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).
When placing the letters for the genes on the outside of the Punnett square, the positioning of male and female does not really matter, as long as it is consistent.
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.
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:
This is a case where all possible offspring genotypes and phenotypes could be shown. The genotypes are 25% BB, 50% Bb, and 25% bb. The phenotypes are 75% brown and 25% blue. This means that when parents of these genotypes have a child, there is a 25% chance it will have blue eyes. Again, remember this is not offspring 1, offspring 2 working around the Punnett square. Each of these genotypes could happen anytime an offspring is produced.
This example answers the original question posed at the opening of this lesson. How is it possible for two brown-eyed parents to have a blue-eyed child? The answer is if they are both heterozygous for eye color!
Different Types of Crosses
So far, we have been examining the simplest type of Punnett square, known as a monohybrid cross. A monohybrid cross is a cross when only one trait is concerned. Think back to the Punnett squares we did for eye color. There was one trait being passed on and each parent could pass on one of two genes, which we placed around the square.
There are also times when two or more traits are passed to the offspring at the same time. These crosses are done the exact same way, but they take a little longer. Also, the number of boxes in the Punnett square needs to be adjusted to handle the additional genes.
For two traits, called a dihybrid cross, the Punnett square has 16 boxes and each parent passes on two traits. In a trihybrid cross, three traits are passed on from each parent and the Punnett square would have 64 boxes, 8 by 8.
The genotype is the genetic makeup of an organism and the phenotype is how the genes are expressed, or what it looks like. In order to know what the phenotype will be, you must know that dominant genes are the ones that are shown and recessive genes are the ones that are hidden by the dominant genes.
We call it homozygous dominant when both genes are for the dominant trait, and we call it heterozygous when one of the genes is dominant and one of the genes is recessive. In both cases, the phenotype will be whatever the dominant gene is. In order for a recessive trait to show, it would have to be homozygous recessive, meaning both genes are recessive.
Punnett squares can help plant and animal breeders know what potential offspring will look like. They are also useful when trying to predict the pattern of inheritance of certain diseases. Knowing if one is a carrier of a disease or will actually express the condition helps medical science to come up with treatments.
You should feel self-assured after finishing this lesson to:
- Define Punnett square
- Contrast genotype and phenotype
- Recall the differences between dominant and recessive genes
- Identify the variety in homozygous dominant, heterozygous and homozygous recessive
- Show how to use the Punnett square to figure out possible genetic outcomes