Two people (both heterozygotes) are able to taste a chemical substance called PTC. The ability to taste PTC is caused by a dominant allele (T). The inability to taste PTC is caused by a recessive allele (t). What proportion of their children would be expected not to be able to taste PTC?
A) 3/4
B) All
C) 1/4
D) 2/3
E) none

Question

Quizplus · Accepted Answer

Both parents are heterozygotes (Tt), which means that they have one dominant allele (T) and one recessive allele (t) each. When they produce offspring, each parent can pass on either the dominant or the recessive allele to their offspring. Therefore, the possible genotypes of their offspring are TT, Tt, and tt, with equal probability for each genotype. 
The ability to taste PTC is determined by the dominant allele (T). Therefore, if an individual has at least one T allele, they will be able to taste PTC. Only individuals with the homozygous recessive genotype (tt) will be unable to taste PTC. Therefore, the proportion of their offspring that will not be able to taste PTC is equal to the proportion of offspring that receive the recessive allele from each parent, which is 1/4 or 25%. Therefore, the correct answer is C.

Two People (Both Heterozygotes) Are Able to Taste a Chemical