Punnett Squares

The Truth Tables of Life

Now that you have undergone walking and crawling, the fun can really start. This is where you will learn the good stuff, the big, juicy facts that you can sink your teeth into. You can begin to learn about the math part of genetics. As you know, genetics is defined as the study of heredity, and, you also know that this section is titled "Heredity." So, as you have probably figured, this will be about heredity. But it will be more than that. Here, you will discover how to mathematically figure out the probabilities of inheriting certain traits from your parents.

Punnett Squares may be a little confusing. Basically, your parents both had two chromosomes that contain a gene that you could receive. These squares will help you figure out which trait you will get from these chromosomes. To take it on a basic level, we will begin by setting up a Punnett Square to determine gender. First, you should set up a square like this:

You then would label the chromosomes from your father (X/Y) on the side, and the chromosomes from your mother (X/X) on the top like so:

Next, you match up the genes on the top with the ones on the side, filling all four spaces of the square like you see below:

Now, this brings me to how to actually figure out probabilities. You have all of the spaces in the square filled in, but you still do not know what it means. Two of the spaces have X/Y, and two others have X/X. Well, since X/Y means a boy, and X/X means a girl, you can conclude that two spaces mean boy and the others mean girl. There is a 50% chance of being male, and a 50% chance of being female. This explains why about half the world is male, and the other half is female. This equilibrium can be used on all traits.

Here is where it gets tricky. Punnett Squares can allow you to determine all sorts of traits. You can even determine two traits at the same time. Say the X chromosome can carry a recessive gene (a) for a blood disorder that makes you anemic. (The normal version would be (A).) The Y chromosome doesn’t carry it. We can set up a Punnett Square to figure out the chance that a child will get the disorder and the probabilities of their gender at the same time. For instance, one of the mother's X chromosomes carried the disorder, and the other didn't. (Since it is recessive, the mother is a carrier, but is not anemic.) The father doesn't carry it. We can set up this square:

Only 25% of the children would get it. The only child that could receive it would be a boy mainly because of their total dependence on one X chromosome. No girls could receive it here. But, if the father had the disorder, too, everything could change. We could set up the disorder for the mother carrier and the anemic father:

50% of the children can get it. Half the children who get it have to be girls, and the other half that get it have to be boys. Because of the male dependence on their only X chromosome, there has to be an anemic father and at least a carrier mother to get anemia. Thanks to the discovery of Punnett Squares, we now can predict how likely it is that our children get diseases, and we can make a more educated decision on if it is safe to have kids.

There are many other uses for Punnett Squares other than just anemia. People have used it for things such as determining hair color. (For example, if both your parents are pure blondes (having only blonde genes for hair color), than you will definitely have blonde hair.) Now that you have learned how to calculate heredity, you are ready for other things on this website, too. I suggest moving on to the next section to learn more about heredity, because this is only a small portion of it.

Introduction
Heredity
Punnett Squares