Punnett Squares — Examples & Diagrams

What Are Punnett Squares?

They are a tool used to figure out the possible allele combinations of a genetic cross. A genetic cross is a fancy way of talking about two parents' genes combining with each other when they create offspring.

Alleles, Genes, and Genetic Crosses

As a reminder, there are two types of allele combinations possible. The first is homozygous, which means two copies of the same allele, like BB for two dominant alleles or bb for two recessive alleles. The other possible combination is heterozygous, which means one of each type of allele, like Bb for one dominant and one recessive allele.

Building A Punnett Square

The first step is to draw a square. We pick a parent—it doesn't matter which one—and write its alleles on the top of the square. We then pick the other parent and write its alleles on the left side of the square.

While drawing this, we have to be careful with the dominant and recessive alleles. Capital letters are typically used for the dominant trait, while lowercase letters are used for recessive traits. If you see a lowercase and an uppercase letter together, the uppercase letter (the dominant one) is the one that will be expressed.

In the example below, yellow coloring is the dominant phenotype for pea plants. Any offspring that inherit the allele for yellow coloring, represented by a capital Y, will have that coloring.

If we want to predict the genotype of future generations, we can just keep making more squares.

In this section, we talked about monohybrid crosses. These crosses focus only on a single trait. If you want to know how two different traits, say plant color and height, are passed down, you will have to construct a dihybrid cross.

$$\color{#8925ae}{\text{Vocabulary Review:}}$$

$\color{#8925ae}{\text{Chromosome}}$ : A chromosome is a long, single, coiled piece of DNA

$\color{#8925ae}{\text{Trait}}$ : A specific characteristic that is different from one individual to another

$\color{#8925ae}{\text{Gene}}$ : A segment of DNA that codes for a specific trait or protein

$\color{#8925ae}{\text{Allele}}$ : Genes of the same type that contain different kinds of information. Alleles can be dominant or recessive

$\color{#8925ae}{\text{Dominant}}$ : The stronger of two expressed genes

$\color{#8925ae}{\text{Recessive}}$ : The weaker of two expressed genes. Only shown if a gene is homozygous recessive

$\color{#8925ae}{\text{Heterozygous}}$ : Two different kinds of allele, one dominant and one recessive

$\color{#8925ae}{\text{Homozygous}}$ : Two of the same type of allele, both dominant or both recessive

$\color{#8925ae}{\text{Punnett square}}$ : A method of determining the probabilities of which traits the offspring of two organisms will inherit

How to Read a Punnett Square

A Punnett square is made to show inheritance patterns in a genetic cross. But you have to know how to read one to understand one. So, let's cover what you need to know.

The Parts of a Punnett Square

Parents

There are many different ways to represent the parents in these diagrams. In this example, the parents are shown as cute little pictures in the upper left corner.

You can see that both parents have the long-beaked phenotype.

We need to know the parents’ phenotypes and genotypes because these tell us what traits the offspring will have and what alleles they will inherit, respectively.

Punnett squares may all show the parent phenotypes differently, but genotypes are always laid out the same way. One parent’s alleles are given in a vertical column, while the other parent’s are given in a horizontal row. Each allele gets its own box because each gamete only gets one allele from each parent. That’s why children don’t look exactly like either parent.

In our bird beak length example, you can see that each parent has a heterozygous genotype.

Offspring

The offspring are the results of the genetic cross between the parents. Just like in a multiplication table, the offspring go in the middle of the table.

Try thinking of it this way: Just like $2 \times 2 = 4$, ${\text{parent}} \times {\text{parent}} = {\text{offspring}}$.

Inheritance Patterns

In our specific example we have two parents with genotype Bb. We know that the alleles separate and then one from each parent gets put together during sexual reproduction to form a baby. So, there are three possible combinations of alleles that the baby bird can have.

${\text{B}} \times {\text{B}} = {\text{BB}}$

${\text{B}} \times {\text{b}} = {\text{Bb}}$ (This genotype is twice as likely for our baby birds than each of the other two possibilities.)

${\text{b}} \times {\text{b}} = {\text{bb}}$

What Does This Table Actually Show Me?

We said earlier that Punnett squares show inheritance patterns for a particular genetic cross. What does that mean? An inheritance pattern is the way that a trait gets passed down from generation to generation. By using diagrams like this, we see how likely it is that a baby will inherit a trait based on the genetics of its parents.

It's also a neat way to illustrate how genotypes and phenotypes are related.

What Is a Punnett Square?

A Punnett square is a method used to predict the outcomes of certain crosses between organisms. This method uses the genotypes of two parent organisms to create a visual for typical Mendelian genetics.

As seen below, a Punnett square puts the genotypes for mother and father on the top and side of the square and creates barriers between each parent's allele. The picture below is an example of a square for eye color.

This diagram allows us to see the possibles outcome of a cross and the predicted percentage of phenotypes. In each individual square, an allele from the side and the top is put together to form a possible genotype that combines both the maternal and paternal DNA.

When talking about Punnett squares, it is important to know what the terms heterozygous and homozygous mean. Heterozygous refers to a genotype with two different alleles (i.e. Bb, Rr, etc.) and homozygous refers to a genotype with two of the same alleles (i.e. BB, bb, etc.). We can make an even further distinction by saying something is homozygous dominant (BB) or homozygous recessive (bb).

(Video) Punnett Square Basics

by 2 Minute Classroom

In this video, 2 Minute Classroom explains how to draw a useful tool for determining offspring genotypes.