Trihybrid Cross - Online Tutor, Practice Problems & Exam Prep

Trihybrid Cross

Trihybrid cross is exactly what it sounds like. It's like a dihybrid cross, but instead of 2 traits, you're looking at 3 traits. Now, you can do a trihybrid cross than just a normal one. You know, you're looking at 3 traits, you're interested in the offspring, so you do a Punnett Square range diagram, and you write out all the little alleles, and figure out what the genotypes of the offspring are. But I'm going to be talking to you more and I'm not going to explain that because it's the exact same as that hybrid. You just add another gene. But I'm going to be talking to you about how to use the results from these crosses to map genetic locations. So for unlinked genes, this is going to follow normal Mendelian inheritance, and then you're not going to be able to do any kind of linkage analysis on it. But for linked genes, which are all on the same chromosome, the recombination frequencies of the offspring, so the genotypes and phenotypes, the offspring chromosome. So instead of just like explaining more, I just want to go through an example and I think it'll make it clear. So I do first want to apologize. So this example is with fruit flies. And I'm doing these examples not because I think that they're interesting or necessarily even because I think that they're easy to remember what all the alleles stand for. I actually think that they're horrible and not very easy to understand and remember, but I'm doing this because these are the examples that you're going to find in your book. So the 3 traits aren't something like height, weight, and color. Instead, it's like very odd things with very odd names, but, because you're in genetics, you probably need to get used to these names anyway. So we'll just go through them. So the first trait is eye color, which luckily is kind of easy. It's red or vermilion. So red, you get red-eyed group wise, if you have the wild type allele. We know it's wild type because of the plus. And you get this Vermilion eye color in the mutant allele, which lacks the plus sign. That's how you know it's mutant. And Vermilion is just like this orange-purple color. It's kind of hard to tell unless you study fruit flies. But, we're not going to be looking at the eyes, so I'm just going to give you the genotype. And so therefore, it hopefully will be easy. Then the second trait is going to be wing veins. So these are wings on the flies. They have veins because they're, you know, actual organs and they need blood supplies and things. So the wild type allele, you have this certain vein called a cross vein. The mutant allele, you don't. And then for the 3rd trait, it's going to be wing shape. The wild type allele is going to be normal, and the mutant is going to be mutated, which means that it has like a cut short wing shape. So the cross that we're going to do, it starts with these parentals. So you start with wild type for eye color, homozygous wild type, and mutant for the vein and the wing shape, and then you cross that with the exact opposite. So this is mutant for eye color and wild type for vein and shape. So the phenotypes are going to be red, which is wild type, crossveinless, which is mutant, and cut wings, which is mutant. And this cross will be mutant for the eye color and wild type for the other traits. So you can see that these are vice versa. And the reason you do that is so that you can easily follow the transmission of alleles, and also, so that the F1 will be a heterozygote, because these are the gametes produced. So from the parental, you get 1 wild type allele and 2 mutants, and the other parent, you get 1 mutant and 2 wild types, exactly what I wrote right here. Now, when you cross these, which is what happens, you get f 1, and I've already said that it's heterozygous. Right? You can see this here. You have 1, wild type allele, 1 mutant. The same here and the same here. Now, notice my notation for this. Right? Do you remember what all this stands for? Do you remember what the dot stands for? Right? That means it's unknown if it's linked, and we're hoping find out. And in this case, you remember what the forward slash means? Right. It means it's on different homologous chromosome. So if we were to write this out in a chromosome diagram,

The following table shows data from a cross (ABC x abc) examining three genes (a, b, and c). Calculate the recombination frequency for A and B 

• A. 20%
• B. 32%
• C. 37%
• D. 9.8%

The following table shows data from a cross examining three genes (a, b, and c). Calculate the recombination frequency for A and C 

• A. 32%
• B. 37%
• C. 20%
• D. 9.8%

The following table shows data from a cross examining three genes (a, b, and c). Determine the order of genes 

• A. A B C
• B. B C A
• C. C A B