Monohybrid Ratios: 3:1 vs 1:1
A 3:1 phenotype ratio comes from crossing two heterozygotes (Tt x Tt); a 1:1 ratio comes from crossing a heterozygote with a homozygous recessive (Tt x tt). The ratio depends entirely on the parents' genotypes.
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The 3:1 ratio
A 3:1 phenotype ratio appears when both parents are heterozygous for the same gene, e.g. Tt x Tt. Each parent produces gametes T and t in equal numbers, giving offspring genotypes 1 TT : 2 Tt : 1 tt.
Because TT and Tt both show the dominant phenotype, three-quarters of offspring show the dominant trait and one-quarter show the recessive trait.
The 1:1 ratio
A 1:1 phenotype ratio appears when a heterozygote is crossed with a homozygous recessive, e.g. Tt x tt. The heterozygous parent produces gametes T and t, but the homozygous recessive parent produces only t.
Half the offspring are Tt (dominant phenotype) and half are tt (recessive phenotype).
Side-by-side comparison
| Cross | Gametes from each parent | Offspring genotype ratio | Phenotype ratio |
|---|---|---|---|
| Tt x Tt | T, t and T, t | 1 TT : 2 Tt : 1 tt | 3 dominant : 1 recessive |
| Tt x tt | T, t and t only | 1 Tt : 1 tt | 1 dominant : 1 recessive |
| TT x tt | T only and t only | all Tt | all dominant (no ratio) |
Why this matters
The offspring ratio is direct evidence of the parents' genotypes. If a cross between a dominant-phenotype parent and a known homozygous recessive gives roughly a 1:1 phenotype split, the dominant parent must be heterozygous, this is exactly the logic used in a test cross.
Worked problem 1 (easy): a 1:1 cross
In rabbits, grey fur (G) is dominant over white fur (g). A heterozygous grey rabbit (Gg) is crossed with a white rabbit (gg).
Work out the phenotype ratio of the offspring.
The grey parent forms G and g gametes in equal numbers, while the white parent forms only g gametes. Half the offspring are Gg (grey) and half are gg (white), so the phenotype ratio is 1 grey : 1 white.
| g | g | |
|---|---|---|
| G | Gg | Gg |
| g | gg | gg |
Worked problem 2 (medium): a 3:1 cross
Two heterozygous grey rabbits (Gg) are crossed. Work out the genotype ratio and the phenotype ratio of the offspring.
Each parent forms G and g gametes in equal numbers. The four combinations give a genotype ratio of 1 GG : 2 Gg : 1 gg.
Because GG and Gg are both grey, the phenotype ratio is 3 grey : 1 white.
| G | g | |
|---|---|---|
| G | GG | Gg |
| g | Gg | gg |
Worked problem 3 (SPM-level): read the ratio backwards
A grey rabbit is crossed with a white rabbit. Among the young produced over repeated matings, about half are grey and half are white.
Deduce the genotype of each parent, with reasoning.
The white parent is gg and passes only g. Roughly half the offspring are white (gg), so those offspring received a g allele from the grey parent as well; the grey parent must therefore carry g and, being grey, is heterozygous Gg.
The cross is Gg x gg, which gives the observed 1 grey : 1 white ratio.
Had every offspring been grey instead, the grey parent would have been homozygous GG, because a GG x gg cross yields only Gg offspring and no white.
| g | g | |
|---|---|---|
| G | Gg | Gg |
| g | gg | gg |
Practice problems
- In tomato plants, purple stem (A) is dominant over green stem (a). A heterozygous purple plant (Aa) is crossed with a green plant (aa). State the expected phenotype ratio of the offspring.
- Two heterozygous purple tomato plants (Aa) are crossed. State the genotype ratio and the phenotype ratio of the offspring.
- A purple tomato plant is crossed with a green one and every offspring is purple. Deduce the genotype of the purple parent and explain why no green offspring appear.
Common mistakes
Source:SRC-DSKP-EN
Frequently asked questions
When does a monohybrid cross give a 3:1 ratio?
When does a monohybrid cross give a 1:1 ratio?
Why don't real offspring always match the expected ratio exactly?
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