TBR: Genetics: Taking into account gender.....

[justadream]

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TBR Bio Book I page 113 #12

"What is the probability that a woman whose father has hemophilia and who marries a normal man will have an affected son"

Answer: 50%

My answer: 25%

Hemophilia is sex-linked recessive. I get that to have an affected son, the woman (who is a carrier) must pass her affected X chromosome to the boy. There is a 50% chance of this.

But what about GENDER? Why don't you account for gender? If you draw out the punnet square, you will see that only one box (25% chance) is for an affected son.

 

[mehc012]

TBR Bio Book I page 113 #12

"What is the probability that a woman whose father has hemophilia and who marries a normal man will have an affected son"

Answer: 50%

My answer: 25%

Hemophilia is sex-linked recessive. I get that to have an affected son, the woman (who is a carrier) must pass her affected X chromosome to the boy. There is a 50% chance of this.

But what about GENDER? Why don't you account for gender? If you draw out the punnet square, you will see that only one box (25% chance) is for an affected son.

I believe when they specify the gender, they are trying to eliminate that step for you. It is more 'if she has a boy, what are the odds that he will be affected?"

 

[justadream]

@mehc012

Okay. I hope it would be more clear on AAMC questions.

I remember doing a similar question in either TPR/TBR a few months earlier where I got it wrong for not taking into account gender (and now I got it wrong for taking into account gender).

 

[mehc012]

@mehc012

Okay. I hope it would be more clear on AAMC questions.

I remember doing a similar question in either TPR/TBR a few months earlier where I got it wrong for not taking into account gender (and now I got it wrong for taking into account gender).

Hopefully the AAMC ones will be clearer, yes, but no guarantee. The MCAT is like any other multiple choice exam - some of the questions, frankly, suck, and are hard to interpret.

 

[kraskadva]

TBR Bio Book I page 113 #12

"What is the probability that a woman whose father has hemophilia and who marries a normal man will have an affected son"

Answer: 50%

My answer: 25%

Hemophilia is sex-linked recessive. I get that to have an affected son, the woman (who is a carrier) must pass her affected X chromosome to the boy. There is a 50% chance of this.

But what about GENDER? Why don't you account for gender? If you draw out the punnet square, you will see that only one box (25% chance) is for an affected son.

You're accounting for gender incorrectly.
(i'll try to do a punnet square here...)

Dad
X Y​

Mom
X
X-h

XX | XY (boy, unaffected)
(girl) |​

--------- |----------------------------

X-h X | X-h Y (boy, affected)​

(girl-carrier)​

so it's a 25% chance that she will have a child affected, but it's a 50% chance that her son would be affected.