Hemophilia is an X-linked recessive trait in humans. Huntington’s Disease is inherited with an autosomal dominant allele.
a. Mr. Y is unaffected by either condition. He marries Ms. X, who is unaffected by hemophilia but shows signs of Huntington’s Disease. Ms. X’s father has hemophilia but is unaffected by Huntington’s Disease. What is the probability that the first child born to Ms. X and Mr. Y will be a son with hemophilia and their second child will be a daughter with Huntington’s Disease and their third child will be a son who is unaffected by either condition?
b. For Ms. X and Mr. Y, what is the probability that the first child will be either a son with Huntington’s Disease but without hemophilia or a daughter that is unaffected by Huntington’s Disease but carries the allele for hemophilia?
c. Mr. U has hemophilia and Huntington’s Disease. Mr. U’s father is unaffected by either condition. Mr. U marries Ms. V, who is unaffected by either condition. Ms. V’s father has hemophilia. Out of three children, what is the probability that two will be girls with hemophilia and Huntington’s Disease?
d. What is the probability that Mr. U and Ms. V will have a son with hemophilia or a daughter with Huntington’s Disease as their first child?
e. What is the probability that Mr. U and Ms. V will have a son with hemophilia and Huntington’s Disease or a daughter with neither condition as their second child?
I have hard time setting up and understanding the problem. Here's what I did:
Huntington(Hnt) is autosomal dominant= H Hemophilia (Hem) is x linked recessive= h
since we're given that Miss X has Hnt then her genotype is H_ (underscore for an unknown 2nd allele). We're given that her dad suffers from Hem thus his genotype must be h(X^h,Y no allele on Y chromosome). Since, Miss X has Hnt (auto dom. does not skip generation usually) then her mom also had one thus her geno is H_. Thus her parental geno(P1) is H_ x h, by carrying out this punnet square we would get 0.25% chance for Hh female, h_ female,H male and a healthy male. Thus the probability of Miss X to be sick is 1/4
Now moving onto the cross between Mr Y and Mrs X
we have --(healthy male) x H_
thus doing the cross we get 0.25 chance to get H_ female,-- female,H male,-- male
thus there is basically 0% chance to get a hem son? this is where i get lost and my logic fails because the mother of Miss X could be Hh(have hemo recessive allele) but in that case:
--(healthy MR Y) x Hh(hetezygote Mrs X)
0.5% to get h-( X^h X) female and 0.5% to get h male(X^h Y). THus if the P to get healthy male is 0% the whole thing becomes zero??
How do I break down the problem more feasibly? as i find myself using the dirty math tricks to tailor my answer to the right one without understanding on the mechanics
correct answer is 1/128 which is (1/4)^3 x 1/2 but i just dont get where the 1/2 and third 1/4 came from (the possiblity to have a healthy male is 0% remember?)
I would appreciate if you could break it down to me