Except for genes on the Y chromosome (and for males, also the X chromosome), if

Question

Except for genes on the  Y chromosome (and for males, also the X chromosome), if an uncommon polymorphic form of a gene has an allelic frequency of a (where a is between 0 and 1), the probability of having this polymorphic form produce a band on the gel obtained from PCR of any given individual is not simply a, but rather 2a (to a close approximation). Explain why this is the case. (It may help to remember that the allelic frequency is defined as the probability that a chromosome carrying a polymorphic gene happens to carry a particular allele for that gene.)

Explanation / Answer

Ans: Create a pedigree of your family tree for the late-onset disease going back at leastthree but as many generations as possible. Based on your pedigree, you will needto determine whether the trait is recessive or dominant, autosomal or X-linked.Use the product rule to determine the probability of your having inherited the trait.Keep in mind that individuals not old enough to show the trait should bediagramed as unknowns on your pedigree and your probability of inheriting thedisease may depend on whether that given individual has the trait.

The Y chromosome determines maleness by the action of the geneSRY, which encodes a protein that initiates a sequence of events prompting the embryonic gonads to develop into testes. The X chromosome in the absence of a Y chromosome results in a female by permitting the conversion of the embryonic gonads into ovaries. We trace the historical progress that resulted in the discovery that one X chromosome in the female is randomly inactivated in early embryogenesis, accomplishing approximate equivalency of X chromosome gene dosage in both sexes. This event results in half of the somatic cells in a tissue containing proteins encoded by the genes of the maternal X chromosome and half having proteins encoded by the genes of the paternal X chromosome, on average, accounting for the phenotype of a female heterozygote with an X chromosome mutation. The hypothesis of X chromosome inactivation as a random event early in embryogenesis was first described as a result of studies of variegated coat color in female mice. Similar results were found in women using the X chromosome-linked gene, glucose-6-phosphate dehydrogenase, studied in red cells. The random inactivation of the X chromosome-bearing genes for isoenzyme types A and B of glucose-6-phosphate dehydrogenase was used to establish the clonal origin of neoplasms in informative women with leiomyomas. Behind these discoveries are the stories of the men and women scientists whose research enlightened these aspects of X chromosome function and their implication for medicine.

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