A monohybrid cross follows one gene. You already know the steps from Mendel's Experiments. Here we make them exact, so you can predict any one-gene cross.
- Write each parent's genotype (for example Aa).
- Write the gametes: each gamete gets one allele of the gene. Aa makes A and a; AA makes only A.
- Fill the Punnett square: each box joins one gamete from the side with one from the top.
- Count genotypes (AA, Aa, aa), then count phenotypes (what the organism looks like).
Key idea
For Aa × Aa the genotype ratio is 1 AA : 2 Aa : 1 aa, and the phenotype ratio is 3 dominant : 1 recessive, because AA and Aa look the same.
Why it works
Each box of the square is equally likely, because each gamete type is made equally often and gametes join at random. So a box is a probability: in a 4-box square, each box is ¼. Counting boxes is the same as adding probabilities.
Worked example: A cross with a heterozygote
In guinea pigs, black fur (B) is dominant to brown (b). Cross Bb × bb. Give the genotype and phenotype ratios.
- Gametes: Bb gives B and b; bb gives only b.
- Boxes: Bb, Bb, bb, bb.
- Genotypes: 2 Bb : 2 bb, which is 1 : 1.
- Phenotypes: 2 black : 2 brown, which is 1 : 1.
Answer: Genotypes 1 Bb : 1 bb; phenotypes 1 black : 1 brown.
Worked example: Two ratios from one square
Cross Tt × Tt (tall is dominant). What fraction are tall, and what fraction are Tt?
- Boxes: TT, Tt, Tt, tt.
- Tall: TT and Tt, so 3 of 4 boxes: ¾.
- Tt alone: 2 of 4 boxes: ½.
Answer: ¾ tall; ½ are Tt.
| Cross | Genotype ratio | Phenotype ratio |
|---|---|---|
| AA × aa | all Aa | all dominant |
| Aa × Aa | 1 AA : 2 Aa : 1 aa | 3 : 1 |
| Aa × aa | 1 Aa : 1 aa | 1 : 1 |
| AA × Aa | 1 AA : 1 Aa | all dominant |
Common mistake
Two slips are common. One is giving the genotype ratio (1:2:1) when the question asks for the phenotype ratio (3:1). The other is counting AA and Aa as different looks. Check: a phenotype ratio for one gene with complete dominance has at most two parts.