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Old 10-01-2011
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Genetics Grandfather and Grandson affected but not the father!

Ectrodactyly is an autosomal dominant trait that causes missing middle fingers (lobster claw malformation). A grandfather and grandson both have ectrodactyly, but the intervening father has normal hands by x-ray. Which of the following terms applies to this family?
A. Incomplete penetrance
B. New mutation
C. Variable expressivity
D. Germinal mosaicism
E. Anticipation
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Old 10-02-2011
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A.Incomplete penetrance
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Old 10-02-2011
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The answer is: A
Incomplete penetrance applies to a normal individual who is known from the pedigree to have an allele responsible for an autosomal dominant trait. Variable expressivity refers to family members who exhibit signs of the autosomal dominant disorder that vary in severity. When this severity seems to worsen with progressivegenerations, it is called anticipation. A new mutation in the grandson would be extremely unlikely given the
affected grandfather. The father could be an example of somatic mosaicism if a back-mutation occurred to allow normal limb development, but there is no reason to suspect mosaicism of his germ cells (germinal mosaicism).
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