4.4: Punnett Squares and Test Crosses
Punnett Squares
Given the genotypes of any two parents, we can predict all of the possible genotypes of the offspring. Furthermore, if we also know the dominance relationships for all of the alleles, we can predict the phenotypes of the offspring. A convenient method for calculating the expected genotypic and phenotypic ratios from a cross was invented by Reginald Punnett. A Punnett square is a matrix in which all of the possible gametes produced by one parent are listed along one axis, and the gametes from the other parent are listed along the other axis. Each possible combination of gametes is listed at the intersection of each row and column. The F 1 cross from Figure \(\PageIndex{6}\)b would be drawn as in Figure \(\PageIndex{7}\). Punnett squares can also be used to calculate the frequency of offspring. The frequency of each offspring is the frequency of the male gametes multiplied by the frequency of the female gamete.
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Figure \(\PageIndex{1}\): A Punnett square showing a monohybrid cro ss. (Original-Deholos (Fireworks)-CC:AN)
Test Crosses
Knowing the genotypes of an individual is usually an important part of a genetic experiment. However, genotypes cannot be observed directly; they must be inferred based on phenotypes. Because of dominance, it is often not possible to distinguish between a heterozygote and a homozgyote based on phenotype alone (e.g. see the purple-flowered F 2 plants in Figure \(\PageIndex{2}\)b). To determine the genotype of a specific individual, a test cross can be performed, in which the individual with an uncertain genotype is crossed with an individual that is homozygous recessive for all of the loci being tested.
For example, if you were given a pea plant with purple flowers it might be a homozygote ( AA ) or a heterozygote ( Aa ). You could cross this purple-flowered plant to a white-flowered plant as a tester , since you know the genotype of the tester is aa . Depending on the genotype of the purple-flowered parent (Figure \(\PageIndex{2}\)), you will observe different phenotypic ratios in the F 1 generation. If the purple-flowered parent was a homozgyote, all of the F 1 progeny will be purple. If the purple-flowered parent was a heterozygote, the F 1 progeny should segregate purple-flowered and white-flowered plants in a 1:1 ratio.
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Figure \(\PageIndex{2}\): Punnett Squares showing the two possible outcomes of a test cros s. (Original-Deholos (Fireworks)-CC:AN)