A nurse is educating a client about genetic screening. The client asks why red-green color blindness, an X-linked recessive disorder noted in some of her family members, is expressed more frequently in males than females. How should the nurse respond?
a. "Females have a decreased penetrance rate for this gene mutation and are therefore less likely to express the trait."
b. "Females have two X chromosomes and one is always inactive. This inactivity decreases the effect of the gene."
c. "The incidence of X-linked recessive disorders is higher in males because they do not have a second X chromosome to balance expression of the gene."
d. "Males have only one X chromosome, which allows the X-linked recessive disorder to be transmitted from father to son."
ANS: C
Because the number of X chromosomes in males and females is not the same (1:2), the number of X-linked chromosome genes in the two genders is also unequal. Males have only one X chromosome, a condition called hemizygosity, for any gene on the X chromosome. As a result, X-linked recessive genes have a dominant expressive pattern of inheritance in males and a recessive expressive pattern of inheritance in females. This difference in expression occurs because males do not have a second X chromosome to balance the expression of any recessive gene on the first X chromosome. It is incorrect to say that one X chromosome of a pair is always inactive in females, or that females have a decreased penetrance rate for this gene mutation. X-linked recessive disorders cannot be transmitted from father to son, but the trait is transmitted from father to all daughters who will be carriers.
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