Monohybrid cross: Punnett square with answers
A monohybrid cross looks at a single gene with two alleles. With a Punnett square you work out which genotypes and phenotypes the offspring have, and how likely each one is.
Rules
- Each parent passes on one of its two alleles (law of segregation)
- Punnett square: one parent’s gametes along the top, the other’s down the side
- Aa × Aa: genotypes 1 : 2 : 1, phenotypes 3 : 1
- Aa × aa: 1 : 1; AA × aa: all Aa (law of uniformity)
- Dominant: the allele shows even in the heterozygote
Your turn
Three problems on this topic. Type probabilities as a percentage number, e.g. 18.75.
Task
Trait: seed colour in peas. Allele A (yellow) is dominant over a (green). The cross is Aa × aa. What is the probability of an offspring with the green phenotype and of a heterozygous offspring? What phenotypic ratio (dominant : recessive) results?
Probability of the recessive phenotype in %
Probability heterozygous (Aa) in %
Phenotypic ratio
Probabilities as a percentage number (25 or 18.75), rounded to 2 decimal places; allele frequencies as a decimal (0.02).
Worked example
Trait: coat colour in guinea pigs. Allele A (black) is dominant over a (white). The cross is AA × aa. What is the probability of an offspring with the white phenotype and of a heterozygous offspring? What phenotypic ratio (dominant : recessive) results?
Worked solution
- 1.Gametes: AA gives A, aa gives a
- 2.Punnett square: Aa 1
- 3.Recessive phenotype only with aa: 0%; heterozygous Aa: 100%
- 4.Phenotypes: all dominant
Typical mistake: Mixing up genotypic and phenotypic ratios: Aa × Aa gives genotypes 1 : 2 : 1, but phenotypes 3 : 1, because AA and Aa look the same.
Common questions
What is the difference between genotype and phenotype?
The genotype is the pair of alleles (AA, Aa, aa), the phenotype is the visible trait. With dominant-recessive inheritance AA and Aa have the same phenotype.
What do Mendel’s laws say?
Uniformity: crossing two pure-breeding parents gives a uniform F1. Segregation: the traits separate again in the F2 (3 : 1). Independent assortment: different genes are inherited independently.