Oh no! Twice the genes and sixteen genotypes – a dihybrid cross seems overwhelming to understand. Never fear though, Punnett squares will save the day! When we were studying coat color, we saw how Mendel’s law of segregation and law of independent assortment could help us predict how chromosomes segregate into gametes.
By applying Mendel’s laws to coat color and ear size, we determined that adults heterozygous at two loci could produce four different gametes.
However, these results are puzzling, right? For starters, the cross produced four different phenotypes.
When we were doing this study with only coat color, the cross only got two. Remember that a Punnett square made it a lot easier to keep track of genotypic and phenotypic possibilities, so let’s see if it can help us here.Why don’t you pause the video and get a piece of paper and a pencil so we can try to do this together? Let’s make sure that we use the same abbreviations for the alleles that we’re going to be talking about. Okay, let’s have big B represent the brown coat, and little b will represent the white coat. For ear size, big E will represent small ears, and little e will represent big ears.
We know that we have parents that are heterozygous at two different genes that can produce four different gametes. To start off, let’s write these gametes down. Let’s write the mother’s gametes up here: BE, Be, bE and be. Alright, we also have the father heterozygous for the same two genes down here, so let’s write those same gametes for the father. Now, we need to draw our Punnett Square with all the squares representing the possible progeny.
There are 16 different boxes here in our Punnet, square meaning that there are 16 different genotypes that this cross can produce. Let’s go through and figure out what those genotypes are. If you recall from our monohybrid cross, that’s pretty simple. All we did was take the letters that appear above the square and the letters that appear to the left of the square and write them down in the square.
So, we’ll do that for all our boxes. In our first square here, that’s going to be BBEE, and the next one here is going to be BBEe and so on. Once we have all our genotypes written out, we can begin to assess what kind of data we expect to get from a cross like this.
Now, this is sort of confusing as a bunch of different letters, right? What we really want to know is what sort of phenotypic output all these genotypes are going to produce. All we have to is go through and figure out what phenotype is going to be produced for each genotype. Now, BB is going to produce a brown coat and EE is going to produce little ears, so let’s go through all 16 boxes here and look at what phenotype we could expect to see from these hamsters.
Don’t forget to keep in mind though that the big B allele is dominant over the little b allele, and the big E allele is dominant over the little e allele.Notice that the Punnett square allows us to predict that we’ll get four different phenotypes. We’re going to get brown hamsters with little ears, brown hamsters with big ears, white hamsters with little ears and white hamsters with big ears. The Punnet square also allows us to predict the ratio at which these different phenotypes will occur.Out of 16 different squares here, I have one square that will produce a white hamster with big ears.
I have three squares that predict white hamsters with little ears. There are also three squares here that predict brown hamsters with big ears. The leaves nine squares that predict brown hamsters with little ears. To summarize, our Punnet square predicts a 9:3:3:1 ratio among our four different phenotypes.
This is indicative of a dihybrid cross.A dihybrid cross is simply a cross between two different individuals heterozygous at two different genes. By applying Mendel’s law of inheritance, we helped Adrian to explain the 9:3:3:1 ratio produced by his experiment. This means that the conclusion we drew from our first experiment that a monohybrid cross produces a 3:1 phenotypic ratio is still correct. Furthermore, our second flying hamster experiment helped us understand chromosome segregation even better.
As a side note, notice that producing data that does not fit with your current hypothesis isn’t necessarily bad; in fact, it could be the basis for the next big scientific discovery.
A dihybrid cross is a cross between two individuals that are heterozygous at two different loci. A dihybrid cross produces a 9:3:3:1 ratio among the four different phenotypes.