In humans, straight thumbs are caused by the dominant allele T, while individuals who are tt have thumbs that curve backward ("hitchhiker's thumb"). Consider a family in which the father has straight thumbs, and both the mother and their only child have curved thumbs. What is the chance that their next child will have curved thumbs?
A) 25%
B) 50%
C) 75%
D) 100%

Question

Quizplus · Accepted Answer

The father must be Tt heterozygous, and the mother is tt homozygous recessive because both she and their child have curved thumbs. Therefore, the father will pass on either a T or a t allele to the next child, and the mother will pass on a t allele. The Punnett square for their offspring would be:

|   | T  | t  |
|---|---|---|
| t | Tt | tt |
| t | Tt | tt |

This shows that there is a 50% chance that the next child will inherit the tt genotype and have curved thumbs.

In Humans, Straight Thumbs Are Caused by the Dominant Allele