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assignment help: genetic cross

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

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assignment help: genetic cross

Postby kamas069 » Sun Jan 02, 2011 3:14 pm

i need help with my genetics assignment:
heres the question

In humans, two genes affect the hands. One gene determines the ability to curve the thumb, the other controls the presence of hair on the middle segments of the fingers. The allele for curved thumb, T, is dominant to the allele for straight thumb, t. The allele for hair on the middle segment of the fingers, H, is dominant to that for an absence of hair, h.

a) draw up a table to how the possible genotypes resulting from a gross between two individuals heterozygous for both of theses genes for male and female gametes.

(genotypes for female gametes and male gametes, male down the side while the female is across the top and 4 gametes each)

Help me please????
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Postby Kezzer » Sun Jan 02, 2011 4:42 pm

Ok for this question you need to do a dihybrid cross to show inheritance of two traits, you are only told that each person is heterozygous for both gametes so you need to show all possibilities, for maternal (female) gametes across the top the four possibilities would be:

TH, Th, tH, th

these are then the same for the male gametes down the side.

You then need to write in the genotypes that result from each heterozygous possibility which is just a case of reading across on the table e.g the first one would be genotype TTHH (curved thumb and hair on the middle segment of the fingers) the second TTHh and so on.

After you have done all the possibilities to fill the table in your genotypic ratio should be 9:3:3:1 (9 curve and hair, 3 straight and hair, 3 curve and no hair, 1 straight and no hair).

Hope this helps,
give me a shout if you get stuck anywhere :)
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Postby kamas069 » Sun Jan 02, 2011 5:28 pm

thank you thats perfect :D
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Postby kamas069 » Sun Jan 02, 2011 5:32 pm

kezzer... i require your help again.

what is the probability that the first born of this couple (who are heterozygous to both genes) would be a female with straight thumbs??

please help again??
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Postby Kezzer » Sun Jan 02, 2011 5:49 pm

I'm not entirely sure about this one sorry but I think the percentage chance of any child to have straight thumbs is (3/16) + (1/16) = 0.25% (as theres are 4 chances out of 16 that a child will have straight thumbs.

As there is a 50/50 chance of having a male/female offspring then I would presume you just half this value, so chance of daughter with straight thumbs 0.125%

Like I said not sure that my answer is correct so don't quote me on it ;)
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Postby kamas069 » Sun Jan 02, 2011 6:37 pm

thanks :D
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Postby Kezzer » Sun Jan 02, 2011 6:39 pm

no problem :)
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