Genetics, Part 4: Punnett Squares

One of my students has been having fun with Punnett Squares. We read about Gregor Mendel's experiments with pea plants and about dominant and recessive genes. We then began making Punnett Squares. A Punnett square is a diagram that is used to predict the outcome of particular genetic traits. You would, for example write the genetic traits of the mother on one side of a square and the paternal traits on top of the square and the square is then divided into four smaller sections and we look at the combinations possible.

After looking at a few Punnett Squares having to do with Mendel's pea plants, I gave him a problem (from My Name is Gene) about a couple, Bill and Jane. Bill's great grandmother had died from sickle cell anemia. Their first child also came down with the disease. What would the Punnett Square for Bill and Jane look like and what is the chance of their next child developing the disease?

All characteristics are noted by the dominate trait with a capital letter and the recessive trait with a lowercase letter. We shall say that they gene for sickle cell anemia is s.

Above is the Punnett Square my student made. Because both Bill and Jane do not have the disease themselves, but one needs ss in order to have the disease, we can only assume that both Bill and Jane have Ss gene traits, or genotype.

My student wrote these along the top and side of the square. (Yeah, I know it is hard to tell upper and lowercase with his handwriting, but you are just going to have to trust me on this one.) Then, making the square inside as a four-squared chart, he then put together the combinations by putting together the letters from the side and top of each individual small square, coming up with SS,Ss,Ss and ss.

From the Punnett Square ss genotype is in one quadrant out of four, which is 25%. Each time they have a child the probability of the child with the disease will always be 25%.

A hands-on way of learning Punnett Squares can be found here.