Dihybrid crosses and Punnett squares with two traits

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Have you ever been working on a Punnett Square and you think, all of a sudden, man I really

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wish there were more than just these 4 squares? Well if you ever have, today is your lucky

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day. And if not, well…prepare to have more squares. Because up until this point we have

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focused on one pair of alleles. Hence, monohybrid w

nced by a pair of alleles (HH,

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Hh, or hh). But guinea pigs have more traits than just hair, right? They have lots of alleles.

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Well if you perform a cross with two pairs of alleles, that’s called a dihybrid. The

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root “di” means two. So, two traits.

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One of us has this cat named Moo. Moo, as in, what the cow says. Because he kind of

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looks like a cow, and we’re not that creative with names. The thing about Moo is that he

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is a cat that loves sinks. You get up in the morning and try to splash water on your face

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and ---oh no you think---there’s a cat in my sink. Try to brush your teeth and---on

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no, it’s a cat in my sink. You have people over and you’re terrified that they’re

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going to flip out because that cat is peeking out of the sink…

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Anyway, not all cats like sinks. But Moo does. Around the clock, chances are, he’s in the

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sink. We cannot find any information about whether this is a genetic trait. We don’t

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know if Moo’s parents liked sinks. But then we realized Moo is not that weird, because

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there is a website called cats in sinks... seriously, google it---you’re going to see

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TONS of cats in sinks. What if this was a genetic trait? There is no research that we

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can find that demonstrates this is genetic, and it’s probably not. Behaviors like that

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can be challenging to study. But for practicing dihybrid squares, let’s imagine what it

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would be like if it was a genetic trait. And that loving sinks---for cats----is a dominant

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trait represented by the allele S and that not loving sinks is a recessive trait with

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the allele s. Let’s also take in account that cats---like guinea pigs in our previous

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video---typically have hair but they can also be hairless. Having hair will be represented

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by the allele H and not having hair would require two recessive h alleles.

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So let’s say we want to cross a cat that is heterozygous for the trait of having hair

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and also for liking sinks. Heterozygous for both traits would be represented by the genotype

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HhSs. Now we want to cross that cat with a hairless cat that does not love sinks. To

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be hairless, the cat must be hh. A dominant allele, capital H in this case, would mean

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it has hair. And if it does not love sinks…a recessive trait in our example…then it is

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ss. A dominant allele, capital S, would mean that it does like sinks. So the second cat

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is hhss.

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Remember how it looks like in a regular monohybrid Punnett square? Here’s an example of what

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it would be like if you were crossing a Hh cat with a hh cat. Remember how you put the

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parents on the top and sides like this? Well when you are doing this, those alleles on

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the top and sides represent the alleles that would be in the gametes of the parents. Gametes

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are sperm cells—if male---and egg cells---if female. And they contain half of the genetic

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material as the cat’s body cell. So it makes sense that if there are two alleles---letters---

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in the Hh parent, then a gamete would only carry one letter---a H or a h. This is known

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as Mendel’s law of segregation. The gametes only carry one allele for a gene.

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Well if you have a cat that is HhSs, there are four alleles there. Two genes—one involving

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hair and one involving sinks---so if gametes carry only one allele—letter-- per gene

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(Mendel’s law of segregation), that means each gamete is going to have two alleles.

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In those gametes, with each of them having two alleles (letters), you have to account

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for every possible combination. Mendel’s law of independent assortment says that those

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alleles are not linked. That means a cat can have hair and love sinks or not have hair

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and love sinks---there is no link.

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So let’s work out a dihybrid with the parent cross of HhSs x hhss. Step 1---write the parent

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cross with your 16 square Punnett square. Step 2---gamete combinations from the parents

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are written along the top and side of the Punnett square. But how did we get these letters

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along time sides? We like the FOIL method to come up with the gamete combinations. FOIL

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stands for FIRST, OUTSIDE, INSIDE, LAST. This is not the only way you can do this---we just

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like this way.

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So when you FOIL HhSs, you get these gamete combos: HS, Hs, hS, and hs. Place those on the top of

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the Punnett square like this. FOIL the other parent hhss, you get these gamete combos:

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hs, hs, hs, hs. Place those on the side of the Punnett square like this. Yes, they are

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all the same, because notice that was all that parent could contribute as far as alleles.

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Remember again---each gamete must have one allele (letter) of each gene. That’s why

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you won’t find a gamete with only H’s or only S’s. One allele of each.

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Step 3----combine the gametes to see what the offspring prediction will be. For formatting

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purposes, because the parents had H’s coming before S’s, we write it that way with the

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offspring as well. For formatting, you also put capitals of each letter type before the

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lowercase.

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So in our example, what is the genotype ratio in the predicted offspring? Remember that

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genotypes are the genetic make-ups---the letters that represent the alleles. So 4/16 (25%)

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are HhSs, 4/16 (25%) are Hhss, 4/16 (25%) are hhSs, and 4/16 (25%) are hhss. That’s

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a 1:1:1:1 ratio.

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What about phenotypes? Well half of the cats have hair here and half of the cats don’t.

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But with dihybrids, often you are asked about both traits. For example, what chance would

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be predicted for a kitten to be born that was like our Moo? Moo has hair and loves sinks.

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Well it’s a 4/16---25% chance----that a kitten would be like Moo.

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We could write out the ratio 4/16 (25%) have hair/love sinks, 4/16 (25%) have hair, dislike

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sinks, 4/16 (25%) hairless/love sinks, and 4/16 hairless/dislike sinks. This is a 1:1:1:1

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ratio.

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Some big things to remember---in our example, the genotype and phenotype ratios were the

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same. This does not always happen. The handout will give you an example of when it is not.

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Just remember the steps to setting up the problem correctly, and you will be fine. Also

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remember that Punnett squares are predictions. This Punnett square is only predicting the

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chances of having offspring with certain genotypes or phenotypes. It’s fascinating really.

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Well that’s it for the amoeba sisters and we remind you to stay curious!

here mono means one. In guinea pigs, for

00:32example, the trait of having or not having hair is influe