3 testcross a wild type fruit fly gray with normal wings bb...

3. TESTCROSS: A wild-type fruit fly (gray with normal wings; b+b vg+vg) was mated with a double mutant black fly with vestigial wings (bb vgvg). a. What would be the expected phenotype/s and phenotypic ratio if body color and wing type were independently assorting? Calculate the phenotypic frequency assuming there were 100 offspring b. What would be the expected phenotypes and phenotypic ratios if body color and wing type were linked such that dominant alleles are located in the same chromosome and recessive alleles on a different chromosome? Calculate the phenotypic frequency assuming there were 100 offspring.

Transcript

00:01 Starting with the f1 generation, we have individuals who are heterozygous, and so having dominant and recessive alleles for each gene.

00:09 And we're saying, you know, if we cross over this f1 generation, what should we expect for the f2 generation? well, to determine that, we're going to do a punnet square.

00:19 And so we have to determine the possible comedic combinations of our f1 generation for that.

00:23 So we could have possible combination of first w, first e.

00:28 Could have first w, second e, first w, or second w, first e, and second of both.

00:40 And because we're crossing two f1 individuals, they're both the same.

00:44 We can write the same possible comedic combinations on the rows as well.

00:53 And then once we have all of these, we should have four for each individual in the cross.

01:00 So that gives us a four by four punnet square.

01:03 And once we have all of that, we can just do our punant square as normal for each probability box looking up and to the left to see what alleles would go in it.

01:12 So we see in our first one, it's all dominant alleles.

01:16 And then it slowly moves out from here.

01:20 Now, this is what we would expect for a f2 generation if these two genes are not linked.

01:29 So we don't have to worry about crossing over.

01:32 They're likely on two different chromosomes and there's no linkage.

01:36 So if the genes are linked, that will definitely throw off your number of expected and observed progeny for the f2 generation.

01:52 Now, when i have a large punnet square like this, i like to make it easier to read by taking a couple colors and say we're going to do green and blue here.

02:03 And then i'll put a green dot for every probability box where i see at least one dominant w and a blue dot for every probability box where i see at least one dominant e allele.

02:17 So that tells me which probabilities have a dominant w phenotype and which ones have a dominant e phenotype and which ones have both.

02:33 That makes it a lot faster to read.

02:35 So that way, once i'm done here, i'm going to go over here and write out all of the possible phenotypes for my f2 generation, i could have both dominant traits.

02:45 I could have the first dominant trait, not the second.

02:47 I could have the second and not the first...

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