The sex linked (X linked) Xg locus in humans has two alleles, a+ and a. The a+ a

Question

The sex linked (X linked) Xg locus in humans has two alleles, a+ and a. The a+ allele causes the presence of Xg surface protein antigen on red blood cells, while the recessive (a) allele does not. The Xg locus maps 10 centimorgans from the STS locus. The recessive sts allele results in the lack of steroid sulfatase activity and homozygous females, as well as sts/Y males, develop ichthyosis, a form of scaly skin. A man with ichthyosis and no Xg antigens has a normal daughter with Xg antigens who is expecting a child.

If the child is a boy, what is the probability that he will lack the Xg antigen and have ichthyosis?

Explanation / Answer

The sex linked (X linked) Xg locus in humans has two alleles, a+ and a. The a+ allele causes the presence of Xg surface protein antigen on red blood cells, while the recessive (a) allele does not. The Xg locus maps 10 centimorgans from the STS locus. The recessive sts allele results in the lack of steroid sulfatase activity and homozygous females, as well as sts/Y males, develop ichthyosis, a form of scaly skin. A man with ichthyosis and no Xg antigens has a normal daughter with Xg antigens who is expecting a child.

What is the probability that the son would have both the Xg antigen and ichthyosis?

The Xg locus maps 10 centimorgans from the STS locus
10%-----------recombinant
90%-----------parental

so 45%-------each -parental

answer
Xg antigen and ichthyosis?=====45%

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