When male mice heterozygous for a small deletion on Chromosome 2 are mated to no

Question

When male mice heterozygous for a small deletion on Chromosome 2 are mated to normal females, deletion-bearing offspring have thin bodies and are slow-moving, while non-deletion-bearing offspring are normal. However, when females heterozygous for the same deletion are mated to normal males, all offspring are normal. a. What mechanism likely explains this pattern of transmission? 1 pt b. Provide an explanation for the observed phenotypes. Why would offspring of females heterozygous for the deletion have normal offspring while those of males heterozygous for the deletion produce a mixture of affected and unaffected progeny? Supplement your explanation with a figure if possible. 2 pts

Explanation / Answer

The general term that you are looking for is called imprinting. You are right that it has to do with methylation of the DNA (gene silencing). Some genes are only expressed from either the maternally or paternally inherited chromosome. In the case outlined by the question, you would assume that the copy inherited from the mother is methylated ("imprinted") so that the genes on that region of chromosome 2 are only expressed from the paternal copy. The methylation of the regions that are imprinted occurs in the gametes of the patents. So, if a male were to inherit the deletion from his mother, he would be normal because the inherited deletion is methylated (not expressed) and the active copy comes from the intact paternal chromosome. However, when this male produces offspring, 50% will be mutant because the deletion chromosome will not be methylated in his sperm and therefore expressed in his offspring. This can be a confusing topic, so let me know if you need some clarification.

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