The XG locus on the human X chromosome has twoalleles, XG+ and XG. The XG+ allele causes the presence of the Xg surface antigen on red blood cells,while the recessive XG allele does not allow antigento appear. The XG locus is 10 m.u. from the STSlocus. The STS+ allele produces normal activity ofthe enzyme steroid sulfatase, while the recessive STSallele results in the lack of steroid sulfatase activityand the disease ichthyosis (scaly skin). A man withichthyosis and no Xg antigen has a normal daughterwith Xg antigen. This daughter is expecting a child.a. If the child is a son, what is the probability he willlack Xg antigen and have ichthyosis?b. What is the probability that a son would have boththe antigen and ichthyosis?c. If the child is a son with ichthyosis, what is theprobability he will have Xg antigen?
The XG locus on the human X chromosome has two
alleles, XG+ and XG. The XG+ allele causes the presence of the Xg surface antigen on red blood cells,
while the recessive XG allele does not allow antigen
to appear. The XG locus is 10 m.u. from the STS
locus. The STS+ allele produces normal activity of
the enzyme steroid sulfatase, while the recessive STS
allele results in the lack of steroid sulfatase activity
and the disease ichthyosis (scaly skin). A man with
ichthyosis and no Xg antigen has a normal daughter
with Xg antigen. This daughter is expecting a child.
a. If the child is a son, what is the probability he will
lack Xg antigen and have ichthyosis?
b. What is the probability that a son would have both
the antigen and ichthyosis?
c. If the child is a son with ichthyosis, what is the
probability he will have Xg antigen?
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