Patterns of genetic inheritance obey the laws of probability. In a monohybrid cross, where the allele*s present in both parents are known, each genotype* shown in a Punnett Square* is equally likely to occur. Since there are four boxes in the square, every offspring produced has a one in four, or 25%, chance of having one of the genotypes shown.
Like flipping a coin, previous matings do not influence the results of subsequent matings. Because of random variation, the actual number of each genotype produced over a series of matings (or crosses) between two individuals will differ slightly from the expected 25% per box.
The illustration above explores how the probabilities predicted by a monohybrid Punnett Square relate to the actual pattern of genotypes and phenotype*s produced from repeatedly crossing two individuals.
Test your understanding of genotype and phenotype probabilities