Genetics: Punnett Square

Mendelian inheritance, genotypes, and phenotype ratios

A Punnett square is a grid used to predict the genotypes of offspring from a genetic cross. Each parent contributes one allele. For a monohybrid cross (Aa × Aa), the predicted genotype ratio is 1 AA : 2 Aa : 1 aa. With complete dominance, the phenotype ratio is 3 dominant : 1 recessive. With incomplete dominance, heterozygotes show a blended phenotype (1:2:1 phenotype ratio). With codominance, both alleles are expressed equally.