Interference occurs when a the number recombinant progeny...

Interference occurs when (a) the number recombinant progeny classes in the testcross of a heterozygote ex- ceeds the number of parental progeny. (b) two genes are far apart on a genetic map. (c) one crossover inhibits another. (d) two genes are assorting independently. (e) a crossover causes the termination of the meiosis event in which the crossover is occurring. (a) The genes A and B are on the same chromosome and closely linked. (b) The genes A and B are on the same chromosome and very far apart. (c) The genes A and B are located on different chromosomes. (d) The genes A and B are probably between 10 and 20 map units apart on the same chromosome.

Transcript

00:02 So we want to be calculating the map distance between genes, and we're only told a little bit of pieces of our progeny of our f1 generation.

00:11 So we're going to go ahead and write down those pieces.

00:13 We know that we're dealing with three genes, so we should have eight different phenotypes.

00:17 We know the two most common are dominant, dominant, recessive.

00:20 There's 250 of those.

00:22 Next most common, recessive, recessive, dominant, and again, there's 250 of those.

00:28 Now, we don't know most of the others, but we do know that the, to least common, there's 50 of each, and one of them is all dominant, the other one is all recessive.

00:44 And so this is all out of a total of 1 ,000 offspring.

00:49 So what does this information give us? well, this information can actually give us quite a bit in terms of middle gene, map distance, and non -recombinates.

00:59 So we're going to look at these two.

01:00 We're told are the two most common.

01:02 So these are going to be your non -recognites.

01:05 So that means that the heterozygous parent has these two chromosomes.

01:14 They have dominant a, dominant b, recessive c, and then vice versa.

01:23 So that's what their chromosomes look like.

01:25 And all of these other phenotypes, even the ones that we don't know, are recomminants.

01:32 Now, we can look at the two non -recognites that we know and the two other phenotypes.

01:40 Phenotypes that we know, which we're told are the least common, so they are double crossovers, and we can compare them to see which gene is in the middle.

01:48 So for example, if i take one of my non -recombinates here, dominant, dominant, recessive, compare it to the closest double crossover.

01:55 Dominant, dominant, there should be one that seems out of place.

02:00 Now that one is going to be our middle gene.

02:01 What happens in a double crossover event is we have our middle gene, right? and then we have the genes on either side.

02:07 Genes on either side will exchange their alleles with the sister, chromatid.

02:12 So these will now be new.

02:15 We'll get a different allele from the sister chromatid, but our middle gene stays put and now seems out of place.

02:21 So since c is the gene that seems out of place, that one is probably our middle gene.

02:25 We're compared the others just to be sure.

02:27 The other non -recommonant, recessive, recessive dominant, other double crossover, recessive, recessive, recessive.

02:34 So again, c is different.

02:35 C is out of place...

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