How to Create a Punnet Square

In this post, I will explain how to create and use a Punnet square, however, I will not be explaining everything for your’s and mine’s sakes.  Explaining everything would take up too much of your time.  So, to get a basic understanding and background of what Punnet squares are used for, go here and here.

There are three steps to creating a Punnet square.  They are, making the square, determining your genes and finally, crossing the genes.  First, making the square!

I. Making the Square

Easily the simplest part of this process, the square is a box divided into four sections. These four sections will represent four people and which gene will be expressed in them.    The end product should look like this.

Punnet Square box



2. Determining Your Gene

Let’s use the gene for hair color as an example. For example, your mother has blonde hair, which we’ll say is the dominant trait.  We represent that as BB.  Your father has black hair, and it is the recessive trait. We show black hair as bb.  Now, using the square, we put your mother’s traits on top and your father’s traits on the side.  It should look like this.  The next and final step in using a Punnet square is crossing the genes.


3. Crossing the Genes

Play a quick game of Battleship with yourself!  Put one of each dominant and recessive trait inside the four boxes and this will determine which trait will be expressed.  A quick note to remember, when crossing genes, remember that dominant trait are always first because, well, they dominate over the recessive traits.  The results of our example should look like this.

Punnet Square

For our example, if a couple has dominant blonde hair traits and recessive black hair traits, and we use the Punnet square, we see that since dominant traits essentially cancel out the effects of a recessive trait,  the probability of their children having blonde hair is high.

Punnet Squares are fun and simple to use.  You can create your own hypothetical situation of crossing two genes and the probability of certain genes being expressed.  This is a tool that can predict likely outcomes, which I think is pretty cool!



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