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How to read a Punnett square

A Punnett square shows every possible combination of parental alleles. Learn to compute genotype and phenotype probabilities in monohybrid crosses.

A Punnett square predicts the offspring of a genetic cross. Each parent contributes one allele per gene, and every box in the square has equal probability.

A monohybrid cross

Cross two heterozygotes: Rr × Rr.

| | R | r | |---|---|---| | R | RR | Rr | | r | Rr | rr |

Reading the results

  • RR: 1 of 4 boxes → probability 1/4=0.251/4 = 0.25
  • Rr: 2 of 4 boxes → probability 1/2=0.51/2 = 0.5
  • rr: 1 of 4 boxes → probability 1/4=0.251/4 = 0.25

If R is dominant, the phenotype ratio is 3 dominant : 1 recessive. Practise on randomised Punnett-square questions below.

Put it into practice

Practise this topic with questions and get an explanation after each answer.

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