A Punnett square is a diagram resembling a grid that is used to predict certain features, traits and characteristics of offspring based on the genotypes of the parents. Named after Reginald Punnett, the creator of the method, it does not guarantee that the offspring will have a certain trait. Rather, it demonstrates the probability of a trait. You can create a Punnett square to examine and predict the outcome of various specific crosses.
Draw a box. Draw a vertical line down the center. Draw a horizontal line, also through the center. This creates four equal boxes within the initial box.
Write the mother’s genotype for the particular trait that you’re examining along the top. Use capital letters for a dominant gene and lowercase letters for a recessive gene. For example, if a mother has one dominant gene and one recessive gene for a certain trait, then one of the boxes will have a capital letter next to it (i.e. “B”) and one of the boxes will have a lowercase letter next to it (i.e. “b”). If she has two recessive genes, there will be two lowercase letters (bb): one alongside each box. If she has two dominant genes, there will be two uppercase letters (BB): again, one alongside each box.
Write the father’s genotype for the particular trait that you’re examining along the left side.
Fill in the four boxes by looking at the column and row that corresponds with each individual box. For the upper left box, you combine the gene on the left for the mother and the gene on the top for the father. Write the combination in the box (i.e. “Bb”, “BB”, or “bb”). Always write the capital letter first, if there is a dominant gene present.
Look at all four of the boxes to determine the probability that offspring will have a certain trait. The presence of a dominant gene means that it’s probable that the offspring will have the trait. Add up the four total probabilities to determine the percentage. For example, if two of the four boxes contain a dominant gene in the maternal and paternal combination, then there is a 50 percent chance overall that the offspring will have the respective trait.