Polydactyly runs in Mikes family. He has an extra toe, his grandchild Victor has two extra fingers. But his child Joe has the normal complement of fingers and toes. That Joe is unaffected is best explained by:

Question

BlackTom AI · Accepted Answer

Autosomal dominant traits can show reduced (incomplete) penetrance, so an individual can carry a dominant disease allele yet display a normal phenotype.

1. Relevant knowledge points and concepts
- Polydactyly is often inherited as an autosomal dominant trait.  
- Penetrance is the proportion of individuals with a given genotype who actually express the phenotype; incomplete (reduced) penetrance means some carriers are phenotypically normal.  
- Expressivity refers to the range or severity of phenotype among affected individuals (variable expressivity can cause different numbers/locations of extra digits).  
- Mechanisms like haploinsufficiency, dominant negative, and gain-of-function describe how a mutant allele produces a phenotype but do not by themselves explain why a carrier would be completely unaffected.

2. Step-by-step reasoning
- Mike has extra toe (affected). His grandchild Victor has extra fingers (affected). Joe (Mike’s child) is phenotypically normal. The trait appears in successive generations but is absent in one generation member.  
- If polydactyly is autosomal dominant, Mike likely carries one mutant allele (genotype Pp). Joe could either have inherited the mutant allele (Pp) or the normal homozygous (pp). If Joe inherited Pp but shows no extra digits, that is a carrier with no phenotype — this is classic incomplete penetrance. If Joe simply inherited pp, that would be non-transmission, but the occurrence in the grandchild despite an unaffected parent is most parsimoniously explained by that parent being a non-penetrant carrier.

3. Intermediate logical steps
- Presence in grandchild despite unaffected parent implies either: (a) parent is a non-penetrant carrier, or (b) new mutation in grandchild. Given trait in grandparent and grandchild, non-penetrance is the simpler explanation.  
- Variable expressivity explains why Mike has an extra toe while Victor has two extra fingers, but does not explain a completely unaffected carrier.

4. Why the correct answer is correct
- Incomplete penetrance directly explains an individual inheriting a dominant allele yet showing no phenotype (Joe unaffected while carrying/ transmitting allele).

5. Why other options are incorrect
- Haploinsufficiency: a mechanism causing dominant phenotype when one functional copy is insufficient; it would typically produce a phenotype in heterozygotes, not readily explain a fully unaffected carrier.  
- Dominant negative mutation: mutant protein interferes with wild-type function; it usually causes a phenotype in heterozygotes, so it does not explain a normal-appearing carrier.  
- Gain-of-function mutation: produces a novel/overactive product that generally manifests in carriers, so it does not explain an unaffected individual.

6. Conclusion
The pattern (affected grandparent and grandchild with an unaffected parent) is best explained by incomplete penetrance. This matches the provided answer: incomplete penetrance.

Polydactyly runs in Mikes family. He has an extra toe, his grandchild Victor has two extra fingers. But his child Joe has the normal complement of fingers and toes. That Joe is unaffected is best explained by:单项选择题

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