Question

In humans, the ability to taste the chemical PTC is inherited as a single gene dominant trait represented by the allele T. A couple who are both tasters have four children, one of which is a nontaster.
Which of the following best predicts what genotypes the parents of these children possess?

• A. Both parents are homozygous dominant for the taster trait (TT).
• B. One parent is homozygous dominant (TT) and the other is heterozygous (Tt).
• C. One parent is homozygous recessive (tt) and the other parent is heterozygous (Tt).
• D. One parent is homozygous dominant (TT) and the other is homozygous recessive (tt).
• E. Both parents are heterozygous (Tt).

Answer 1

The correct option is E Both parents are heterozygous (Tt).

Taste blindness of PTC is a genetic trait. PTC (Phenyl Thiocarbamide) is a compound of nitrogen, carbon and sulphur with sour taste. This trait is transmitted by a dominant gene T. The genotypes TT and Tt are tasters of PTC, while tt are non tasters or taste blind persons.

In the question it is given that the couple are both tasters. This means their genotype could be either TT or Tt. But one of their four children is non taster i.e. his genotype is tt. For this genotype, the t allele is inherited from both the parents. So, both the parents have to be heterozygous for PTC i.e. they should have the genotype Tt. The cross can be represented as shown in the figure.

So, the correct answer is 'Both parents are heterozygous (Tt)'.