## Dihybrid cross involving two sex-linked genes

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### Dihybrid cross involving two sex-linked genes

Hi! Can anyone help me with this: Does Mendel's 2nd law apply in cases of two sex-linked diseases (e.g. hemophilia and daltonism), since both genes reside on the X chromosome and therefore non-independent? Can we use Punnett square to predict offsprings?
doramk
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Since they're both on the same chromosome, Mendel's second law (the law of independent assortment) doesn't apply in this case. But, Punnett square may be used in some situations like in a and can't be used in others like b:

a) If a man has both hemophilia and daltonism > X^hd Y, no other possibility.

It's also easy to predict genotypes of a woman who has both illnesses or none (while not being carrier).

b) But, it's difficult in the case of a woman being carrier for both of the illnesses. i.e. Is she (X^Dr X^dR) or (X^DR X^dr)?

But, I think if you're given genotypes (not only phenotypes) you can still use a punnett square in this case.

DRT23
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### Re: Dihybrid cross involving two sex-linked genes

Thanks! This means that we cannot predict percentages according to Mendel's law, even if we know the exact genotypes. A Punnett square can give possible combinations of gametes, but Mendel' s law does not apply since the genes reside on the same chromosome. So why use a Punnet square?
doramk
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Well if you know the distance between the genes you can actually estimate the percentages and use the punnet square accordingly.
Patrick

Science has proof without any certainty. Creationists have certainty without
any proof. (Ashley Montague)

canalon
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### Re: Dihybrid cross involving two sex-linked genes

I actually don't, since this is a high-school issue, but thanks!...
doramk
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I tried to find it and managed throughOMIM.
And to be honest it was not completely evident how to get the info. And there are multiple form of both color blindness and hemophilia just to make things a bit more muddy. Any how in the end it appears that genes causing a common form of both diseases are located in the same region of the chromosome X (Xq28 if you want to look at the map) and the gene coding for the factor VIII (cause of hemophilia A) and one coding for one of the Opsin (causing one of the form of color blindness) are just about 562 thousand bp apart. If we take teh wiki definition that put one cM at ca. 1 million bp, that mean that there is only a 0.6% chance of the genes being inherited independantly. In other word and doing a massive simplification, it can be assumed that the genes are fully linked when it comes to drawing a punnet square.
Patrick

Science has proof without any certainty. Creationists have certainty without
any proof. (Ashley Montague)

canalon
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### Re: Dihybrid cross involving two sex-linked genes

doramk wrote::D

Thanks! This means that we cannot predict percentages according to Mendel's law, even if we know the exact genotypes. A Punnett square can give possible combinations of gametes, but Mendel' s law does not apply since the genes reside on the same chromosome. So why use a Punnet square?

So why use a Punnet square?

Simpy, these genes are on the same chromosome and out of Mendel's second law. So, punnett square shouldn't be used. But, we can still use it in some cases. Let me give you an example:

A man has both illnesses (hemophilia and daltonism) and his wife is not ill or carrier for both of them.

So, Man is (X^hd Y) and Woman is (X^HD X^HD).

Punnett Square:

------------X^hd / Y
X^HD......X^hd X^HD / X^HD Y
X^HD......X^hd X^HD / X^HD Y

All girls are carrier for both of the illnesses and all boys are healthy.

One thing we should consider is recombination and as canalon says this is a low probability for these genes and can be ignored. Also, a recombination in the woman above would result in the same combination (X^HD X^HD again) and it can't be in the man for X chr (there is only 1 X). So, this answer is correct for all cases.

On the other hand, the example above would be more complicated for the case that the woman is a carrier for both illnesses [(X^Dr X^dR) or (X^DR X^dr) and possible recombination].

DRT23
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