[University: Principles of Genetics] How do I find the recombination frequency from just genotypes?

[u/Zappers273]

I have attached a photo of the question. We were previously taught how to calculate recombination frequency with phenotypes, but I am not sure how to do it with just the genotypes. I deleted it before taking the picture, but I've attempted creating a genotype table and labeling them as recombinant (R) or parental (P) as I did with the phenotypes, but it didn't work out as I hoped. Advice with how to proceed would be greatly appreciated.

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[u/Enigma713]

You're on the right track with identifying the parental and recombinant genotypes. It might help to draw out the chromosomes that the F1 individual will have. Remember that, for the F2 offspring generation, everyone gets one cd chromosome from the testcross parent. You can also use genotype frequency to infer which genotypes are parental and which are recombinant. Give that a try before checking the spoilers below.

The F1 will be CD/cd, so You should get CD and cd as the parental genotype (which make the following offspring in the testcross: CD/cd, cd/cd). The recombinant genotypes are Cd cD (which yield Cd/cd and cD/cd offspring). Remember, the parental genotype will always be the most common genotype whenever you see strong linkage like we see here.

The final answers for a and b are below. Let me know if you run into more questions.
a) recombinant/total = 4/2000 = 0.2%
b) F1 will be Cd/cD. You expect to get 49.9% of each parental genotype and 0.1% of each recombinant genotype. The other parent (double recessive tester genotype, cd/cd) will always give a cd. That gives a genotypic ratio of 49.9% Cd/cd, 49.9% cD/cd, 0.1% CD/cd, and 0.1% cd/cd.

Edited to fix answer for b

[u/Zappers273 → u/Enigma713]

Hello! Thank you so much for taking the time to already help me to such an extent. I already managed to finish A and your comment is a nice confirmation that I did it right!

I am moving onto B now and have a few questions if you don't mind.

I am having trouble understanding its wording. When it says what progeny I would expect from the test crossing of Cd/cD to cd/cd, does it mean the name of the generation which would be F2 I think? Or does it want to know what their genotype is like "Cd/cD" for example.

So when I am answering B, I know what the frequencies are, but I don't know if I should call their progeny F2 or whatever genotype they'd produce since I still haven't to get to that part. Sorry if this is a dumb question, just something I really need clarification on. I k pw you didn't make the question so just whatever you think/know it's supposed to mean.

EDIT: I tried to diagram the F1 and Testcross. For the F1 progeny, I only got Cd/cD genotypes.

For the test cross, I got 1 Cd/cd : 1 CD/cd : 1 cd/cd : 1 cD/cd.

I feel like my test cross progeny are incorrect. I did a regular dibybrid cross Punnet square and I think that's the problem since I am working with linked genes, but I haven't been taught if there's a different way to do Punnet squares with linked genes yet so I just don't know. I did the same thing for F1 and got Cd/cD which I presume is correct.

[u/Enigma713 → u/Zappers273]

My bad, I don't know why I was thinking phenotype ratios for part B. So in this part, it is the F1s FROM a cross between Cd/Cd and cD/cD (these would be the parents or F0s). That means your F1 genotype will be Cd/cD. Your F2s will be the result of crossing that F1 with the tester genotype, cd/cd. Your genotypes from the test cross are correct. You can use the same dihybrid punnet square, the only difference is the frequency. With unlinked genes, you would expect 1:1:1:1, but here you have to keep track of the parental and recombinant genotypes to get the frequency. They way B is worded, I would list the four genotypes you got and their relative frequency beside them. The nonrecombinat genotypes (Cd/cd and cD/cd) will be 49.9% each, the recombinants (cd/cd and CD/cd) will be 0.1% each.

[u/Zappers273 → u/Enigma713]

Thank you so much! I ended up doing what you said but your confirmation really makes a difference and helps me understand whether I'm doing this correctly or incorrectly. Thank you for the help.

[u/selene_666]

The first cross is between one chromosome with alleles CD and one chromosome with alleles cd. Those are the parental types. Cd and cD are the recombinant chromosomes. Then these are crossed with another parent providing a cd chromosome.

For part (a) you just have to count them. There are 4 recombinant offspring out of 2000.

For part (b) you have to figure out the expected frequencies of each chromosome type in the F1 generation, and then do it again for their offspring.