The ability to taste PTC, a bitter chemical, is inherited as an autosomal dominant trait. A woman who is a nontaster has children with a man who is a PTC taster. Their first child is born a nontaster, as is their second child.
What is the probability that their third child will be born a nontaster?

The correct option is A 0.5
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 given question, the woman is non taster. This means she has genotype tt. The man is a PTC taster which means his genotype could be either TT or Tt. Since, they are having a non taster child, this means the child has tt genotype and t allele is inherited from both the parents. So the man has to be heterozygous i.e. Tt genotype. Thus the cross can be represented as shown in the figure.
Hence the probability that their third child will be born a nontaster = No. of children as nontaster/ Total number of children
= 2/4

= 0.5

So, the correct answer is '0.5'.