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I'm having trouble answering this problem:
The ability to taste the chemical phenylthiocarbamide (PTC) is due to a dominant gene
(T) in man. The inability to taste PTC is due to its recessive allele (t). A man who can
taste PTC, but whose mother could not, marries a woman who can taste PTC, but whose
father could not.
a.
b.
c.
d.
What is the genotype of the man? Explain?
What is the wife’s genotype? Explain?
What types of gametes will the man & woman produce?
What proportion of their children will probably have the ability to taste PTC?
_____ : ______
For this question I know I would have to draw a Punnet square and do a test cross
Solution:
A dominant phenotype for tasting PTC can have two genotypes, TT or Tt. So the
nontaster’s genotype is for certain recessive tt.
Man’s genotype:He can taste PTC but his mother cannot. If his mother cannot taste, it’s obvious that she
has a recessive genotype tt and his father must have been either TT or Tt for tasting.
Let us test cross to distinguish between the homozygous dominant and heterozygous
genotypes since they are the same when viewed as a phenotype. Note that a test cross
parent is always homozygous recessive for the gene under consideration.
So
T- x tt – [T- = genotype incompletely known, tt = non taster]
Gametes: T and – x t
Offsprings: Tt
The male parent [father] must be producing two kinds of gametes and so he is
heterozygous Tt. The son [man in question] must also be heterozygous.
Woman’s genotype:Woman can taste PTC while her father cannot. The above explanation is applicable to
this condition as well except the change in sex of the person during testcross. So the
woman is also heterozygous for the trait.
Types of gametes that the man and woman would produce:-
Parents: Man Tt x Woman Tt
Gametes: Man – T, t Woman – T, t
Offsprings:
Gametes – Male / Female
T
t
T
TT
Tt
The offsprings will have the following phenotypes and genotypes:
¼ TT – Tasters
½ Tt – Tasters
¼ tt – non tasters
So, ¾ will be tasters and ¼ will be non tasters amongst the offsprings.
t
Tt
tt