Direct comparison
Dominant vs recessive
A dominant allele shows its effect with just one copy; a recessive allele needs two copies to show.
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Side-by-side comparison
| Dimension | Dominant allele | Recessive allele |
|---|---|---|
| When the trait shows | With one or two copies present. | Only when both copies are recessive. |
| Copies needed | One is enough. | Two are needed. |
| Usual symbol | A capital letter, e.g. B. | A lower-case letter, e.g. b. |
| Effect on the other allele | Masks (overrides) a recessive partner. | Is masked when a dominant allele is present. |
| Genotypes that show it | BB (homozygous) and Bb (heterozygous). | Only bb (homozygous recessive). |
| Carrier possible? | A dominant trait is always visible. | Yes — a Bb individual carries it without showing it. |
| Example | Brown eyes can come from BB or Bb. | Blue eyes need bb. |
| In a cross of two carriers (Bb x Bb) | About three in four show the dominant trait. | About one in four show the recessive trait. |
Why one copy can be enough
Every gene comes as a pair of alleles, one inherited from each parent. A dominant allele is "louder": if it is present, its version of the trait appears, whether the organism has one copy or two. A recessive allele is "quieter" and can only be heard when there is no dominant allele to drown it out — that is, when both alleles are recessive. This is why a recessive trait can skip generations: two parents who each carry one hidden recessive allele (and so show the dominant trait) can have a child who inherits the recessive allele from both and shows the recessive trait. Dominant does not mean more common or stronger overall — it just means one copy is enough to show.
Common questions
FAQ
Does dominant mean a trait is more common?+
No, this is a frequent misunderstanding. Dominant describes how an allele behaves — one copy is enough to show the trait — not how common it is in a population. A recessive trait can be far more common than a dominant one if the recessive allele is widespread. Dominance is about masking, not frequency or strength.
How can two parents without a trait have a child who shows it?+
If both parents carry one recessive allele hidden behind a dominant one (genotype Bb), neither shows the recessive trait, but each can pass the recessive allele on. A child who inherits the recessive allele from both parents (bb) will show the trait. This is why some recessive traits appear to skip a generation.
How do you write dominant and recessive alleles?+
By convention, the dominant allele is given a capital letter and the recessive allele the same letter in lower case — for example B for brown eyes and b for blue. An organism then has one of three genotypes: BB, Bb or bb. Both BB and Bb show the dominant trait; only bb shows the recessive trait.
Going deeper
