Punnett Square: Definition & Example

Instructor: Jeffrey Sack

Jeff is a Biology teacher and has a Doctorate in Educational Leadership

This lesson will define what a Punnett square is and show several examples of how one can be used. It will also briefly describe patterns of inheritance.

What Are Punnett Squares?

Two parents with brown eyes have a child with blue eyes. How is this possible? When studying genetics, it's important to remember that there are all kinds of variations in the ways genes express themselves. It's often one gene or a combination of genes that give an organism a particular trait. Since there are so many possibilities when the genes of two parents combine, it is helpful to use a tool called a Punnett square. A Punnett square is a graphical way of determining all the possible genetic outcomes when a cross is performed. In essence, it is a probability box that shows the odds of each possible trait happening.

When traits are expressed by an organism, they result from two copies of a gene: one from the mother and one from the father. Genes can appear in one of two forms: dominant or recessive. 

The recessive form will be masked if a dominant form is present. Since each parent may have a different combination of the genes, three possible conditions may occur:

BB is called homozygous dominant. Both genes are the same and dominant. The dominant trait is expressed.

Bb is called heterozygous. The organism has a dominant gene and a recessive gene. The dominant trait usually masks the recessive, so the dominant trait is expressed.

bb is called homozygous recessive. Both genes are the same and recessive. The recessive trait is expressed.

When offspring are produced, parents can pass on either of their genes. This is why a Punnett square is a helpful tool.

Making a Punnett Square

To determine the possible genetic combinations of the offspring, the first thing that must be done is to identify the trait that is to be studied. Once this is done, the trait should be assigned a letter to represent it. It's always best to stay away from letters where the capital and the lowercase look similar, like with S and W.

For example, let's say you want to determine the possible outcomes of a cross for eye color. Use the capital letter 'B' to represent brown, as more people have brown eyes than blue, which will be shown by a lowercase 'b':

B is brown

b is blue

The next thing to do is draw the square. Since you are only investigating one trait and each trait has two genes, then the Punnett square should have four boxes. It should look like this:

Once this is complete, place each parent's genes on the outside of each square. For this example, start with a cross between two parents who are both homozygous dominant (or capital B, capital B).

Then, just like in math, cross-multiply. You do this by putting the first letter of the vertical parent in the first box. Then you add to that the first letter of the second parent. Then you fill in the next box with the first letter of the vertical parent with the second letter of the horizontal parent. Then you do the same thing with the second letter of the vertical parent. When you are finished, there should be two letters (or genes) in each box.

Once you have filled in all the boxes, it's time to look at what the possible outcomes are.

Genotypes and Phenotypes

The two things a Punnett square can tell you are the genotypes and phenotypes of the offspring. A genotype is the genetic makeup of the organism. This is shown by the three genetic conditions described earlier (BB, Bb, bb). The phenotype is the trait those genes express. Eye color, hair color, pod shape, and flower position are all examples of phenotypes.

In this example, it asked you to do a cross between two parents who were homozygous dominant for eye color. Looking at the possible offspring, each box (or possible offspring) has two copies of the dominant gene. This means there is a 100% chance of the offspring having brown eyes, or being BB.

It's important to note here that each box represents a possible offspring. It does not follow the pattern of offspring 1, offspring 2, etc. Every time these parents have a child, they have a 25% chance for each genotype or, in this case, 100% BB.

Examples

Let's do another example. This time, do a cross between a parent who is homozygous dominant for brown eyes and one who has blue eyes. Remember that brown is dominant to blue, so the parents genotypes will be BB crossed with bb. Once you draw the Punnett square and do the cross, it should look like this:

Just like in the last example, each parent only has one type of gene to pass on. However, in this case, one parent can pass on a dominant gene, while the other parent can only pass on a recessive. The genotypes for this cross would be 100% Bb and the phenotypes would be 100% brown. Even though each possible offspring is carrying a recessive gene, it is masked by the dominant one.

Here is one more example to get a hang of how the Punnett square works to predict genetic probabilities. In this cross, two heterozygous parents are crossed. Remember, to be heterozygous means an organism has both a dominant and a recessive gene. Usually, the dominant gene masks the expression of the recessive one. Once the Punnett square has been filled out, the results look like this: