Punnett squares are used to help you determine what genes the child of two parents will have. Everyone has 2 copies of a certain gene (called an allele): 1 copy comes from your mom and 1 copy comes from your dad. But since your mom and dad each have 2 copies, how do you know which ones you will get?
Scientists often use letters to look at genes. Let’s look at eye color and call the alleles here B or b (each one is a different allele, but both are part of the same gene). If your 2 copies are B and B (we’ll write it BB), you will have brown eyes. If you have b and b (bb), you will have blue eyes. If you have a mix of the two (Bb) you will still have brown eyes (this is because B is dominant and b is recessive — we can go over that later if you would like!).
Let’s say your mom has brown eyes (BB) and your dad has blue eyes (bb). What color eyes might you have? We can draw a Punnett square which is just a box with four tiles. We’ll put each allele for your dad at the top (b and b) and each allele for your mom on the left side (B and B). Then for each of the 4 tiles, we look at the top and side letter and combine them. It would look like this:
So you could only have brown eyes! But what if your mom had brown eyes because she had Bb? That would look like this:
So you could have either blue or brown eyes. Because the Punnett square is made up of 4 squares and 2 of them are blue and 2 of them are brown, this means you have a 50% chance of having blue or brown eyes. What would be your chance of having blue eyes if only one square was blue? What kind of alleles would your parents have to have to make only 1 blue square?
Here your parents could both have brown eyes, but you would have a 25% chance of having blue eyes! Try it out yourself — what color eyes do your parents have? Fill in a Punnett square and see what color eyes you might have had.
What if you have green eyes? Then it gets a bit trickier. Most traits (like eye color) aren’t controlled by just one gene. Instead, there might be two or more genes that work together to produce eye color. If you have green eyes, it’s probably because you have bb and at another gene (we’ll use the letter G), you will have GG. So the combination would look like bbGG. If you have anything else besides bb, you won’t be able to get green eyes. So you could have BbGG and still have brown eyes. Punnett squares for more than one gene get to be a bit complicated. Take a look: