A dominant phenotype is the physical trait you see when at least one copy of a dominant allele is present in a person’s genes. If you inherit one dominant allele and one recessive allele for the same trait, the dominant one determines what shows up. You only see the recessive trait when both copies are recessive. This core idea, first described by Gregor Mendel in the 1860s, remains one of the foundations of genetics.
How One Allele Overrides Another
You carry two copies of most genes, one from each parent. These copies are called alleles, and they can be different versions of the same gene. When one allele’s effect completely masks the other, the visible result is the dominant phenotype. The hidden version is recessive. A person who carries one dominant and one recessive allele (heterozygous) looks identical to someone carrying two dominant alleles (homozygous dominant). That’s what “complete dominance” means: you can’t tell the two apart just by looking.
Dominance only affects what you see, not how genes get passed down. A parent who carries one dominant and one recessive allele still passes each one to offspring with equal probability. The recessive allele doesn’t weaken or disappear across generations. It simply stays hidden whenever a dominant allele is present alongside it.
The 3:1 Ratio and How to Predict It
Mendel figured this out by crossing pea plants. When he crossed two plants that each carried one dominant and one recessive allele, about three-quarters of the offspring showed the dominant phenotype and one-quarter showed the recessive one. That 3:1 ratio became a hallmark of single-gene inheritance.
A Punnett square makes this easy to visualize. Say both parents are heterozygous, carrying alleles “B” (dominant) and “b” (recessive). The four possible offspring combinations are BB, Bb, bB, and bb. Three of those four contain at least one capital “B,” so three out of four offspring display the dominant phenotype. Only the bb combination, where both alleles are recessive, produces the recessive phenotype. That gives you a 75% chance of the dominant trait appearing in any single offspring.
If one parent is homozygous dominant (BB), every child will show the dominant phenotype regardless of the other parent’s genotype, because every child inherits at least one “B.”
Examples in Humans
Several human traits follow straightforward dominant inheritance. Wet earwax is dominant over dry earwax. “Achoo syndrome,” the reflex that makes some people sneeze when they look at bright sunlight, is a dominant trait. Advanced sleep phase syndrome, which causes people to wake up unusually early, also follows a dominant pattern.
Some traits that were once taught as simple dominant-recessive examples have turned out to be far more complex. Eye color is the classic case. For decades, students learned that brown eyes are dominant over blue. The reality is that eye color is polygenic, meaning multiple genes influence it. A three-letter genetic variation in a single gene accounts for much of the difference between blue and brown eyes, but the currently identified genes explain only about 50% of eye color variation overall. The genetics of eye color are considerably more complex than the simple Mendelian model suggests.
Dominant Alleles in Genetic Disorders
Dominant inheritance also applies to disease. In autosomal dominant disorders, a single copy of the altered gene is enough to cause the condition. Huntington’s disease is one well-known example. A person who inherits just one copy of the Huntington’s allele will develop the disease. Marfan syndrome, which affects connective tissue, and BRCA1-related hereditary breast and ovarian cancer also follow dominant inheritance patterns.
Sometimes a person with a dominant disorder is the first in their family to carry the genetic change. It doesn’t always trace back through generations. A new mutation can arise spontaneously and then follow dominant inheritance from that point forward.
When Dominance Isn’t Complete
Mendel’s pea plants showed clean, all-or-nothing dominance. But not every trait works that way. Two other patterns are common.
Incomplete dominance produces a blended phenotype. In snapdragons, crossing a red-flowered plant with a white-flowered plant produces pink offspring. Neither allele fully overrides the other, so the heterozygote lands somewhere in between. There is no single “dominant phenotype” in these cases.
Codominance is different from blending. Both alleles are fully expressed at the same time. A well-studied example involves markers on the surface of red blood cells. A person who inherits one “M” allele and one “N” allele doesn’t get a blend. Instead, their red blood cells display both M and N markers in equal numbers. Both alleles show up simultaneously rather than one masking the other.
Why the Distinction Matters
At the molecular level, dominance often comes down to what proteins the alleles produce. A dominant allele typically codes for a functional protein that does its job even when only one copy is present. The recessive allele may produce a nonfunctional version or no protein at all. As long as one working copy generates enough protein to produce the trait, that’s what you see. This is why carrying just one dominant allele is sufficient.
Understanding dominance helps make sense of inheritance patterns in families. If a trait appears in every generation, it’s likely dominant, because only one copy needs to be passed down for the trait to show. If it skips generations, that’s a hallmark of recessive inheritance, where both parents can silently carry an allele without displaying it. Knowing whether a trait or condition follows dominant inheritance changes the probability calculations for whether children will be affected, which is a practical question for anyone looking at family health history.