What is the genotype for Sally’s father?

Question

Huntington’s disease is degenerative disease of the nervous system caused by a lethal dominant allele that has no obvious phenotypic effect until the individual is about 35 to 45 years old. (

Sally’s father and sister both have Huntington’s disease. Sally also has a brother that was tested negative for Huntington’s disease. It is unknown whether Sally’s mother had Huntington’s disease.


a)What is the genotype for Sally’s father? Explain your answer.


b)What are the chances that Sally’s mother was homozygous dominant for Huntington’s disease? Explain your answer.


c) What are the chances that Sally will have Huntington’s disease? Explain your answer.

Answer 1

a) Sally's father Hh (or whatever letter you want to use). Since he suffers from Huntington's he must have at least one dominant allele. Since he has a child who does not suffer from it, he must also have one recessive allele (if he was homozygous dominant all children would suffer regardless of the mother's genotype).

b) 0% - if she was homozygous dominant, regardless of what Sally's father is, all of the children would suffer from Huntington's. Again, since there is a child who does not suffer from it, being homozygous dominant is ruled out.

c) Regardless of what anyone says, you cannot calculate an exact percentage here as there is not enough information in the question to ascertain whether Sally's mother is heterozygous (and does suffer) or homozygous recessive (and does not suffer). The information provided only allows you to say that the father is heterozygous. However, for the sake of giving complete answers, if Sally's mother is heterozygous, Sally has a 75% chance of being affected. If her mother is homozygous recessive, Sally has a 50% chance of being affected.

Answer 2

a. Sally's father must be Hh. Sally's brother is negative, so bro is hh, and he must have gotten an h from Dad. But Dad has Huntington's, so his other allele is H.
b. Sally's mother can not have been homozygous dominant for Huntington's disease (HH) because Sally's brother tested negative (hh) and he had to get the h allele from Mom.
c. The most likely scenario is that Sally has 1/2 chance of having Huntington's disease because:
-- her dad is Hh, so Sally has 1/2 chance of having the H allele from him.
-- her mom is undetermined, and Huntington's is pretty uncommon in the population, so it's most likely that Mom is hh.
---- If her mom turns out to be Hh, then Sally's parents are Hh x Hh, and Sally's chance of having Huntington's jumps from 1/2 to 3/4.

Answer 3

Punnet squares are very helpful when you need to figure out these kind of questions.

a) X^h Y : If Sally's father has the disease then his X allele is contaminated.

b) Very slim because if she was homozygous for both alleles, then the boy would have to have the disease too. So she is a carrier of the disease.

c) She wouldn't have the disease but she would be a carrier. One of her X alleles would be affected.