Trihybrid Cross: Study with Video Lessons, Practice Problems & Examples

A trihybrid cross examines the inheritance of three traits simultaneously, utilizing a Punnett Square to determine offspring genotypes. For linked genes on the same chromosome, recombination frequencies reveal genetic distances, essential for mapping. By analyzing parental and recombinant gametes, students can calculate recombination frequencies using the formula: $(r_{\mathrm{f}})$ = $\mathrm{number\; of\; recombinant\; offspring}/\mathrm{total\; offspring}$. Understanding these concepts is crucial for genetic mapping and analysis.

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

Here’s what students ask on this topic:

What is a trihybrid cross and how is it different from a dihybrid cross?

A trihybrid cross examines the inheritance of three traits simultaneously, whereas a dihybrid cross looks at two traits. In a trihybrid cross, you use a Punnett Square to determine the genotypes of offspring for three different genes. This involves more complex calculations and a larger Punnett Square compared to a dihybrid cross. The principles of Mendelian inheritance still apply, but the analysis includes more combinations of alleles, making it more intricate. Understanding trihybrid crosses is essential for genetic mapping and analyzing linked genes.

How do you calculate recombination frequencies in a trihybrid cross?

To calculate recombination frequencies in a trihybrid cross, use the formula:

$\mathrm{rf}=\frac{\mathrm{number} \mathrm{of} \mathrm{recombinant} \mathrm{offspring}}{\mathrm{total} \mathrm{offspring}}$

First, identify the parental and recombinant gametes. Count the number of recombinant offspring for each gene pair and divide by the total number of offspring. This gives you the recombination frequency, which can be used to determine genetic distances and map the genes on a chromosome.

What is the significance of recombination frequencies being less than 50% in a trihybrid cross?

Recombination frequencies less than 50% indicate that the genes are linked and located on the same chromosome. This is because linked genes tend to be inherited together, resulting in fewer recombinant offspring. A recombination frequency of 50% or more suggests that the genes are unlinked and assort independently, following Mendel's law of independent assortment. Understanding these frequencies helps in mapping the genetic distances between genes and determining their relative positions on a chromosome.

How do you determine the order of genes in a trihybrid cross?

To determine the order of genes in a trihybrid cross, first calculate the recombination frequencies between each pair of genes. The gene pair with the highest recombination frequency is placed farthest apart on the chromosome map. The third gene is then placed based on its recombination frequencies with the other two genes. The gene with the lowest recombination frequency is closest to the middle gene. This method helps in accurately mapping the relative positions of the genes on the chromosome.

What is the role of double crossovers in a trihybrid cross?

Double crossovers occur when two separate crossover events happen between three genes. They can complicate the calculation of recombination frequencies because they may not be immediately apparent. To account for double crossovers, you need to identify and count them separately, often by looking at the lowest frequency recombinant types. Double crossovers provide more accurate genetic distances and help refine the gene map, ensuring that the calculated distances between genes reflect the true genetic linkage.