Cross Aa × Aa with complete dominance. What fraction of offspring show the recessive phenotype?

Punnett square gives 1 AA : 2 Aa : 1 aa. Only aa is recessive = 1/4 = 25%.

Cross Aa × aa. What genotype ratio do you expect?

1 Aa : 1 aa (50% heterozygous, 50% homozygous recessive).

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Genetics: Punnett Square

Mendelian inheritance, genotypes, and phenotype ratios

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How it works

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.

Step-by-step

Select genotypes for both parents and choose a dominance pattern.
The Punnett square fills automatically.
Click 'Breed' to generate offspring and compare observed ratios to predicted ratios.
Increase the offspring count to see the law of large numbers in action.

Key formulas

$P(\text{genotype}) = \frac{\text{favorable outcomes}}{4}$ Probability of each genotype from a monohybrid cross

Frequently asked questions

Cross Aa × Aa with complete dominance. What fraction of offspring show the recessive phenotype?: Punnett square gives 1 AA : 2 Aa : 1 aa. Only aa is recessive = 1/4 = 25%.
Cross Aa × aa. What genotype ratio do you expect?: 1 Aa : 1 aa (50% heterozygous, 50% homozygous recessive).