Question

The following couples plan to marry and want to know the chance of having a child with the following diseases. You are a genetic counselor who needs to answer their questions. Write out the genotypes of the crosses and perform a Punnett square to determine the probability of the child inheriting the disease.

            a. an affected father and carrier mother for color blindness (X-linked recessive)

            b. an unaffected father and a carrier mother for hemophilia (X-linked recessive)

            c. an unaffected father and an unaffected mother for color blindness

            d. an affected father and an unaffected mother for color blindness

Answer 1

a) Genotype of affected father for color blindness  - X^cY.

Genotype of carrier mother for color blindness - X⁺X^c.

Cross between them X^cY X   X⁺X^c.

Gametes formed    X^c and Y X⁺ and X^c

Next generation   

Male/Female	X+	Xc
Xc	X+Xc(unaffected carrier female)	XcXc(colorblind female)
Y	X+Y(unaffected male)	XcY(colorblind male)

The probability of the child inheriting the disease - 1/2 .

b)  an unaffected father and a carrier mother for hemophilia.

Genotype of unaffected father for hemophilia - XY.

Genotype of carrier mother for hemophilia - X^hX.

Cross between them XY X X^hX

Gametes formed X and Y X^h and X.

Next generation

Male/Female	X	Y
Xh	XhX(unaffected carrier female)	XhY(affected male)
X	XX(noraml female)	XY(unaffected or normal male)

The probability of the child inheriting this disease - 1/4.

c) an unaffected father and an unaffected mother for color blindness.

Genotype of unaffected father for color blindness  - XY.

Genotype of unaffected mother for color blindness - XX.

All are unaffected, then the probability of the child inheriting this disease - zero 0%.

d) an affected father and an unaffected mother for color blindness.

Genotype of affected father for color blindness  - X^cY.

Genotype of unaffected mother for color blindness - XX.

Cross between them X^cY X XX

Gametes formed   X^c and Y X and X.

Next generation

Male/Female	Xc	Y
X	XcX(unaffected carrier female)	XY(unaffected male)
X	XcX(unaffected carrier female)	XY(unaffected male)