help needed please!

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zatom
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help needed please!

Post by zatom » Tue Mar 20, 2007 2:01 pm

hi I have been wondering about this question for some time, if a disease (such as sickle cell) is inherited as an X-linked recessive gene in humans and an unaffected woman with no history of the disease marries a man who has the disease, what propotion of their sons and their daughters are expected to be affected?..

Is it still a simplictic 3:1 ratio?

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Revenged
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Re: help needed please!

Post by Revenged » Tue Mar 20, 2007 2:42 pm

zatom wrote:hi I have been wondering about this question for some time, if a disease (such as sickle cell) is inherited as an X-linked recessive gene in humans and an unaffected woman with no history of the disease marries a man who has the disease, what propotion of their sons and their daughters are expected to be affected?..

Is it still a simplictic 3:1 ratio?


Firstly sickel cell disease is not an X-linked recessive condition - it is an autosomal recessive condition... In the offspring there is a 25% chance of having disease, 50% of carrier, and 25% of not having disease... i think this is what you are referring to as 'the 3:1 ratio'...

An X-linked recessive condition would be something like Haemophilia...

Here is an example... The effected allele I've put in red

Male with haemophilia XY vs. Normal female XX

Offspring

50% chance of female carrier XX
50% chance of normal male XY

And so, none of the offspring would suffer from the condition in this case...

Hope that helps...

mjolnir
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Re: help needed please!

Post by mjolnir » Thu Mar 22, 2007 2:50 am

zatom wrote:hi I have been wondering about this question for some time, if a disease (such as sickle cell) is inherited as an X-linked recessive gene in humans and an unaffected woman with no history of the disease marries a man who has the disease, what propotion of their sons and their daughters are expected to be affected?..

Is it still a simplictic 3:1 ratio?


The example I gave you in your takehome quiz used G6PD deficiency, which is X-linked. Sickle cell anaemia isn't.

sportysunnyfunny
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Post by sportysunnyfunny » Fri Mar 30, 2007 6:51 am


hi dude :D
see your question is pretty valid and a good one that mite confuse many
but it has a simple approach
see what happens is that a male contributes only X chromosome to daughter and a Y chromosome to the son.
so it means that the other half is the contribution of the mother.
and if the male is suffering frm the disease so what will happen,he would have a progeny that has following

sons:- they will not carry the disease as its an X-linked disease and the father or the male doesnt contribute X chrmosome to his sons.

daughters:- in this case the daughters will be the carriers of the disease as they get an x- chromosome from the father amd the other half from their mother.

so thats what u get

Darby
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Post by Darby » Sat Mar 31, 2007 5:41 pm

I ask essentially this question for one of my laboratory exercises, and the wrong answer of Revenged is often given - it's an easy mistake to make.

sportysunnyfunny has it right, but somehow the answer - no offspring affected, but all females carriers - doesn't seem right. I've seen many students cross out the right answer and substitute a wrong one.

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Post by Revenged » Sat Mar 31, 2007 6:30 pm

Darby wrote:I ask essentially this question for one of my laboratory exercises, and the wrong answer of Revenged is often given - it's an easy mistake to make.

sportysunnyfunny has it right, but somehow the answer - no offspring affected, but all females carriers - doesn't seem right.


My answer wasn't wrong... It was EXACTLY the same as what sportysunnyfunny said...


"sons:- they will not carry the disease as its an X-linked disease and the father or the male doesnt contribute X chrmosome to his sons.

daughters:- in this case the daughters will be the carriers of the disease as they get an x- chromosome from the father amd the other half from their mother."


is exactly the same as my answer of...

"Offspring

50% chance of female carrier XX
50% chance of normal male XY

And so, none of the offspring would suffer from the condition in this case..."


:wink:

Saneth
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Post by Saneth » Tue Apr 10, 2007 8:53 am

The ratio you assumed is 3:1 but according to my mind it is not possible to get 3:1 ratio from a XX & XY cross.I think you are confuced with Aa & Aa cross.(dihybrid cross between heterozygous genotypes)

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Post by Revenged » Tue Apr 10, 2007 4:55 pm

Saneth wrote:The ratio you assumed is 3:1 but according to my mind it is not possible to get 3:1 ratio from a XX & XY cross.I think you are confuced with Aa & Aa cross.(dihybrid cross between heterozygous genotypes)


ok, i see... i did wonder why people were getting confused... and yes, i was assuming that both the parents were carriers in my autosomal recessive ('3:1') example... i just didn't say it so, which is why i confused people!...

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