Which statement about X chromosome dosage in mammals is true?

• A. It balances gene dosage by activating both X chromosomes
• B. It only affects autosomal genes
• C. It equalizes gene dosage between males and females
• D. It increases gene dosage in females

Answer: (C)

X chromosome dosage in mammals is equalized through X-chromosome inactivation, where one of the two X chromosomes in female cells is largely silenced. This means that most X-linked genes are expressed from a single active X, bringing female gene expression from the X in line with males, who have only one X. If both X chromosomes remained active, females would have higher levels of X-linked gene products. The story centers on X-linked genes, not autosomal ones, and while some genes can escape inactivation and be expressed from both Xs, the overall effect is to balance dosage between the sexes. So the true statement is that X chromosome dosage is equalized between males and females.