Hi! My name is Mary Poffenroth. I'm an adjunct professor of biology,

and today we're going to have a little fun with Mendelian Punnett squares.

Now, before we started and jump on in to our Punnett square problems,

let's set down some ground rules. First, I'm going to talk about alleles.

Alleles are going to be like different flavours of genes.

So, you go into the ice cream shop, there's 31 flavours of ice cream. Even though they're all different flavours, they're all still ice cream, right?

Alleles are going to be those different flavours of genes that you have available to you.

The next thing I want to talk about is dominant and recessive.

In any relationship, some of those alleles are going to be dominant, and some of those alleles are going to be recessive.

Those dominant alleles, if they're present, they're going to have more of a say in what the outcome of that cross is going to be,

and the recessives, well, they're going to have little to no say unless there are two recessives that are available.

Also, in each of our crosses, we're going to have one allele from our mother, and one allele from our father.

Now, let's jump into our crosses.

Today, we're going to do a four-square Punnett square.

So, you want to start out by just drawing a square. In that square, you're going to draw a line right down the middle from the top to the bottom, and a line from left to right.

Now, you have four individual squares. Each of those squares is going to be a probability,

and each of those squares is one fourth, right? Or 25%.

Next, we're going to put our parents' alleles on the Punnett square.

Now, it doesn't really matter if you put male or female on the top, but generally people put male on top, female to the left.

For our problem today, we're going to use eye colour.

The eye colour alleles are going to be for our father, are going be brown phenotypically-expressed alleles.

That means that that father has brown eyes, but his genotype, or the genes at play, his alleles are going to be big B, little b.

That means that he's heterozygous, or he has two different alleles, for that trait of eye colour.

It's a little important to note at this time that all of these letters are completely arbitrary.

You can make up any letters you want. However, in any relationship, a capital letter, or a big letter, like, say, big B, is going to be for the dominant,

and a little letter, or a lowercase letter, in this case, little b, is going to be for the recessive.

OK, so we have the father, big B, little B. He's heterozygous, meaning he has two different alleles. What about the mom?

The mom has blue eyes, so phenotypically, her expression that you see on the outside is blue eyes.

Her genotype, or the genes at play, are going to be little b - little b, or homozygous recessive.

All right, it's time to do our cross.

Now, we're going to take one from the top, and one from the left, and we're going to bring them down into those squares.

So we have on our top, big B-little b, then bring another big B-little b down. In our right square at the top, we have little b-little b, and our bottom right square we have little b-little b.

OK, what does that mean for the probability of getting an offspring with what kind of eye colour?

Now remember, each of those squares is going to be 25%.

So, we have a 25 plus 25, or 50% chance of getting an offspring with brown eyes,

and we have a 25 plus 25, or 50% chance of getting an offspring with blue eyes.

And for a fun little extra problem when you're sitting around the dinner table, take the eye colours or the phenotypes of your parents.

Even though you won't know the exact genotypes, or the genetic makeup, behind what you're seeing on the outside,

do some crosses, and see if you can find out the probability that you could have actually had a different eye colour.

Thanks for watching, and if you want to learn more about this subject, click on the link below, or if you want to learn more biology, feel free to click on any links around me.

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Thanks for learning!