By John Brennan
Updated Aug 30, 2022
In a diploid organism such as a human, each individual inherits one copy of every gene from each parent. Populations often contain multiple versions of a gene—each distinct variant is called an allele.
A Punnett square visualizes every possible allele combination you might receive from your mother in the columns, and every possible combination from your father in the rows. The grid allows quick calculation of genotype and phenotype ratios.
Decide how many rows and columns your square needs. Each row represents one possible gamete from one parent, and each column represents one possible gamete from the other parent. For a single gene with two alleles, you’ll use a 2 × 2 grid. For a dihybrid cross involving two genes, each with two alleles, a 4 × 4 grid is appropriate. Punnett squares are generally reserved for these simpler cases; larger crosses become unwieldy.
Draw the grid. Label the top of each column with one of the mother’s possible allele combinations, and the left side of each row with one of the father’s possible combinations. Alleles are conventionally denoted by letters: a capital letter for a dominant allele and a lowercase letter for its recessive counterpart.
Fill each cell with the combined alleles from the corresponding column and row. For example, if a column is labeled Yh and a row is labeled yh, the resulting genotype in that cell is Yyhh.
Scan the grid to identify distinct genotypes. Treat genotypes such as Yy and yY as the same, because allele order is irrelevant in diploid organisms.
Count how many times each genotype appears and express the counts as a ratio. For instance, a grid that contains one YY, two Yy, and one yy yields a genotypic ratio of 1 : 2 : 1.
Translate genotypes into observable traits (phenotypes). Dominant alleles usually mask recessive ones, so a genotype containing at least one dominant allele will display the dominant phenotype. For example, red flowers (R) dominate white flowers (r); only rr plants show white. Some traits exhibit incomplete dominance, producing an intermediate phenotype (e.g., red + white = pink). Others show codominance, where both alleles are expressed simultaneously (e.g., AB blood type). Cystic fibrosis is a classic recessive disorder: only individuals with two CF alleles manifest the disease.
Count the phenotypes that appear in the grid. Using the earlier example, the 1 : 2 : 1 genotypic ratio corresponds to a 3 : 1 phenotypic ratio if Y is dominant, because YY and Yy produce the same phenotype. If the trait exhibits incomplete dominance or codominance, each genotype yields a distinct phenotype, so the phenotypic ratio matches the genotypic ratio.
