BIO genetics

[d wannabe]

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Hi
Can someone help me with this question?
I will attach the pic here

View attachment genetic.bmp

Q: In the pedigree, the darkened figures indicate an individual with hemophilia, a sex linked recessive disease. The genotype of the Femaile marked A is:

a) X_H X_h
b) X_H X_H
c) X_h X_h
d) X_h Y

The answer is a) but I thought it is c



Since 3rd/4th generation have no hemophilia,
i thought female (A) should be X_h X_h becaue her husband is hemophilia (X_H Y)

Am I wrong?

 

[Mstoothlady2012]

You have to realize that A is one of the daughters of the parent that is hemophilic (first cross). So A gets one of her X from the mother who is not hemophilic and another X from the father who is hemophilic. Hemophilic is x-linked recessive, so the father is XhY. So A's genotype is XH (from mother) Xh (from father)

 

[Mstoothlady2012]

oh and if A would be Xh Xh, which means she is hemohpilic then all the children (3rd generation) would be hemophilic as well; which is not true in the pedigree.

 

[Orgodox]

You have to realize that A is one of the daughters of the parent that is hemophilic (first cross). So A gets one of her X from the mother who is not hemophilic and another X from the father who is hemophilic. Hemophilic is x-linked recessive, so the father is XhY. So A's genotype is XH (from mother) Xh (from father)

Thats backwards I think circle female square male

 

[Orgodox]

Firt generation the fathers X is normal otherwise he would be hemophiliac. He gives that XH to A
Now the quesiton is which X does the mother in 1st generation give? we know A is XHX_
In the 3rd generation all the females get the Xh from their father, the only way they can all be normal i if the mother gives an XH otherwise at least one of the 3 females in generation 3 should have hemophilia since they already have one Xh. Truthfully you cant tell from here because it COULD be chace that the mother is XHXH and thats why none of her kids are affected or she could be XHXh and happenes to give XH too all her daughters and son otherwise they would also be homophilliacs.

 

[drgreen]

I originally guessed that the answer was 'c' as well, but the answer is in fact 'a'. I think the point of confusion is that XH is dominant (not hemophilia) while Xh is recessive (hemophilia).

A's father is XH Y
A's mother is Xh Xh
(since she is phenotypically a hemophiliac she must have Xh Xh, if she was XH Xh, she would not be shaded black)
A gets one X from dad and one X from mom. This makes her automatically XH Xh.

but wait! A's children are all phenotypically not hemophiliacs!!!
They must just all be lucky, getting mom's XH.

Hope this helped

 

[Orgodox]

Truthfully you cant tell from here because it COULD be chace that the mother is XHXH and thats why none of her kids are affected or she could be XHXh .

But since only one of these is a possibility theres your answer

 

[ryry]

Drgreen got it right, i think.
During the first cross, you cross affected female (circle) with normal male (square).
Thats XH_Y*Xh_Xh

If you do the square, you get following genotypes:

XH_Xh - All daughters are carriers
Xh_Y - all sons are affected

That leaves you with only answer choice A.

Say, you crossed one of Daughters from the cross with an affected male,

XH_Xh * Xh_Y

In this case, you get:

XH_Xh - carrier, female
XH_Y - normal
Xh_Xh - affected female
Xh_Y - affected female

In this case we get the 1:1:1:1 ratio. So it is possible that all of their children will show no phenotype for the disease (theres 50% chance of that happening)

You wrote, " Since 3rd/4th generation have no hemophilia,
i thought female (A) should be X_h X_h becaue her husband is hemophilia (X_H Y)"

Hemophilia is a recessive trait, so her husband should be Xh_Y (he is affected)
And it is possible to have 3/4th generation show no hemophilia because hemophilia is a recessive trait and therefore can skip generations.

But either way, only thing the question asked for is whats the genotype of A, so only cross you really have to worry about is the 1st one between affected Female and normal male.

i hope it helped.

 

[Mstoothlady2012]

Thats backwards I think circle female square male

you are right! Didn't look at the pedigree properly.

Sorry the explanation is still right just switch the parents. A's father is normal (XH Y) and mother is hemophilic (Xh Xh). So A is a carrier but not hemophilic ( XH Xh)...answer being A

 

[Mstoothlady2012]

Firt generation the fathers X is normal otherwise he would be hemophiliac. He gives that XH to A
Now the quesiton is which X does the mother in 1st generation give? we know A is XHX_
In the 3rd generation all the females get the Xh from their father, the only way they can all be normal i if the mother gives an XH otherwise at least one of the 3 females in generation 3 should have hemophilia since they already have one Xh. Truthfully you cant tell from here because it COULD be chace that the mother is XHXH and thats why none of her kids are affected or she could be XHXh and happenes to give XH too all her daughters and son otherwise they would also be homophilliacs.

How is that possible? She gets 1 X from each parent. She has to get Xh from her hemophilic mother.

 

[Orgodox]

How is that possible? She gets 1 X from each parent. She has to get Xh from her hemophilic mother.

Yup sorry she gets Xh from mom, and Dad only has X which must be XH so thats it... shes XHXh. And the kids are all really lucky like green said but not very probable.