The following progeny are obtained from a test cross of ...

Question

The following progeny are obtained from a test cross of a trihybrid wild-type plant to a plant with the recessive phenotypes compound leaves (c), intercalary leaflets (i), and green fruits (g). (Traits not listed are wild type.) The test-cross progeny are as follows:

Phenotype Number
Compound leaves 324
Compound leaves, intercalary leaflets 32
Compound leaves, green fruits 5
Compound leaves, intercalary leaflets, green fruits 51
Intercalary leaflets 3
Intercalary leaflets, green fruits 309
Green fruits 42
Wild type 49
815

How many double-crossover progeny are expected among the test-cross progeny? Calculate the interference for this cross.

Accepted Answer

Hi, everyone. Welcome back. Let's look at our next problem. It says after crossing organisms with big A big A big B, big B, big C, big C and little, a little, a little B, little B, little C, little C genotypes. We obtain offspring from F one progeny that are heterozygous for all traits. Big a little A big B, little B, big C little C, We then cross the F1 tri hybrid with another organism that is homozygous recessive for the three traits and obtain offspring that have the following genotypes shorthand. Now, these genotypes are described as shorthand because it only lists a single allele for each gene A B and C and it's the all that are inherited from the heterozygous parent. This is because since the second pair is, is homozygous recessive for all three traits, the alley from that parent will always be little, a little B, little C. So rather than write that down for every single offspring, since we know they'll all have those recessive alls as their second set, we just look at the, all that differ inherited from the hetero parent. So we have this table with these shorthand genotypes. And it says big A big B big C 420 little A little B little C 389. Big A Little B Big C93 Little A Big B Little C 82. Big A big B little C 62 little A little B big C 68 little A big B big C 12 and big A little B little C seven. Then our question says which of the following shorthand genotypes are considered double crossovers. And we have the answer choices. A big A big B big C and little A little B little C. Choice B big A little B big C and little A big B little C choice C big A big B little C and little A little B big C and then choice D little A big B big C and big A little B little C. So when we want to look for double crossovers and we have a list of the numbers of offspring in each genotype, there's actually a pretty straightforward way to go about it. We don't even have to look at crossover diagrams or anything like that because double crossovers will be the least frequently occurring. Um gamete, they would represent the lowest number of offspring since that will be less likely to have two crossovers occur than just no crossovers, the parental genotype or one crossover. So we can look at our table here and just select it's listed in order of the number of offspring. So the last two rows here, little A big B, big C with 12, big A little B, little C with seven. And we can just go down to our answer choices. And we see that choice D represents those two things and we have our answer. However, just a little side note here because you might be asked this question in a slightly different way. Um If we look at that and know that that has two crossovers, we can tell from that, that our genes are not in the order ABC in terms of their physical location on the chromosome. So they cannot be in the order ABC. If we had assumed they were then a double crossover. In that order, you would have big, a big, big B big C from one parent, little, a little B, little C from the other parent, which we know for sure because we're told in our problem that the F one generation was a result of a cross between an individual that was homozygous dominant for all three. And another parent who was homozygous recessive for all three. And if it were in that order, a double crossover would have resulted in the genotypes. Big A little B big C, the middle alley is swapped in a double crossover and then little A big B little C. But when we look at the number of offspring, we see that those aren't the lowest numbers and therefore, that can't be the double crossover genotype. But you can see looking at this, that the result of a double crossover is that middle alley is swapped. And the 1st and 3rd all remain the same as in the parental genotype. So when we look at our two genotypes that we know are double crossovers, we see that B and C remain the same or remain the parental genotypes while the A has been swapped. So we know that the order of the genes must be B AC. So I've drawn big A big B, big B big A big C and then if I put underneath it, little B little A little C and if I show two crossovers, XS in between B and A and A and C, that would give me those two genotypes if double crossovers occurred. So we didn't need that for this problem, but it's just sort of an important thing or if you win about the problem trying to list out the genes and you automatically wrote them ABC. Our problem didn't tell us they were in that order. And in this case, our genotype numbers tell us that it couldn't have been. So just a little side note for these types of problems if you encounter it in a slightly different format in the future. So again, our answer for which of the following shorthand genotypes are considered double crossovers. This choice D little A big B big C and big, a little b, little C see you in the next video.

Key Concepts

Test Cross

Crossover and Double Crossover

Interference

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