Pedigree problem

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thewax
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Pedigree problem

Post by thewax » Sun Dec 14, 2008 6:42 am

A question in AP Biology by Deborah T. Goldberg - "A cross was made between two fruit flies, a white-eyed female and a wild mae (red eyed). One hundred F1 offspring were produced. All the males were white eyed and all the females were wild. When these F1 flies were allowed to mate, the F2 flies were observed and the following data was collected. Females MalesP: White eyed X Wild (red eyed)
F1: 59 wild 51 white eyed
F2: 24 wild 23 wild
26 white eyed 27 white eyed
What is the most likely pattern of inheritance for the white-eyed trait?
(a) autosomal dominant
(b) autosomal recessive
(c) sex-linked dominant
(d) sex-linked recessive
(e) holandric"


The solution is..."(D) Here is the first cross. [A Punnett Square without the boxes, as I can't seem to create them] X Y
X- X-X X-Y
X- X-X X-Y

All the female offspring are carriers (X-X), and all the male offspring have white eyes (X-Y). Here is the second cross. [Again a Punnett Square]
X- Y
X- X-X- X-Y
X X-X XY

There is a 50 percent chance that a male will be white eyed and a fifty percent chance he will be red eyed. There is a fifty percent chance a female will be white eyed and a 50 percent chance she will be red eyed (a carrier)." I have two questions:(1) Could you explain the solution and why & how it works (how would you know how to construct the Punnett square with so many variables - whether it is sex-linked at all or autosomal and then whether it is dominant or recessive)? I mean, when I look at the solution, it makes sense. But how do you get to that solution? (2) Couldn't you assume it was autosomal and construct a Punnett Square for F1 generation like this:Let A be the dominant allele and a be the recessive allele... (note that a and A are not assigned specifically what the dominant and the recessive allele is)

A a
a Aa aa
a Aa aa

Darby
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Re: Pedigree problem

Post by Darby » Mon Dec 15, 2008 4:22 pm

Your first batch of offspring tell you everything: it's sex-linked, or you wouldn't get traits in all females different from traits in all males; it's recessive, or your first generation females would show it.

thewax
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Re: Pedigree problem

Post by thewax » Wed Dec 24, 2008 10:53 pm

Darby wrote:Your first batch of offspring tell you everything: it's sex-linked, or you wouldn't get traits in all females different from traits in all males; it's recessive, or your first generation females would show it.


Thanks Darby!!!!!!!!!!!!!!!! :)

I just have one question:
But can you explain to me WHY would it be sex-linked??????

Darby
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Re: Pedigree problem

Post by Darby » Thu Dec 25, 2008 5:09 pm

I'm not sure what you're asking, so I'll answer one possibility - being linked to the X chromosome would make it sex-linked (or on the Y chromosome, but only male-related genes are on there anyway), which also makes single recessives show up in the males but (no second X chromosome in males to carry a covering dominant).

Was that the proper "why"?

thewax
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Post by thewax » Thu Dec 25, 2008 11:15 pm

That answered my question.
Thanks!!!!!!!!!!!!!!!!! :)

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Re: Pedigree problem

Post by Darby » Sat Dec 27, 2008 8:34 pm

Good - now I'm trying to figure out how a bit of one sentence got left out.

It should say after the parenthesis...not the female, whose single recessive is covered by the dominant on the other X.

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Post by thewax » Sun Dec 28, 2008 4:51 pm

ohhh... I see...
Thanks!!!!!!!!!!!!! :)

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