How to Complete a Punnett Square

Punnett squares work by crossing parental genotypes. A monohybrid cross is the simplest version of a Punnett square. A monohybrid cross looks at one specific gene and predicts the possible genotypic and phenotypic combinations that can occur in an offspring. Read the steps below to see how to set up a monohybrid Punnett square. Turn on video captions for detailed instructions.

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The genotypic and phenotypic ratios in monohybrid crosses are often predictable depending on what parental genotypes are being crossed. In other words, specific crosses always result in the same ratios. This can help us determine if we completed a Punnett square correctly, as well as to determine parental genotype if they are unknown. When two heterozygous parents are crossed, there is always a genotypic ratio of 1:2:1 and a phenotypic ratio of 3:1, as seen in the example above. Another cross that always provides the same ratios is a cross between two homozygous parents. This cross always results in a genotypic and phenotypic ratio of 4:0. If the parental phenotypes of a cross are unknown, but there is a 1:2:1 genotypic ratio observed, it can be concluded that both parents are heterozygous for the trait. This method for determining parental genotypes can be used for any of these known genotypic or phenotypic ratios.

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