A Punnett square is a graphical representation of the possible genotypes of an offspring arising from a particular cross or breeding event. It is a square diagram that is used to predict the genotypes of a particular cross or breeding experiment. The Punnett square is named after Reginald C. Punnett, who devised the approach in 1905. The diagram is used by biologists to determine the probability of an offspring having a particular genotype. The Punnett square is a tabular summary of possible combinations of maternal alleles with paternal alleles. These tables can be used to examine the genotypical outcome probabilities of the offspring of a single trait (allele), or when crossing multiple traits from the parents. The Punnett square is a visual representation of Mendelian inheritance.
Creating a Punnett square requires knowing the genotypes of the parents. The known genotypes of each parent are shown to help predict the possible genotypes of their offspring. Capital letters represent dominant alleles and lower case letters represent recessive alleles. The Punnett square shows how alleles are inherited or passed on to offspring from parents. The Punnett square is a tool that helps to show all possible allelic combinations of gametes in a cross of parents with known genotypes in order to predict the probability of their offspring possessing certain sets of alleles.
Two types of Punnett squares are commonly used. The first is relevant when a single trait determined by one genetic locus is being observed. This is called a monohybrid cross and examples include some of Mendel’s original experiments, where he chose true-breeders for a single trait and crossed them with members carrying a different allele. For a monohybrid cross, these are 2X2 squares with four boxes, each representing one fertilization event between the parent gametes. The second type of Punnett square is used when two traits are being observed. This is called a dihybrid cross and examples include Mendel’s experiments with pea plants that differed in two traits, such as seed color and seed shape. For a dihybrid cross, these are 4X4 squares with 16 boxes, each representing one fertilization event between the parent gametes.
While Punnett squares are a convenient tool to understand Mendelian genetics, they cannot be used in many situations involving complex genetic inheritance. For example, they are not effective in estimating the distribution of genotypes and phenotypes when there is linkage between two genes. Genetic linkage is a phenomenon where two genes exist on the same chromosome and are inherited together more often than not.
