What is a Punnett Square?
Punnett squares are grids used to determine the probability of offspring receiving a certain trait.
A blank Punnett square looks like this:
A blank Punnett square looks like this:
On top of the square go one parent's genes. On the side goes the other parent's genes.
After you've set up the square, you start filling it in. In the upper left square, there would be two T's because there are two big T's on either side. In the upper right, there would be a little t and a big T because those are the genes on either side.
Let's say we're talking about butterflies. The dominant trait R will represent the gene for red wings. The recessive trait r will represent the gene for blue wings. You have two parent butterflies, both with the genotype Rr. You want to find the probability of the offspring having red or blue wings. You can make a Punnett square, or you can just wing it. Feel free to pun-it like we do.
The completed Punnett square looks like this:
After you've set up the square, you start filling it in. In the upper left square, there would be two T's because there are two big T's on either side. In the upper right, there would be a little t and a big T because those are the genes on either side.
Let's say we're talking about butterflies. The dominant trait R will represent the gene for red wings. The recessive trait r will represent the gene for blue wings. You have two parent butterflies, both with the genotype Rr. You want to find the probability of the offspring having red or blue wings. You can make a Punnett square, or you can just wing it. Feel free to pun-it like we do.
The completed Punnett square looks like this:
What does it all mean?
After you fill in the square, you have all of the possibilities of inheritance. In the square above you have: RR, Rr, Rr, and rr. The capital letter is the dominant trait, so any square with at least one big R receives red wings. A square must have two little r's to receive the recessive trait, blue wings. In this example, the offspring have a 3/4 chance of getting red wings and a 1/4 chance of getting blue wings. In this Punnett square, there is a 75% chance that the offspring will have red wings. There is a 25% chance that the offspring will have blue wings.
Bigger Punnett Squares
Punnett squares aren't always used for only one trait at a time. They can look like this!
Or this!
Some of them have more pairs of genes than your bedroom closet.
For a Punnett square with two traits, you need to have four combinations of the two genes (for different traits) given by each parent on the sides. Each combination will be there once for a 1/4 chance, twice for a 1/2 chance, three times for a 3/4 chance, and four times for a total chance, so there will be four written combinations for each parent.
Let's take a look at this one again.
For a Punnett square with two traits, you need to have four combinations of the two genes (for different traits) given by each parent on the sides. Each combination will be there once for a 1/4 chance, twice for a 1/2 chance, three times for a 3/4 chance, and four times for a total chance, so there will be four written combinations for each parent.
Let's take a look at this one again.
The parents both have the genotype RrTt. Filling out the big Punnett square is basically the same as it is for the little Punnett square. Each box should have two genes for each trait. Let's say that R is red wings, r is blue wings, T is tall wings, and t is tall wings. There is a 9/16 of offspring having red, tall wings, a 3/16 chance of them having red, short wings, a 3/16 chance of them having blue, tall wings, and a 1/16 chance of the offspring having blue, wide wings.