Geneticists use Punnett squares to determine all the possible genotypes and phenotypes that offspring can inherit from their parents. A genotype is a combination of genes and may be made up of dominant or recessive genes or a combination of the two. Whenever a dominant gene is part of the genotype, it hides the trait carried by the recessive gene, resulting in the same phenotype, or displayed trait, as a genotype with all dominant genes. By examining the frequency that each combination of genes appears in the Punnett square, geneticists can determine the probability of an offspring inheriting a particular genotype or phenotype.
Draw a square, then divide it into four boxes.
Write the genotype of one parent across the top of the square with one gene over each column.
Write the genotype of the other parent down the side of the square with one gene next to each row.
Write the combination of genes represented by the column heading and the row heading in each box. If one gene is dominant, write it first. For example, if the genotype Aa is written across the top and down the side of the square, the first box in the first row would contain AA. The second box in the first row would contain Aa. The third box would contain Aa and the fourth box would contain aa.
Analyze the boxes to determine the probability of the parents' offspring inheriting a particular genotype. For example, one out of the four boxes, or 25 percent, contains the genotype AA. Two boxes, or 50 percent, contain the genotype Aa. One box, or 25 percent, contains the genotype aa. Therefore the parents' offspring have a 25 percent chance of inheriting an AA or aa genotype and a 50 percent chance of inheriting an Aa genotype.
Analyze the dominant and recessive genes represented in each box to determine the probability of the offspring inheriting a certain phenotype. For example, three out of the four boxes, or 75 percent, contain a dominant gene. Only one out of the four boxes, or 25 percent, contains all recessive genes. Therefore the parents' offspring have a 75 percent chance of displaying the dominant trait and a 25 percent chance of displaying the recessive trait.