How to Solve a Punnett Square

1. Determine the genotypes (letters) of the parents. Bb x Bb
2. Set up the Punnett square with one parent on each side.
3. Fill out the Punnett square middle
4. Analyze the number of offspring of each type.

In pea plants, round seeds are dominant to wrinkled.

The genotypes and phenotypes are:

RR = round
Rr = round
rr = wrinkled

If a heteroyzous round seed is crossed with itself (Rr x Rr) a punnett square can help you figure out the ratios of the offspring.


3/4 round, 1/4 wrinkled

Genetics Word Problems - what if you aren't given the genotypes?


Example: In horse, black coats are recessive to brown. If a heterozygous brown horse is crossed with a black horse, what are the expected phenotypes?

First of all, this problem doesn't make it easy. Start by assigning genotypes and phenotypes. It doesn't matter what letter you pick, but it may be easiest to pick a letter that represents the dominant trait. In this case, use the letter B for brown.

BB = brown
Bb = brown
bb = black

Now use the key to figure out your parents. . Once you've figured that out, the cross is simple!

Bb x bb

Punnett square

Practice Genetics

Simple Genetics Practice – using mendelian genetics and Punnett squares

Peas, Please – practice setting up squares for basic Mendelian traits in pea plants

Bunny Genetic Crosses with two traits – basic crosses, uses Punnett squares

Practice Punnett Squares with Skinny Pigs – hairless guinea pigs are the result of a recessive gene (Key, TpT)