Monohybrid cross — one trait, two alleles.
A Punnett square predicts offspring genotypes from parental crosses. For a monohybrid cross (one gene, two alleles): each parent contributes one allele to each offspring. Aa × Aa yields 1 AA : 2 Aa : 1 aa (genotype ratio) and 3 dominant : 1 recessive (phenotype ratio). Aa × aa yields 1 Aa : 1 aa (1:1 ratio) — this is a test cross used to determine if a dominant phenotype is AA or Aa. AA × aa yields all Aa (100% heterozygous). Developed by Reginald Punnett in 1905, this tool remains fundamental in genetics education, breeding programs, and genetic counseling for predicting inheritance of single-gene traits.