How Do You Do Punnett Squares?


Quick Answer

Complete a basic Punnett square by determining the parents' genotypes and then drawing a square divided into four sections. The goal is to cross the parents' genotypes to determine the offspring's genotype and phenotype.

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  1. Determine the parents' genotypes

    The parents' genotypes are usually written as capital letters, lowercase letters or a combination of both. A capital letter indicates that the gene is dominant, and a lowercase letter indicates that the gene is recessive.

  2. Draw a Punnett square

    The basic Punnett square has four sections. More complex Punnett squares, such as a four-by-four square, are used for more than one pair of alleles.

  3. Write the genotypes along the sides of the square

    Write one parent's genotype along the top of the Punnett square. Each allele, or letter, goes over one box. For example, a genotype of "Aa" means that the capital "A" goes over one box and the lowercase "a" goes on the next box. Write the other parent's genotype along the left side of the Punnett square in the same manner.

  4. Cross the parents' genotypes

    To cross the parental genotypes in a basic Punnett square, start with the letter above the upper-left box. Cross this letter with the letter to the left of the upper-left box. If one of the letters is capitalized, it is written first. Write this two-letter combination in the upper-left box. Cross the letter above the upper-right box with the letter to the left of the upper-left box. This two-letter combination goes in the upper-right box. Complete the rest of the Punnett square by crossing the letters from the top with the letter to the left of the lower-left box and writing each two-letter combination in the lower-left box and the lower-right box.

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