Punnett Squares are used in Genetics to determine the possible combinations of genes that will occur at fertilization. It consists of squares, which are used to create combinations of genes, such as recessive and dominant.
Step 1
Begin by drawing up a simple table that has four boxes, two in each column and two in each row.
Step 2
For example, let's use the scenario of determining whether someone can roll their tongue. The gene that allows someone to roll their tongue is dominant which means that the individual will be able to roll their tongue even if only one of their alleles codes for tongue rolling. This scenario shows a mating of two individuals that each have one dominant tongue rolling allele and one recessive allele. As each gamete (egg or sperm) only receives one of the two alleles the boxes help us to see the possible combinations that may occur when the possible sperm join with the possible egg, forming a zygote.
Step 3
Next to each row, and above each column, write R (for dominant) and r (for recessive).
Step 4
Write the letter from the top of the column in both squares of that column. Then write the letter from the left of the row in both squares of that row. You should end up with a something like this.
Step 5
If a box has a capital R in it it means that there is a dominant gene in that zygote, and the resulting person will be a Tongue-Roller. The outcome of this exercise shows that although 3/4 of the possible offspring will be able to roll their tongue, the non-rolling gene remains hidden in half of the offspring. Also, 1/4 of the offspring will not receive any tongue rolling gene and will be non-rollers.
Tips
- This is a very simple Punnett square, they can conceivably have many fields, depending on the number of alleles of the gene you are dealing with.
Warnings
- If you are dealing with dominant and recessive make sure dominant is always ahead of recessive. So that means big letter over little letter.
