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Brachydactyly type D is human autosomal dominant condition in which the thumbs are abnormally short and broad. In most cases, both thumbs are affected, but occasionally just one thumb is involved. The pedigree above shows a family in which brachydactlyly type D is segregating. Filled circles and squares represent females and males who have involvement of both thumbs. Half-filled in symbols represent family members with just one thumb affected?  (Picture attached)

There is evidence of variable expressivity and incomplete penetrance in this family. Which individual is most likely nonpenetrant for the trait?


A) III-11

B) IV-1

C) IV-5

D) III-10

E) II-4

3 4
6 789 10 11
1 2

Image Transcriptionclose

2 3 4 6 789 10 11 3_4 1 2 IVI

Step 1

Brachydactyly type D is a genetic abnormality, which occurs due to a dominant gene mutation in the autosomes. Hence, it is an autosomal dominant condition in which the thumbs are abnormally short and broad. 

Step 2

In the given pedigree, the offspring III-10 shows the least chance of penetrance. Though this individual might carry the allele of the disease, but as shown in the pedigree, the individual failed to express the trait. So, option (D) III-10, is correct.

Step 3

The individual III-11 is not a part of the family and is an outsider. He is normal and does not carry the disease. So, option (A) III-11 is incorrect.

The symbol for the individual IV-1 is completely filled, and thus is an example of penetrance. So, option (B) IV-1 is incorrect.

The symbol for the individual IV-5 is half-filled, and thus is...

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