"Here is the question i was given: A man with hemophilia (a recessive, sex-linked condition)has a daughter who

"Here is the question i was given: A man with hemophilia (a recessive, sex-linked condition)has a daughter who is of a normal phenotype. She marries a man who is normal for the trait. What is the probability that a daughter of this mating will be a hemophiliac?Or a son who is hemophiliac? If they have 4 sons, what is the probability that all 4 will be born with hemophilia? i do not understand how to get the probability."

"Here is the question i was given: A man with hemophilia (a recessive, sex-linked condition)has a daughter who
well I guess it is linked to the X cromosone. then you can calculate from there.


if they get a daugther (she is XX) then she has a 50% risk to be a carrier like her mother, but not ill.


If they get a son (he would be XY) then he would have a 50% risk to get the condition.


This is because the farther ,who is 50% of the genetic material, have no condition, a man cannot have an recessive sex-linked condition without being effected.


The mother, who is the other 50% is carrying the condition in one of her X cromosones, so there is a 50-50 chance/risk of their children to get the condition.


so each son have a 50% risk of being efected, so for 4 sons to be effected is 0,5^4*100=6,25%


the risk is hafed each time.
Reply:ok, here goes... To be hemophiliac, both X chromosomes must carry the gene. Daughter is normal phenotype, so she has one normal X and one carrier X ( let's call this one x). Her husband is XY since he is not a hemophiliac. Since his daughters will inherit a normal X from him, none of them will be hemophiliacs, but could be carriers if they inherit mom's x instead of her X. A 50/50 shot. Since the sons will inherit their Y from dad, he is out of the equation and EACH son has a 50/50 shot of inheriting mom's X or x. This can be expressed as x=1/2. Or a one in two chance. Therefore the probability that ALL four sons will be hemophiacs is the product of all of the probabilities. Or ... Total prob= son#1 prob * son#2 prob * son#3 prob * son#4 prob. Or... 1/2*1/2*1/2*1/2.... This equals a 1/16 chance ( or 6.25%) that ALL fours sons will be hemophiliacs ( this is also the probability that all four will NOT be also).