A mitochondrial mutation is detected in a man. This mutation is only lethal
beyond the age of 70. He marries a woman who does not carry the mutation,
and they then have children. What percentage of his grand-daughters would be
carriers for this mutation
100%
0%
25%
33%
Mitochondrial mutations follow strict maternal inheritance patterns in humans, where fathers do not transmit mitochondrial DNA (mtDNA) to offspring due to degradation of sperm mitochondria during fertilization. In this scenario, a man carries a late-onset lethal mutation, marries a mutation-free woman, and has children; the question asks the percentage of his granddaughters who would be carriers.
Inheritance Pattern
Human mtDNA, encoding 37 genes essential for energy production, resides in mitochondria inherited almost exclusively from the egg, as sperm contribute negligible mtDNA diluted or eliminated post-fertilization. The man’s mutation cannot pass to his children (sons or daughters), who inherit the wife’s normal mtDNA; thus, none become affected or carriers. Daughters of these children (granddaughters) receive mtDNA solely from their mothers, who lack the mutation, ensuring no transmission across generations from the paternal grandfather.
Option Analysis
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100%: Incorrect, as this assumes full transmission to all offspring like autosomal dominant traits, ignoring mtDNA’s maternal exclusivity; no children inherit the mutation.
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0%: Correct, since the man’s mtDNA mutation stops with him—children get normal mtDNA from the mother, and granddaughters inherit only from their unaffected mothers.
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25%: Incorrect, typical of recessive autosomal inheritance (1/4 affected), but mtDNA bypasses nuclear Mendelian ratios and paternal lines entirely.
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33%: Incorrect, akin to some X-linked patterns (1/3 carrier daughters), but mtDNA transmission is cytoplasmic and female-only, not chromosomal.
CSIR NET Relevance
This question tests understanding of non-Mendelian inheritance for exams like CSIR NET Life Sciences, where mitochondrial genetics appears in molecular biology units. Late-onset lethality (post-70) emphasizes heteroplasmy thresholds but does not alter transmission rules. Rare biparental cases exist but are negligible for standard pedigree analysis.
