The Sunny Side of Punnett Squares
Ever been curious of what determines the color of fruit flies? How about why baldness is almost exclusively a male affliction? Can't figure out why some flowers in a patch are tall, and others short, yet all are from the same species? Utilizing Punnett squares can make these seemingly elusive facts very apparent.
Why not Punnett Circles?
A Punnett square is a mathematical device used by geneticists to show combinations of gametes and to predict offspring ratios.
Before jumping right into Punnett squares, there are a few fundamental concepts that must be understood for them to predict correctly.
Each trait is controlled by two genes
Due to the binary nature, genes can be represented by letters
An uppercase letter denotes the wild type (usual, dominant) form of the gene
A lowercase letter denotes the mutant (unusual, recessive) form of the gene
In monohybrid crosses (Punnett square configurations) there are three distinct combinations; e.g. TT,Tt, and tt
If a dominant gene is paired with a recessive gene, the dominant's trait will take effect
To set up a Punnett square, draw a large square, and then divide it into 4 equal sections (also squares). It should look something like this:
Now you need two parents to mate, ones with a known genotype. For example, a red flower (genotype Rr) and a white flower (genotype rr). Place one of the parents on top, and one on the left. You should get a something similar to this:
Finally, take each letter in each column and combine it with each letter from each row in the corresponding square. You should now have a picture close to this:
The two-letter combinations are your possible offspring. They are: Rr, Rr, rr, and rr. From this it is possible to determine the probability that a flower will be red (2/4 or 50%) or white (2/4 or 50%).
How Do You Like Your Punnett Squares?
Now that you have a basic understanding of how Punnett squares work, we can venture into the depths of the dihybrid cross. It is from these that we are able to find out why there are sex-linked traits.
Over Easy, Please
To make a dihybrid Punnett square, all that is necessary is the analyzing of two different sets of genes at the same time. Sound difficult? It really isn't. We'll look at how hemophilia is predominantly male through Punnett squares. Set up the Punnett square the same way as in the first example but instead of setting it up for two flowers mating, set it up for two humans mating. If you don't know how, set one side up so that it has X and Y (the male side) and the other so it has X and X (the female side). The blood clotting gene is designated by an H. The mutant gene (defective, thus preventing blood from clotting and developing the condition hemophilia) designated by an h.
We shall mate a carrier female (a female with the mutant gene, but who is unafflicted because she has the normal gene on her other X chromosome) with a normal male. The Punnett square should look like this:
The o attributed to the Y chromosome is placed because the gene is not on the Y chromosome.
After completing the square, it should be similar to this:
From the Punnett square results, it is evident that the possible offspring are: female carrier, female normal, male normal, and male hemophiliac. Due to the male total dependence on their X chromosome, males have a much greater chance of getting hemophilia than females. For a female to get hemophilia, she has to have either a hemophiliac father, carrier mother and bad luck, or hemophiliac mother and hemophiliac father.
Punnett Squares For All!!!
Punnett squares are a very useful tool to use that are simple to set up and quite quick to solve. If only everything else was this easy!