All female mammals have one active X chromosome per cell instead of two. What causes this to happen? A) Activation of the XIST gene on the X chromosome that will become the Barr body. B) Activation of the BARR gene on one X chromosome, which then becomes inactive. C) Inactivation of the XIST gene on the X chromosome derived from the male parent. D) Attachment of methyl (-CH₃) groups to the X chromosome that will remain active.

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Quizplus · Accepted Answer

During development of female mammals, both X chromosomes are active initially. However, one X chromosome is randomly inactivated in each cell, and this is mediated by the expression of the XIST gene on the inactive X chromosome. XIST produces a long noncoding RNA that coats the X chromosome and recruits chromatin-modifying complexes to silence gene expression. The inactive X chromosome forms a compact structure called a Barr body, which is visible under a microscope. Therefore, female mammals only have one active X chromosome per cell, and the other X chromosome is inactivated and forms a Barr body.

All Female Mammals Have One Active X Chromosome Per Cell