Mendel's Dihybrid Cross Example: Practice & Ratio

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  • 0:02 Coat Color and Ear Size
  • 0:43 Predicting Genotype…
  • 4:27 Predicting Phenotypic Ratios
  • 5:24 Lesson Summary
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Lesson Transcript
Instructor: Greg Chin
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.

The 16 genotypes this cross can produce
Punnett Square Genotypes

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.

The possible phenotypes resulting from this cross
Punnett Square and Phenotypes

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