What is a Punnett square?

• A. A diagram that shows possible genotypes of offspring
• B. A chart of evolutionary relationships
• C. A method for sequencing DNA
• D. A way to measure gene frequency in populations

Answer: (A)

A Punnett square is a diagram used to predict the genotypes of offspring from the parental alleles. It lay outs the possible gametes from each parent along the top and side, and fills in each box with the resulting zygote genotype, showing the chances of each genotype in the offspring. For example, crossing two heterozygotes (A carrying a and a carrying a) yields AA, Aa, Aa, aa, illustrating a 1:2:1 genotype pattern. If the dominant allele A also determines the phenotype, you’d expect three individuals with the dominant trait and one with the recessive trait in the next generation. This tool is specific to Mendelian inheritance for a single gene with two alleles and isn’t used to map evolutionary relationships, sequence DNA, or measure gene frequencies in populations.