In the following example, we are going to discuss a situation where genomic imprinting occurs on the X chromosome. You are studying a female (XX) mouse that has a genomically imprinted gene on the X chromosome called ‘Bluefur’. The ‘Bluefur’ gene is normally expressed on the maternal X chromosome, and silenced on the paternal X chromosome. The female mouse you are studying has a mutation in only one allele of the ‘Bluefur’ gene so that the mutant allele can’t produce blue pigment that makes the fur of the mouse blue. The female mouse you are studying has patches of blue fur. This is because:

Question

In the following example, we are going to discuss a situation where genomic imprinting occurs on the X chromosome. You are studying a female (XX) mouse that has a genomically imprinted gene on the X chromosome called ‘Bluefur’.  The ‘Bluefur’ gene is normally expressed on the maternal X chromosome, and silenced on the paternal X chromosome. The female mouse you are studying has a mutation in only one allele of the ‘Bluefur’ gene so that the mutant allele can’t produce blue pigment that makes the fur of the mouse blue. The female mouse you are studying has patches of blue fur. This is because:

BlackTom AI · Accepted Answer

Let’s parse the scenario carefully and evaluate each option in turn.
Option 1: 'X chromosome inactivation repressed expression of only the maternal X chromosome' — This statement suggests that inactivation specifically targeted the maternal X for repression in all cells. In reality, X inactivation is a random, cell-by-cell event that can silence either the maternal or paternal X in different cells, not exclusively the maternal one. Moreover, the question describes a mosaic (patchy) phenotype, which is exactly what random X inactivation would produce across cells. So this option is incorrect because it implies a uniform repression pattern that doesn’t explain mosaic blue patches.
Option 2: 'The mutant Bluefur allele is on the maternal X chromosome' — If the mutant allele were on the maternal X, then cells in which the maternal X remains active would express the mutant and fail to produce blue pigment, while cells where the paternal X (which is normally silenced for Bluefur) is the active one would still be unable to express Bluefur due to imprinting, potentially leading to a non-blue outcome in those cells as well. This does not neatly explain why patches would appear blue; in fact, it would more likely produce uniform non-blue coloration if maternal is the active one in most cells. Thus this option does not align with the described imprinting pattern and mosaic blue patches.
Option 3: 'The mutant Bluefur allele is on the paternal X chromosome' — This fits the model of X-linked imprinting with mosaicism. Bluefur is expressed only from the maternal X, while the paternal X is normally silenced for this gene. If the paternal X carries a mutant allele that cannot produce blue pigment, then in cells where the paternal X is the active one, the pigment cannot be produced (non-blue). In cells where the maternal X is active, the functional maternal allele can produce blue pigment. Since X inactivation occurs randomly in each cell, you get a mixture of blue and non-blue patches across the fur, exactly as described. This option directly aligns with the mechanism of mosaic expression due to random X inactivation combined with paternal X carrying the mutant allele. Therefore, this is the correct explanation.
Option 4: 'X chromosome inactivation repressed expression of only the paternal X chromosome' — This would imply that only the paternal X is ever silenced, which would lead to a uniform expression pattern, not a mosaic patchwork of blue and non-blue fur. It ignores the standard imprinting pattern described (maternal X active for Bluefur). This option misstates how imprinting and inactivation interact in this scenario.
Option 5: 'X chromosome inactivation repressed expression of the mutant Bluefur allele in all cells' — If inactivation always repressed the mutant allele, all cells would express the normal maternal allele regardless of which X chromosome carries the mutation, producing uniform blue fur. That contradicts the observed patchiness and the fact that a mutation on the paternal allele leads to loss of blue in cells where that X is active. This makes the option incorrect.

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