A genetic defect in humans results in the absence of sweat glands in the skin. Some men have this defect all over their bodies, but in women it is usually expressed in a peculiar way: A woman with this defect typically has small patches of skin with sweat glands and other patches without sweat glands. In women, the pattern of sweat-gland distribution can best be explained by __________.

• A. a mutation
• B. linkage
• C. recombination
• D. nondisjunction
• E. X chromosome inactivation

Answer: (E)

Verified step by step guidance

1. Understand that the problem involves a genetic defect related to the absence of sweat glands, which is expressed differently in men and women.
2. Recognize that the defect is expressed uniformly in men but in a patchy manner in women, suggesting a mechanism that affects gene expression differently in females.
3. Recall that females have two X chromosomes, and one of them is randomly inactivated in each cell during early development, a process known as X chromosome inactivation.
4. Consider that X chromosome inactivation leads to a mosaic pattern of gene expression in females, where some cells express genes from one X chromosome and other cells express genes from the other X chromosome.
5. Conclude that the patchy distribution of sweat glands in women with this defect is due to X chromosome inactivation, where some patches have the active X chromosome with the normal gene, and others have the X chromosome with the defect.