A calico cat has patches of orange fur and patches of black fur, with the pattern unique to each individual cat. The gene for coat color is X-linked, with one allele producing orange and the other producing black. What best explains the patchy distribution?
AThe cat has three copies of the X chromosome (XXX), expressing all three alleles in alternating patches
BDuring embryogenesis, X-inactivation randomly silences either the maternal or paternal X in each cell; all descendants of that cell maintain the same inactive X, producing visible patches where one allele or the other is expressed
CSomatic mutations during development switch the active allele in individual cells, producing a mosaic pattern
DThe orange and black pigment genes are on separate chromosomes that segregate independently during cell division, creating alternating patches
This is the classic visible demonstration of X-inactivation mosaicism. The calico cat is female (XX) and heterozygous for the coat color gene. Early in embryogenesis, each cell independently and randomly silences either the maternal X (orange allele) or the paternal X (black allele). Once silenced, all daughter cells inherit the same inactive X. Because groups of cells descend from a single precursor, they form contiguous patches of one color or the other. The pattern is unique to each cat because the initial silencing events are random. Male cats (XY) cannot be calico — they have only one X and express a single coat color.
Question 2 Multiple Choice
A researcher proposes that if X-inactivation always silenced the paternal X instead of being random, females heterozygous for X-linked recessive diseases would always be unaffected carriers. Is this reasoning correct?
AYes — if the paternal X (carrying the disease allele from the father) were always silenced, all cells would express the normal maternal X, fully protecting the carrier
BNo — the disease allele could be on either the maternal or paternal X in any given carrier female, so selective silencing of one parental X would still leave carriers with all cells expressing the mutant allele in half of cases
CNo — even if the paternal X were always silenced, the inactivation would be reversed in some tissues, exposing the recessive allele
DYes, but only if the disease allele is fully recessive — dominant X-linked mutations would still cause disease regardless of which X is silenced
The researcher's logic is internally consistent but ignores that X-linked mutations can arise on either the maternal or paternal X. A carrier female may have inherited the disease allele from her father (paternal X) or from her mother (maternal X). If inactivation always silenced the paternal X: carriers with the mutation on the paternal X would be fully protected; carriers with the mutation on the maternal X would be fully affected — just as severely as a hemizygous male. Imprinted X-inactivation (silencing always the same parental X) does not guarantee protection; only random inactivation with skewing toward the normal X provides partial protection in some carriers.
Question 3 True / False
Xist RNA acts in cis — it coats and silences only the chromosome from which it is transcribed, not the other X chromosome in the same cell.
TTrue
FFalse
Answer: True
Cis action is mechanistically crucial and was initially surprising — RNA molecules can diffuse, so why does Xist stay on its chromosome of origin? Xist is thought to be retained in proximity to its transcription site through nuclear architecture, and it spreads outward from the X-inactivation center (Xic) along the chromosome rather than drifting to the other X. The active X also expresses a non-coding RNA (Tsix) that blocks Xist coating in cis, creating a mutually exclusive regulation where only one X accumulates enough Xist to trigger silencing. If Xist acted in trans, it would silence both X chromosomes — lethal for females.
Question 4 True / False
Once X-inactivation is established in a somatic cell lineage, the inactive X can easily be reactivated by ordinary cell division or differentiation signals.
TTrue
FFalse
Answer: False
X-inactivation is epigenetically stable and heritable through cell division — it is designed to persist through the lifetime of a somatic lineage. Maintenance depends on multiple reinforcing mechanisms: CpG methylation of promoters on the inactive X, repressive histone marks (H3K27me3, H3K9me2), and continued Xist RNA coating. These marks are faithfully copied during DNA replication. X-inactivation is only reversed in a specific biological context: germ cells, where both X chromosomes must be active for oogenesis. This reversal requires active demethylation and chromatin remodeling — it does not occur spontaneously during ordinary somatic differentiation.
Question 5 Short Answer
A woman is a carrier for Rett syndrome (caused by a loss-of-function mutation in MECP2, an X-linked gene). Explain why she might show mild neurological symptoms even though she has one functional copy of MECP2.
Think about your answer, then reveal below.
Model answer: X-inactivation is random — in each cell of the early embryo, either the X carrying the mutant MECP2 or the X carrying the normal MECP2 is silenced, and that choice is inherited by all daughter cells. If, by chance, the normal X is silenced in a disproportionate fraction of cells in the brain, more neurons express only the mutant allele. If this skewing is severe enough, the woman may have a substantial proportion of neurons lacking functional MECP2, producing symptoms. The severity depends on the ratio of cells expressing the normal vs. mutant allele — a matter of statistical chance during early development. This explains the highly variable expressivity of Rett syndrome in heterozygous females, ranging from asymptomatic to severely affected.
This question connects the mechanism of X-inactivation directly to clinical genetics. The mosaic nature of female X-inactivation is not just an abstract property — it has direct consequences for how X-linked diseases manifest. Males with MECP2 mutations are typically severely affected or lethal because every cell expresses the mutant allele. Females can range from unaffected to severely affected depending on their somatic mosaicism pattern, which is why Rett syndrome is almost exclusively diagnosed in females (males rarely survive to diagnosis).