Monohybrid inheritance is the inheritance of a single characteristic controlled by one gene with two alleles. An allele is a version of a gene. Genetic diagrams let us predict the offspring of a cross.
Key terms
- Dominant allele: shown even when only one copy is present (capital letter, e.g. T).
- Recessive allele: only shown when two copies are present (small letter, e.g. t).
- Genotype: the alleles present (e.g. Tt). Phenotype: the observable feature (e.g. tall).
- Homozygous: two identical alleles (TT or tt). Heterozygous: two different alleles (Tt).
Key idea
A dominant allele is expressed in both the homozygous (TT) and heterozygous (Tt) states. A recessive allele is expressed only in the homozygous recessive state (tt).
A monohybrid cross
Worked example
Cross two heterozygous tall pea plants: Tt × Tt. The gametes are T and t from each parent. The Punnett square gives TT, Tt, Tt, tt — a ratio of 3 tall : 1 short, i.e. a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio.
Test cross
To find whether a tall plant is TT or Tt, cross it with a homozygous recessive (tt). If any short offspring appear, the tall parent must be Tt.
Remember
- Capital = dominant, small = recessive.
- Tt × Tt → 3:1 phenotype ratio.
- Ratios are predictions, not guarantees for small numbers.