Download Dihybrid Cross Questions

Biology 621 - Genetics Problems
1. In guinea pigs, black hair colour (B) is dominant and brown hair colour (b) is recessive. Long hair (L) is dominant
and short hair (l) is recessive. Answer the following questions:
(a) Diagram the cross: BbLl x BbLL
(b) What are the phenotypes of the parent generation?
(c) What is the phenotypic ratio of the F1 generation?
2. About 70% of Canadians get a bitter taste from the chemical phenyl thiocarbamide (PTC), while the other 30%
do not. The ability to taste this chemical (T) is a dominant characteristic, while taste-blindness to it is recessive (t).
Tongue-rolling ability is dominant (R), while the inability to roll the tongue is recessive (r). A tongue-rolling
woman who is taste-blind for PTC has a father who could not roll his tongue but could taste the PTC chemical.
She marries a man who can taste PTC but cannot roll his tongue. His mother was taste-blind to the chemical.
Show the possible children this couple could produce. Use a Punnett square to illustrate your answer.
3. Colour-blindness is the result of an X-linked recessive allele, Xc. The allele for normal eyesight is X
(a) A woman with normal colour vision whose father was colour-blind marries a colour-blind man. Give the
genotypes and phenotypes of their children. What ratio of their children can be expected to be colour-blind?
Explain using a Punnett square.
(b) A man with normal colour vision whose father was colour-blind marries a woman carrier of the colour-blind
allele. What is the likelihood that their children will be colour-blind? Carriers of the trait? Show your work using a
Punnett square.
4. Male pattern baldness is inherited on the X chromosome. The allele for baldness is designated X b, while the
allele for normal hair pattern is X. Neither of your parents has the condition, but your mother’s brother does.
What is the chance that you will inherit the recessive gene? Explain.
5. For the pedigree shown below, show how a child with adrenoleukodystrophy (ALD) could be born to parents
not affected by ALD. X represents the normal gene and is dominant over Xa the allele for ALD. Show the genotype
and phenotype for the parents and siblings (all possibilities).
6. A pedigree is a useful tool for determining human inheritance patterns. Use the pedigree below to answer the
following questions.
(a) Is the trait controlling the condition dominant or recessive? Explain.
(b) Is the trait sex-linked or not? Explain.
(c) Determine the carriers of the trait. Explain how you know.