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AB x A (Blood type) Cross Issues and Trihybrid Phenotypes

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

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AB x A (Blood type) Cross Issues and Trihybrid Phenotypes

Postby Iceshard » Tue Feb 03, 2009 10:08 pm

I've just started my genetics class and I'm having difficulty understanding a few things. I'm pretty sure I understand the Phenotype crosses, but i'm about ready to pull my hair out with the Blood type problems. I've read over our book, and the class notes and still can't figure things out. These are the questions I had and the answers I came up with. I could certianly use some imput to atleast see if i'm on the right track.

1. In peas, tall (T) is dominant to short (t), yellow (Y) is dominant to green (y), and round (R) is dominant to wrinkled (r). From a cross of two triple heterozygotes, what is the chance of getting a plant that is
a. tall, yellow, round? (I got 27/64 for this)
b. short, green, wrinkled? (I got 1/64 for this)
c. short, green, round? (and 3/64 for this one)
I used the fork line method for this trihybrid problem and came up with the 27:9:9:3:9:3:3:1 ratio, i'm confident I got the answers right but someone checking and giving me a nudge in the right direction if i'm wrong is always helpful.

(This one I don't really understand no matter how much information I go over)
2. A woman of blood group AB marries a man of blood group A whose father was
group O. They have two offspring. What is the probability that
a. their two children will both be group A?
b. one child will be group B and the other group O?
c. the first child will be a son of group AB and their second child a son of group B?

I can't really figure out what signifigance the father being type O would have to deal with this problem, other then it would make the man AO? So there would be a cross between AB x AO. If that's right, then I don't understand how to do the Punnett square or fork line method for that cross. I did try, and came up with the results of 2/4 being A, 1/4 being B and 1/4 being AB and none being O. Doesn't seem right...I know one would have to show up being O but that's not what the data is telling me. For question (a) I came up with that there is a 2/4 chance both will be group A, and for (b) I gathered that 1/4 chance of one being B and no chance for one being group O. And for (c) I can't find any information about the gender being signifigant to the problem, so the chance of their first child being group AB is 1/4 and their second being of group B is 1/4. This doesn't seem right, and i'm completely confused, can anyone shed some light on my issues?
Iceshard
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Postby domwood » Mon Feb 09, 2009 7:59 pm

There's nothing wrong with your answers for the first one. As for the second one you're on the right track. Why doesn't it seem right that that none are O. Remember that the blood groups refer to surface antigens of blood cells, A group just has A antigen, B just has B antigen, AB has both, and O has neither. If someone is O it means they have no alleles that produce A or B antigens whatsoever. So if one parent is AB... As for the last part, I'm not aware that blood groups are sex-linked, I feel you're probably expected to incorporate the probability of gender as well, so a punnet square with AX, OX, AY, OY, etc...

Hope this helps.
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