in men than in women (because females need to have both X chromosomes with the abnormal allele (homozygous), which is of course less likely.
 
 
 
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