If one of the parents is a homozygote for one or more traits, the Punnett Square still contains the same number of boxes, but the total number of unique allele combinations is 2 raised to the power of the number of traits for which the parent is heterozygous.Ī commonly discussed Punnett Square is the dihybrid cross. If the same genotype is present in two boxes, its probability of occurring doubles to 1/8 (1/16 + 1/16). The probability of a cross producing a genotype in any box is 1 in 16. The genotype in each box is equally likely to be produced from a cross. With one row or column for each allele combination, the total number of boxes in a Punnett Square equals the number of rows times the number of columns. If the genes for the two traits are on the same chromosome, alleles for each trait will always appear in the same combinations (ignoring recombination). Independent assortment typically means the genes are on different chromosome *s. Three traits produce 8 combinations (2^3). For two traits, an individual can produce 4 allele combinations (2^2). Assuming that all traits exhibit independent assortment, the number of allele combinations an individual can produce is two raised to the power of the number of traits. Adding more traits increases the size of the Punnett Square. A single trait Punnett Square tracks two alleles for each parent.
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