Dihybrid genotypic and phenotypic problem help needed

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DiscoJen
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Dihybrid genotypic and phenotypic problem help needed

Post by DiscoJen » Mon Apr 02, 2007 3:58 pm

Got some more intro to bio homework to do but I having some trouble with a dihybrid formula that I am working on. I'll write the problem down the way it was given to me and maybe someone here could give me a hand.

The position of the flower on the stem of garden pea is governed by a pair of alleles. Flowers growing in the axils are produced by the dominant T, flowers growing at the tip are produced by the recessive t. Colored flowers are produced by the dominant C, and white flowers by the recessive c.

A dihybrid plant with colored flower at the axils is crossed to a pure breeding strain of the same phenotype. What genotypic and phenotypic ratios are expected in the F1 progeny?


I believe I need to be able to show my work on a Punnet Square but I don't even know where to begin. I have narrowed down the first plant to being a TtCc, but the 2nd plant that is a pure breeding strain is throwing me...we haven't discussed pure breeding strain, my guess would be it's a TTCC but the probability of my guessing wrong would be high. :shock: hehe

Any help is greatly appreciated. I've been struggling with it all weekend and it's due tuesday at 1pm.

Thanks in advance,
Jen

2Loula
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Post by 2Loula » Mon Apr 02, 2007 4:32 pm

Hey,

Yup, the first plant is TtCc, and the pure breeding strain will be TTCC.

Basically, when it says pure breeding, it means is homozygous, and will probably mention somewhere else in the question if it is pure breeding dominant (like your question), or pure breeding recessive.

A dihybrid punnett square will end up having 16 outcomes, that usually conform to a 9:3:3:1 ratio. However, when a pure breeding is involved the ratio is often different.

For the punnett square, you need to take whatever gametes you will have from both parents, in order to work out the possibilites of the F1 generation.

ie. From TtCc, you get TC Tc tC tc
From TTCC, you get TC (4 times)

If you do your punnett square, I believe that all offspring will show dominant charactestics for both genes.

I hope this helps.

DiscoJen
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Ratios

Post by DiscoJen » Mon Apr 02, 2007 6:05 pm

Cool, so at least I started off on the right track. :) Your definition was extremely helpful!

So I am now trying to break it all down from there. Since one of the donors is essentially unknown homo or heterzygous, we can't really do a Punnetts Square right? At this point for F1, isn't this phase really just determining the possible gametes for each then figuring out the offspring using a Two-Trait Testcross?

Here is what I think the outcome will be.
_________________________________________________________

TTCC - known homozygous
TtCc - unknown homozygous or heterozygous

Possible gametes - TC, Tc, tC, tc
Offspring of these gametes with TC - TTCC, TTCc, TtCC, TtCc
_________________________________________________________
If my diagram above is right, then what would my genotypic ratio be? Is the phenotypic ratio 100%? Cause they would all have TC physical characteristics. My textbooks don't show any examples of such a combination so my confidence level is not really high.

Thanks again for your help 2Loula!

DiscoJen
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Cool - I did pretty ok

Post by DiscoJen » Tue Apr 03, 2007 8:17 pm

Just an update, I turned in my homework and we just went over it in class.

For the most part I was right, thanks in large part to 2Loula. I just forgot to put down my genotypic ratio duh, which was 25% TC, 25% Tc, 25% tC, 25% tc. It also would have been accepted doing a two type testcross chart or a punnett chart.

At least I will get a large portion of the 10 possible points added to my next test grade as extra credit! Plus I have a better understanding and will also do better on the test itself. :)

Thanks a bunch,
Jen

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