Dominant and recessive, genotype and phenotype
When the two alleles of a gene differ, the one that shows is dominant and the one that is hidden is recessive. We write the dominant allele with a capital letter and the recessive one with the small form of the same letter, taking the first letter of the dominant trait: for purple flowers, P and p. Complete dominance means that the dominant trait fully hides the recessive one. Homozygous means two identical alleles (PP or pp). Heterozygous means two different alleles (Pp). The genotype is the pair of alleles, and the phenotype is the trait we see. So PP and Pp have the same phenotype, purple, but different genotypes, while pp is white. The recessive trait appears only with pp. Careful: dominant does not mean stronger, better or more common. We use pea plants and invented organisms for our crosses, because human traits such as eye or hair colour depend on several genes and our one-gene models are only simple models.
| Genotype | Alleles | Phenotype |
|---|---|---|
| PP | homozygous dominant | purple |
| Pp | heterozygous | purple |
| pp | homozygous recessive | white |
| Trait | Dominant | Recessive |
|---|---|---|
| stem | tall | short |
| flower colour | purple | white |
| seed shape | round | wrinkled |
| seed colour | yellow | green |
Genotype cards. Make cards with the genotypes TT, Tt and tt for tall (T) and short (t) pea plants. Under each card write the phenotype and tick homozygous or heterozygous. Shuffle and sort. Then find the mistake in: “a dominant allele is the one that is more common”. Draw the seven pea traits as small symbols.