1. 7. The ability to taste the chemical PTC is determined by a single gene in humans with the ability to taste given by the dominant allele T and inability to taste by the recessive allele t. Suppose two heterozygous tasters (Tt) have a large family. Predict the proportion of their children who will be tasters and nontasters. What is the likelihood that their first child will be a taster

3/4 taster; 1/4 non-taster

The likelihood that their first child will be a taster: 75 %

Explanation:

Complete dominance occurs when a dominant gene variant or 'allele' completely masks the expression of the recessive allele in heterozygous individuals (i.e., individuals carrying one copy of the dominant allele and one copy of the recessive allele). In this case, both parents are heterozygous for a single gene trait (i.e., the ability to taste) which is ruled by complete dominance. In consequence, the expected phenotypic ratio in the progeny (F1) will be:  

- Alleles: T (dominant taster allele); t (recessive non-taster allele)

- Parental cross: Tt x Tt

- Punnett Square from this cross:

         T        t

T       TT     Tt

t        Tt       tt

Expected F1 phenotypic ratio: 3/4 taster (i.e., 1/4 TT genotype + 1/2 genotype Tt = 3/4 or 75%); 1/4 non-taster (tt genotype = 1/4 or 25%). Moroever, the chance that the first child has the taster phenotype is 3/4 (75%).