Question

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 child will be born a nontaster?
A. 0.5
B. 0.25
C. 0.75
D. 0.125
E. 0.1666666666666667

Answer 1

Hint: Taste blindness of PTC (Phenyl Thicarbamide) is a genetic trait and is composed of nitrogen, carbon, and sulphur with sour taste which is an autosomal disorder which arises due to gene mutation in autosomes.

Complete answer: The PTC trait is transmitted by TT and Tt genotypes where T is the dominant trait. The woman is nontaster which means the genotype would be tt and as the first child is nontaster it means the child’s genotype would be also tt.
As man is PTC taster and the first child’s genotype is tt, it means the genotype of man would be Tt
Parents: Tt x tt
Gametes: T, t, t, t
Punnett square :

	T	t
t	Tt(taster)	tt(non taster)
t	Tt(taster)	Tt(non-taster)

Offspring formed: Tt ( Taster), Tt ( Taster), tt (non-taster) and tt (non taster)
Probability: No. of children non-taster/ total no of children
                    = 2/4
                    = 0.5

Genotype: The genotype of an individual refers to the sum total of genes inherited from both the parents irrespective of whether they expressed or not.
Phenotype: The phenotype of an individual refers to the expressed or observable structural and functional traits produced by the interaction of genes and the environment.

So, the correct answer is “Option A”.

Additional Information: Homozygous organisms: An organism in which both the alleles of a character at the corresponding loci in homologous chromosomes are identical is said to be homozygous or genetically pure for that character. Homozygous individuals breed true for the specific trait i.e. produce offspring with similar genotype and phenotype on inbreeding.
They are also called pure individuals.
Heterozygous organism: An organism in which two alleles of a character at the corresponding loci in homologous chromosomes is unlike is said to be heterozygous for that character.
Heterozygous individuals do not breed true and are said to be hybrid for that character.

Note: According to the law of dominance one factor in a pair may express itself (i.e. appear as an observable trait in the organism) and prevent the expression of the other.
The factor expresses itself was described by Mendel as the dominant factor and the unexpressed one as a recessive factor.