When a dominant allele and a recessive allele are together in a heterozygous individual, the dominant allele’s effect is what shows up in the organism’s observable trait (phenotype), while the recessive allele’s effect is masked but still present in the genotype and can be passed to offspring.

Basic idea

  • In a heterozygote (for example, genotype AaAaAa), there are two different versions of the same gene: one dominant (A) and one recessive (a).
  • The dominant allele is defined as the one whose trait appears in the heterozygote’s phenotype; the recessive allele’s trait does not appear, even though the allele is still there.
  • The recessive allele can still be inherited by the next generation and will only be visibly expressed if an individual has two recessive copies (aa).

How they “interact”

In simple Mendelian dominance:

  1. The dominant allele produces a functional product (usually a protein) at a level that is sufficient for the normal trait.
  2. The recessive allele often produces no product or a nonfunctional version, but one working copy from the dominant allele is enough to give the dominant phenotype.
  1. As a result, the heterozygote looks phenotypically the same as a homozygous dominant individual, even though genetically it carries both alleles.

Important twist: not always so simple

While classic textbook examples say “dominant masks recessive,” real genetics can show more nuanced patterns:

  • Incomplete dominance : the heterozygote shows an intermediate trait (like pink flowers from red and white parents), meaning neither allele fully masks the other.
  • Codominance : both alleles are expressed distinctly in the heterozygote (such as AB blood type, where A and B are both visible).
  • Context-dependent dominance : one allele can be dominant to one partner but recessive to another, so “dominant vs recessive” is really a relationship between particular allele pairs, not an inherent property of an allele in all situations.

Simple take-home

  • In the classic case, dominant + recessive in a heterozygote → only the dominant trait is seen; the recessive is hidden but transmissible.
  • In more complex, but very common, real-world cases, both alleles can contribute to the phenotype in different ways (blended, side‑by‑side, or context‑dependent).

In short: they share the same gene “slot,” but the dominant allele usually decides what you see, while the recessive quietly rides along in the background.