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Have you ever been working on a Punnett Square and you think, all of a sudden, man I really
wish there were more than just these 4 squares? Well if you ever have, today is your lucky
day. And if not, well…prepare to have more squares. Because up until this point we have
focused on one pair of alleles. Hence, monohybrid where mono means one. In guinea pigs, for
example, the trait of having or not having hair is influenced by a pair of alleles (HH,
Hh, or hh). But guinea pigs have more traits than just hair, right? They have lots of alleles.
Well if you perform a cross with two pairs of alleles, that’s called a dihybrid. The
root “di” means two. So, two traits.
One of us has this cat named Moo. Moo, as in, what the cow says. Because he kind of
looks like a cow, and we’re not that creative with names. The thing about Moo is that he
is a cat that loves sinks. You get up in the morning and try to splash water on your face
and ---oh no you think---there’s a cat in my sink. Try to brush your teeth and---on
no, it’s a cat in my sink. You have people over and you’re terrified that they’re
going to flip out because that cat is peeking out of the sink…
Anyway, not all cats like sinks. But Moo does. Around the clock, chances are, he’s in the
sink. We cannot find any information about whether this is a genetic trait. We don’t
know if Moo’s parents liked sinks. But then we realized Moo is not that weird, because
there is a website called catsinsinks.com. Seriously, google it---you’re going to see
TONS of cats in sinks. What if this was a genetic trait? There is no research that we
can find that demonstrates this is genetic, and it’s probably not. Behaviors like that
can be challenging to study. But for practicing dihybrid squares, let’s imagine what it
would be like if it was a genetic trait. And that loving sinks---for cats----is a dominant
trait represented by the allele S and that not loving sinks is a recessive trait with
the allele s. Let’s also take in account that cats---like guinea pigs in our previous
video---typically have hair but they can also be hairless. Having hair will be represented
by the allele H and not having hair would require two recessive h alleles.
So let’s say we want to cross a cat that is heterozygous for the trait of having hair
and also for liking sinks. Heterozygous for both traits would be represented by the genotype
HhSs. Now we want to cross that cat with a hairless cat that does not love sinks. To
be hairless, the cat must be hh. A dominant allele, capital H in this case, would mean
it has hair. And if it does not love sinks…a recessive trait in our example…then it is
ss. A dominant allele, capital S, would mean that it does like sinks. So the second cat
is hhss.
Remember how it looks like in a regular monohybrid Punnett square? Here’s an example of what
it would be like if you were crossing a Hh cat with a hh cat. Remember how you put the
parents on the top and sides like this? Well when you are doing this, those alleles on
the top and sides represent the alleles that would be in the gametes of the parents. Gametes
are sperm cells—if male---and egg cells---if female. And they contain half of the genetic
material as the cat’s body cell. So it makes sense that if there are two alleles---letters---
in the Hh parent, then a gamete would only carry one letter---a H or a h. This is known
as Mendel’s law of segregation. The gametes only carry one allele for a gene.
Well if you have a cat that is HhSs, there are four alleles there. Two genes—one involving
hair and one involving sinks---so if gametes carry only one allele—letter-- per gene
(Mendel’s law of segregation), that means each gamete is going to have two alleles.
In those gametes, with each of them having two alleles (letters), you have to account
for every possible combination. Mendel’s law of independent assortment says that those
alleles are not linked. That means a cat can have hair and love sinks or not have hair
and love sinks---there is no link.
So let’s work out a dihybrid with the parent cross of HhSs x hhss. Step 1---write the parent
cross with your 16 square Punnett square. Step 2---gamete combinations from the parents
are written along the top and side of the Punnett square. But how did we get these letters
along time sides? We like the FOIL method to come up with the gamete combinations. FOIL
stands for FIRST, OUTSIDE, INSIDE, LAST. This is not the only way you can do this---we just
like this way.
So when you FOIL HhSs, you get these gamete combos: HS, Hs, hS, and hs. Place those on the top of
the Punnett square like this. FOIL the other parent hhss, you get these gamete combos:
hs, hs, hs, hs. Place those on the side of the Punnett square like this. Yes, they are
all the same, because notice that was all that parent could contribute as far as alleles.
Remember again---each gamete must have one allele (letter) of each gene. That’s why
you won’t find a gamete with only H’s or only S’s. One allele of each.
Step 3----combine the gametes to see what the offspring prediction will be. For formatting
purposes, because the parents had H’s coming before S’s, we write it that way with the
offspring as well. For formatting, you also put capitals of each letter type before the
lowercase.
So in our example, what is the genotype ratio in the predicted offspring? Remember that
genotypes are the genetic make-ups---the letters that represent the alleles. So 4/16 (25%)
are HhSs, 4/16 (25%) are Hhss, 4/16 (25%) are hhSs, and 4/16 (25%) are hhss. That’s
a 1:1:1:1 ratio.
What about phenotypes? Well half of the cats have hair here and half of the cats don’t.
But with dihybrids, often you are asked about both traits. For example, what chance would
be predicted for a kitten to be born that was like our Moo? Moo has hair and loves sinks.
Well it’s a 4/16---25% chance----that a kitten would be like Moo.
We could write out the ratio 4/16 (25%) have hair/love sinks, 4/16 (25%) have hair, dislike
sinks, 4/16 (25%) hairless/love sinks, and 4/16 hairless/dislike sinks. This is a 1:1:1:1
ratio.
Some big things to remember---in our example, the genotype and phenotype ratios were the
same. This does not always happen. The handout will give you an example of when it is not.
Just remember the steps to setting up the problem correctly, and you will be fine. Also
remember that Punnett squares are predictions. This Punnett square is only predicting the
chances of having offspring with certain genotypes or phenotypes. It’s fascinating really.
Well that’s it for the amoeba sisters and we remind you to stay curious!