I'm having major issues understanding the sexlinked genetics part of my bio class. I've understood the regular genetics section including how to work the punnet square.
A woman whose maternal grandfather suffered from hemophilia has parents that are normal. The woman's husband is normal. She has a hemophilic son. What are the chances that their next son will be normal? Will any daughters be hemophiliac? Will any be carriers? Hemophilia is sex linked recessive.
I dont know where to begin. Please carefully walk me through how to solve this.
A woman whose maternal grandfather suffered from hemophilia has parents that are normal. The woman's husband is normal. She has a hemophilic son. What are the chances that their next son will be normal? Will any daughters be hemophiliac? Will any be carriers? Hemophilia is sex linked recessive.
I dont know where to begin. Please carefully walk me through how to solve this.
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Hemophilia is carried by both males and females and usually expressed in males...
OK, assuming that the father does not carry the gene, it's coming through the mother. The chances that future sons will have the disorder are 50-50, and the chance that any daughters will be affected are less than 1 in 4.
It's been a long time since school!
OK, assuming that the father does not carry the gene, it's coming through the mother. The chances that future sons will have the disorder are 50-50, and the chance that any daughters will be affected are less than 1 in 4.
It's been a long time since school!
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If you know how to do Punnett squares, then sex-linked genetics should not be difficult if you just consider them another phenotype. The Punnett square rules are exactly the same - gametes on the axes, progeny on the inside.
The specifics (that you don't need to memorize, but you will notice when you do the Punnett square properly) is that males are "hemizygous," because the Y chromosome doesn't have the h.
Using good symbolism helps - X^h (X superscript h) for example would be how to write the hemophiliac. X's and Y's can pair. XX = female, XY = male.
So I'll do your example for you.
The husband is normal. He is XY, and therefore must be XHY.
Likewise the son is XhY, because he is hemophiliac.
So you can do the Punnett square backwards...which means the woman must be XHXh (we can assume she is normal). She must be a carrier because the son had to get the Xh from somewhere.
So now we just do the Punnett square of the cross:
XHXh x XHY = 25% XHXH, 25% XHXh, 25% XHY, 25% XhY (1:1:1:1). Now the problem here is these are probabilities for children in general. [1]
But figuring out the probabilities for sons or daughters is not hard.
If we only look at the sons, it is 50% XHY, 50% XhY (1:1) [2]
If we look at the daughters, it is 50% XHXH, 50% XHXh (1:1). [3]
So probability of a son being normal (XHY) is 50%, given that we know the child is a boy. [2]
The probability of a hemophiliac daughter is also 50%, given that we know the child is a girl. [3]
The probability of a carrier (XHXh...only girls can be carriers) is 25%, without being given any information about the sex of the child. [1]
The specifics (that you don't need to memorize, but you will notice when you do the Punnett square properly) is that males are "hemizygous," because the Y chromosome doesn't have the h.
Using good symbolism helps - X^h (X superscript h) for example would be how to write the hemophiliac. X's and Y's can pair. XX = female, XY = male.
So I'll do your example for you.
The husband is normal. He is XY, and therefore must be XHY.
Likewise the son is XhY, because he is hemophiliac.
So you can do the Punnett square backwards...which means the woman must be XHXh (we can assume she is normal). She must be a carrier because the son had to get the Xh from somewhere.
So now we just do the Punnett square of the cross:
XHXh x XHY = 25% XHXH, 25% XHXh, 25% XHY, 25% XhY (1:1:1:1). Now the problem here is these are probabilities for children in general. [1]
But figuring out the probabilities for sons or daughters is not hard.
If we only look at the sons, it is 50% XHY, 50% XhY (1:1) [2]
If we look at the daughters, it is 50% XHXH, 50% XHXh (1:1). [3]
So probability of a son being normal (XHY) is 50%, given that we know the child is a boy. [2]
The probability of a hemophiliac daughter is also 50%, given that we know the child is a girl. [3]
The probability of a carrier (XHXh...only girls can be carriers) is 25%, without being given any information about the sex of the child. [1]