Monohybrid crosses
The Punnett square
- A Punnett square predicts the offspring of a cross involving one gene.
- Steps:
- write each parent's genotype, then their gametes (one allele each),
- combine them in a grid to find the offspring genotypes and phenotypes.
The classic ratios
- Bb × Bb → a 3:1 ratio of dominant to recessive.
- Bb × bb → a 1:1 ratio.
- A pedigree diagram (family tree) lets you follow a feature and work out genotypes.
- (Supplement) A test cross finds an unknown genotype: cross it with a homozygous recessive — if any offspring show the recessive feature, the unknown was heterozygous.
Practice
Crossing two heterozygous parents (Bb × Bb) gives a phenotype ratio of:
Bb × Bb gives BB, Bb, Bb, bb — a 3:1 ratio of dominant to recessive.
Practice
Crossing Bb × bb gives a phenotype ratio of:
Bb × bb gives Bb, Bb, bb, bb — a 1:1 ratio of dominant to recessive.
Practice
A test cross uses a homozygous recessive partner to reveal an unknown genotype.
If the unknown is crossed with bb and any recessive offspring appear, the unknown carried a recessive allele (was heterozygous).
You've got it
Key idea
- a Punnett square combines each parent's gametes to predict offspring
- Bb × Bb → 3:1; Bb × bb → 1:1
- (Supplement) a test cross (× homozygous recessive) reveals an unknown genotype