Mendel's monohybrid cross
Crossing pure tall and pure dwarf peas gives all tall F₁ plants. Selfing the F₁ gives an F₂ ratio of 3 tall : 1 dwarf.
The cross, step by step
- 1Parents (P): pure tall TT × pure dwarf tt.
- 2Gametes: the tall parent gives only T; the dwarf parent gives only t.
- 3★F₁ generation: all plants are Tt and all are tall. Dwarfness is hidden.
- 4F₁ plants are self-pollinated: Tt × Tt.
- 5Each F₁ plant makes T and t gametes in equal numbers.
- 6F₂ generation: TT, Tt, Tt and tt (see the checkerboard).
- 7★Phenotypic ratio 3 tall : 1 dwarf. Genotypic ratio 1 TT : 2 Tt : 1 tt.
★ key step
Checkerboard (Punnett square) for the F₂
| Female gametes ↓ / Male gametes → | T | t |
|---|---|---|
| T | TT (tall) | Tt (tall) |
| t | Tt (tall) | tt (dwarf) |
Law of dominance
When two contrasting alleles come together, only one of them shows. This is the dominant allele (T). The one that stays hidden is recessive (t). This is why all F₁ plants are tall. The dwarf trait comes back in the F₂, so it was hidden, not lost.
Law of segregation (law of purity of gametes)
The two alleles of a gene stay together in the plant. But they separate (segregate) when gametes form. Each gamete gets only one allele, T or t, so gametes are always pure. At fertilisation, gametes join at random. This gives the 1 : 2 : 1 genotypic ratio.
Using the 3 : 1 ratio
In the F₂ of a monohybrid cross, 800 pea plants grew. How many are expected to be tall and how many dwarf?
- Ratio tall : dwarf = 3 : 1, so there are 4 parts in all.
- Dwarf = 1/4 of the plants = 800 × 1/4 = 200.
- Tall = 3/4 of the plants = 800 × 3/4 = 600.
- Check: 600 + 200 = 800.
About 600 tall and 200 dwarf plants. Of the 600 tall plants, about 200 are TT and 400 are Tt.
The short, strong-stemmed rice and wheat varieties of the Green Revolution were bred using Mendel's rules. Plant breeders still use these ratios to predict how many plants will show a wanted trait.
1 marks: “The phenotypic ratio of a monohybrid F₂ generation is ______.” 3 : 1.
2 marks: “What is a monohybrid cross?” A cross between two parents that differ in one pair of contrasting characters, e.g. tall × dwarf pea.
7 marks: “Explain Mendel's monohybrid cross with a checkerboard. State the laws it shows.” P: TT × tt; gametes; F₁ all Tt, tall; selfing; checkerboard; F₂ 3 : 1 phenotype and 1 : 2 : 1 genotype; law of dominance and law of segregation.
Mono gives 3 : 1 on the outside, and hides 1 : 2 : 1 on the inside.