Answer:
75%
Explanation:
This question involves a single gene coding for the ability to taste the chemical PTC. The allele coding for the ability to taste (T) is dominant over the allele coding for the inability to taste (t). This means that an heterozygous individual will have the ability to taste PTC.
According to this question, if two heterozygous tasters (Tt) cross to have a large family i.e. Tt × Tt, the following gametes will be produced by each parent;
Tt = T and t
Using these gametes in a punnet square (see attached image), the following proportion of offsprings will be produced:
TT, Tt, Tt - Tasters (3/4 or 75%)
tt - Non-taster (1/4 or 25%)
Note that based on the question asked, there is a 75% probability that their first child will be a taster because there is equal segregation of alleles into gametes.
