Apentix/AP Biology/Unit 6: Gene Expression and Regulation/Question
AP Biologyhardmcq1 pt

A female mouse heterozygous for a coat-color allele shows patches of two different colors across her fur. Which explanation accounts for the patchwork?

A.Coat-color alleles mutate independently in neighboring skin clones throughout adulthood
B.Each embryonic cell randomly condensed one X chromosome into a Barr body, and descendant clones kept whichever allele stayed active
C.Skin cells delete the unused color allele whenever it would conflict with pigmentation needs
D.Pigment cells blend the products of both alleles wherever both chromosomes remain active

Explanation

Core Concept

Early embryos silence one X chromosome per cell by coating it with Xist RNA and packing it into heterochromatin. Inactivation is random with respect to parental origin and then inherited through mitosis. Neighboring lineages therefore expose different alleles, producing the mosaic pattern.

Correct Answer

BEach embryonic cell randomly condensed one X chromosome into a Barr body, and descendant clones kept whichever allele stayed active

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