biology punnett

Question

4a) Hemophilia is an x-linked disease of the blood. What is the probability of producing an unaffected son by a father who is unaffected and a mother who is a carrier of hemophilia?

4b) Using a Punnett square, explain why a male cannot be a carrier of the hemophilia allele.

do it by hand

Answer 1

a) Alleles for Hemophilia:

H - Dominant Allele (Normal)

h - Recessive Allele (Diseased)

A female can be:

X^HX^H (Genotype) - Normal (Phenotype)

X^HX^h (Genotype) - Normal and Carrier (Phenotype)

X^hX^h (Genotype) - Diseased (Phenotype)

A male can be:

X^HY (Genotype) - Normal (Phenotype)

X^hY (Genotype) - Diseased (Phenotype)

In this case,

Father is unaffected; so, the genotype of dad is X^HY

Mother is a carrier; so, the genotype of mom is X^HX^h

Cross: X^HY. x. X^HX^h

X^H Y

X^H X^HX^H X^HY. <----- 1/2 sons unaffected

X^h X^HX^h X^hY <----- 1/2 sons diseased

The probability of producing an unaffected son by a father who is unaffected and a mother who is a carrier of hemophilia is 1 out 2.

b) Based on the above Punnett square, we can clearly see that a male cannot be a carrier of the hemophilia allele. This is because hemophilia is an X-linked recessive disease. An X-linked recessive disease is a disease when the recessive defective/diseased allele is located on the X-chromosome. Males only have one X-chromosome. Therefore, a male could either carry the normal (H-dominant allele) or the diseased (h-recessive allele). Therefore, a male cannot be a carrier of the hemophilia allele.