The Xg locus on the human X chromosome has two alleles, a+ and a. The a+ allele

Question

The Xg locus on the human X chromosome has two alleles, a+ and a. The a+ allele causes the presence of the Xg surface antigen on red blood cells, while the recessive a 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 lac,k 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, who is expecting a child.
a. If the child is a son, what is the probability he will lack antigen and have ichthyosis?
b. What is the probability that a song would have both the antigen and ichthyosis?
c. If the child is a son with ichthyosis, what is the probability that he will have Xg antigen?

Explanation / Answer

This problem requires an understanding of how linkage affects the proportions of gametes. First designate the genotype of the individual in which recombination during meiosis affects the transmission of alleles: in this problem, the daughter.
The X chromosome she inherited from her father (who had icthyosis and no Xg antigen) must be sts a.

(No recombination could have separated the genes during meiosis in her father since he has only one X chromosome.)

Because the daughter is normal and has the Xg antigen, her other X chromosome
(inherited from her mother) must contain the Sts and a+ alleles.
Her X chromosomes can be diagrammed as:

sts____________a
sts____________a+



Because the Sts and Xg loci are 10 m.u. apart on the chromosome, there is a 10% recombination frequency. Ninety percent of the gametes will be parental: sts a or Sts a+ (45% of each type) and sts a Sts a+ 10% will be recombinant: sts a+ or Sts a (5% of
each type). The phenotype of a son directly reflects the genotype of the X chromosome from his mother. Therefore, the probability that he will lack the Xg antigen and have icthyosis (genotype: sts a / Y) is 45/100.

b. The probability that he will have the antigen and ichthyosis (genotype: sts a+ / Y) is 5/100.

c. There are two classes of gametes containing the
ichthyosis allele: sts a (45%) and sts a+ (5%). If
the total number of gametes is 100, then 50 will
have the sts allele. Of those gametes, 5 (or 10%)
will have the a+ allele.
Therefore there is a 1/10
probability that a son with the sts allele will have
the Xg antigen.

Answer and more info here: http://highered.mcgraw-hill.com/sites/dl/free/0072848464/387006/har48464_ch05.pdf
Page 157

Hope this helps!

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