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Punnett Square Question

Genetics as it applies to evolution, molecular biology, and medical aspects.

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Punnett Square Question

Postby NiceOnTheIce » Thu Jan 13, 2011 4:35 pm

In rats, a black-tailed mutant was discovered. When it was crossed to a pink tailed rat, 4 offspring were black-tailed and 3 were pink tailed. Two of these black-tailed rats were crossed and they produce 6 black tailed and 3 pink-tailed rats. These experiments were repeated several times with approximately the same results. Never was a case seen where a mating between two black-tailed rats produced all black-tailed rats. Explain the black X pink tail and black X black crosses in terms of the alleles of the gene involved in tail colour and the genotypes of the rats in the crosses and their offspring.

This is one of the questions on my assignment. I know that the first cross will have to result in a 1:1 phenotypic ratio of black to pink-tailed rats (the prof gave me this info), and that the second cross will have to result in a 2:1 ratio of black to pink-tailed rats. I also know that the only way to get a 1:1 ratio for the first cross will be with the cross of a heterozygote with a homozygote, but no matter how I manipulate the genotypes of either type of rat, I can't seem to get a 2:1 phenotypic ratio for the second cross, only a 3:1 ratio. Help please!
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Postby magicsiew » Thu Jan 13, 2011 9:53 pm

Is there more information? Sex influence? lethal gene?

If XbXb is lethal, then can get the ratio.
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Postby NiceOnTheIce » Thu Jan 13, 2011 11:35 pm

There isn't any more information, but I'm pretty sure this assignment doesn't cover the variations to Mendel's Principles. Just the basic, "pea plant" scenarios. (I'd thought of lethal genes too.)
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Re:

Postby magicsiew » Fri Jan 14, 2011 1:40 am

NiceOnTheIce wrote:There isn't any more information, but I'm pretty sure this assignment doesn't cover the variations to Mendel's Principles. Just the basic, "pea plant" scenarios. (I'd thought of lethal genes too.)


If you are saying pea plant scenarios, then u are saying Mendelian (Mendel's principles), non-mendelian is something totally different and more complicated.
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Postby NiceOnTheIce » Fri Jan 14, 2011 2:43 am

Yes, exactly. We have learned about the variations to Mendel's principles, but they aren't being applied in this first assignment. Only the basic "Mendelian" should be used in this case.

What I thought was, since my prof said that the 7 F1 offspring (4 black:3 pink) was a 1:1 ratio (crossing a heterozygote black with a homozygote recessive pink), then perhaps the 9 F2 offspring (6 black:3 pink) is supposed to represent a 3:1 ratio? (crossing 2 F1 heterozygote blacks to get 3:1, 3:1, then 1 pink?) (As in, maybe he didn't plan the offspring genotypes perfectly to fit a Punnett square. Maybe he's expecting us to assume there would obviously be some randomness in the 9 offspring's genotypes, because although it's improbable, there is still 1/4 chance that any one of them would be pink?)
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Postby magicsiew » Fri Jan 14, 2011 7:33 am

Maybe u can count ur hypothesis in a chi square test, if the hypothesis is accepted, then u can argue in that way.
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