Question

QUESTION 6

Red-Green colorblindness is a X-linked recessive disorder. Huntington’s is an autosomal dominant disorder. Susan’s father is color blind and has Huntington’s disease and is heterozygous for the Huntington’s allele.   Her mother is normal and has no history of colorblindness or Huntington’s in her family. Her brother has developed Huntington’s but is not colorblind. Her husband Greg is healthy and has no incidence of either disorder in his family.   Susan is not colorblind and has not yet be tested to see if she has inherited the Huntington’s allele. What is the chance that a child of Susan and Greg will be a boy with both colorblindness and Huntington’s?

a.

1/4

b.

1/8

c.

1/32

d.

1/16

e.

1/64

Answer 1

c. 1/32

It is given that Susan’s father is color blind and has Huntington’s disease and is heterozygous for the Huntington’s allele, so his genotype is, “Hh xY.” Her mother is normal and has no history of colorblindness or Huntington’s in her family, but her brother developed Huntington’s but not colorblind, means, her mother is a carrier of Huntington’s disease, but not the colorblindness. So, Susan’s mother’s genotype is, “Hh XX.”

Susan’s husband has no history of colorblindness or the Huntington’s disease, so his probable genotype is, “HH XY.” Susan is not colorblind and does not know whether she had the Huntington’s allele or not.

Assume that Susan’s husband is a carrier of Huntington’s disease. So, the chances of a child of Susan and Greg will be a boy with both colorblindness and Huntington’s is,

= The probability of being a boy* the probability of inheriting the Huntington’s allele from the father* the probability of inheriting the Huntington’s allele from the mother* the probability of inheriting the colorblind allele from the mother

= ½*1/2*1/4*1/2 = 1/32