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So I have a confession to make. For my classroom, I always wanted a hairless guinea pig. They
sell them at some pet stores but they’re so hard to find. And while I doubt they’d
do so well in the wild (suggest picture of freezing guinea pig which is problem they
have), their genetics are fascinating. And so are guinea pigs. We spend quite some time
on LOLCats but we’d spend even more time if someone would make a LOLguinea pigs.
Most guinea pigs have hair. This is coded for in their DNA. DNA makes up genes, and
guinea pigs receive genes from both their mother and their father. An allele is a form
of a gene and often represented by a letter, in this case, we’ll use the letter h for
hair. A hairless guinea pig has two recessive alleles for the trait of having or not having
hair. A recessive allele is usually represented by a lowercase allele. By recessive, it means
that the allele will not usually show up ---the only way it will show up is if there is no
dominant allele present. A dominant allele is represented by a capital letter and is
an allele that will show up. Think of it as---dominating.
So a hairless guinea pig does not have a dominant gene for hair and that’s why the recessive
trait---not having hair----shows up. A hairless guinea pig’s genotype---that’s the genetic
makeup of an organism---can be represented as lowercase hh. A guinea pig that does have
hair can be represented as HH or Hh. It only takes one dominant allele (“H”) for a
trait to show up. That dominating allele means the recessive is hidden.
We’re not quite done with the vocab here. A genotype of HH or hh is considered homozygous.
The root in this word (please underline “homo”) means “same” and they are the same case.
HH are both capital and hh are both lowercase. HH is homozygous dominant, because of the
capitals. And hh is homozygous recessive because of the lowercase. A genotype of Hh is considered
heterozygous. The root in this word (please underline “hetero”) means “different”
so they are different cases. One is capital and one is lowercase.
If I were to show you a guinea pig with hair, would you know its genotype? Well you would
know it’s not hh because then it would be hairless. But remember, a guinea pig with
hair could be HH or Hh. We don’t really know. We could do a test cross to eventually
determine this (the handout will be all about this) but you can’t tell by just looking
at it. Now if we had a hairless guinea pig, we do know it’s hh. Because if it had a
capital letter---even one---that DOMINATING allele would take over and it would have hair.
So let’s try a monohybrid cross. The root “mono” means 1 because it focuses on one
trait—in this case hair--and that means a Punnett square would need to be created
like this with 4 squares like this. In our cross, we are going to cross two heterozygous
So first step---(Step 1)---figure out the genotypes of the parents. Heterozygous means
Hh. (Step 2) Place one parent along the top of the punnett square, outside of the boxes
like this. Place the other parent along the left of the square, outside of the boxes like
this. (Step 3) Cross them! Like this. For formatting purposes, we always put the capital
letter first. The results you get in the squares would be the offspring---the babies.
Now if I were to ask you the genotypes of the babies, you could list them out: 1 HH,
2 Hh, and 1 hh. I could even turn that into a genotype ratio: 1HH: 2 Hh: 1 hh. Or a percentage
25% HH, 50%Hh, and 25% hh.
What if I asked the phenotypes? What does that mean? I like to think that “pheno”
sounds like “physical” so the phenotype would be the physical traits of that organism.
And in this example, it’s whether they have hair or not. Remember any babies that have
a capital “H” have a DOMINATING allele, and they will have hair. So the babies that
are HH or Hh---they have hair! So 3 of them---the 1 HH baby and the 2 Hh babies---all have hair.
The hh baby has no dominant allele present so this little guy with be hairless. You could
say the phenotype ratio is 3 have hair: 1 hairless or a phenotype percentage could be
75% hair, 25% hairless.
One thing to emphasize about Punnett squares is that they are predictions. These are probabilities.
This means that it’s not necessarily exactly what you are going to get. For example, it’s
a probability that a child has a 50/50 chance of being born a boy or girl but we all know
a family that only has children that are girls or a family that only has children that are
boys. Probabilities are predictions. Another reason to love biology---it’s exciting.
That’s it for the Amoeba Sisters and we remind you to stay curious!