Dominance in biology describes how one version of a gene can control your visible traits even when a second, different version is present. You carry two copies of most genes, one from each parent. When those copies differ, dominance determines which one shows up in your body. A dominant allele produces its effect whether you have one copy or two, while a recessive allele only shows its effect when both copies match.
How Dominance Works at the Gene Level
Your cells read genes as instructions for building proteins. When you inherit two identical copies of a gene (homozygous), the outcome is straightforward: your cells follow those matching instructions. Things get interesting when your two copies differ (heterozygous). In many cases, one allele produces enough functional protein on its own to determine your trait, effectively masking the other allele. That’s dominance in its simplest form.
Consider a gene where one allele codes for a fully functional protein and the other codes for a broken or inactive version. If a single working copy produces enough protein to do the job, the functional allele is dominant. The broken version is recessive because it can only reveal itself when no working copy is around to compensate. This is why many harmful mutations are recessive: one good copy of the gene is often sufficient to keep things running normally.
Complete Dominance
Complete dominance is the classic pattern Gregor Mendel described in his pea plant experiments. One allele fully overrides the other, so a heterozygous individual looks identical to someone who carries two copies of the dominant allele. If you carry one allele for brown eyes (dominant) and one for blue (recessive), complete dominance means you have brown eyes, period. There’s no visible trace of the recessive allele in your appearance, even though it’s still in your DNA and can be passed to your children.
Several quirky human traits follow this pattern. Achoo syndrome, the tendency to sneeze when stepping into bright sunlight, is dominant. So is having wet (rather than dry) earwax. Advanced sleep phase syndrome, which causes people to wake up unusually early, also follows a dominant inheritance pattern. In each case, a single copy of the relevant allele is enough to produce the trait.
Incomplete Dominance
Not all dominance relationships are all-or-nothing. In incomplete dominance, the heterozygous phenotype falls somewhere between the two homozygous parents, essentially a blend. The classic example comes from snapdragon flowers: crossing a homozygous red plant with a homozygous white plant produces pink offspring. Neither allele fully wins. The pink flowers carry one red allele and one white allele, and the result is an intermediate color.
This happens because a single copy of the red-pigment allele doesn’t produce enough pigment to make the flower fully red. You get half the pigment production, which translates to a lighter shade. The key distinction from complete dominance is that you can tell heterozygous individuals apart from both types of homozygous individuals just by looking at them.
Codominance
Codominance takes a different approach: instead of one allele masking the other or blending together, both alleles are fully expressed at the same time. The result isn’t an intermediate trait but rather both traits appearing simultaneously.
The ABO blood group system is the most familiar example. The A and B alleles are codominant with each other, while the O allele is recessive to both. If you inherit one A allele and one B allele, your red blood cells display both A-type and B-type markers on their surface. You don’t get some intermediate “AB-lite” blood type. You get full A and full B, expressed equally. That’s why AB is its own distinct blood type rather than a weakened version of A or B.
Sickle cell trait offers another example. People who carry one normal hemoglobin allele and one sickle hemoglobin allele produce both types of hemoglobin protein. Their blood contains a mixture of normal and sickle-shaped red blood cells. Both alleles are actively making their respective proteins at the same time.
Dominance in Human Disease
When a disease-causing allele is dominant, inheriting just one copy from either parent is enough to cause the condition. Huntington’s disease is one of the most well-known autosomal dominant disorders. It’s caused by an abnormal repetition of a specific DNA sequence in a single gene. Normally, this sequence repeats 10 to 35 times. People with 40 or more repeats almost always develop the disease, while those in the 36 to 39 range may or may not show symptoms.
This “one copy is enough” principle can work through several molecular mechanisms. Sometimes a single working gene copy simply can’t produce enough protein to maintain normal function, a situation called haploinsufficiency. Other times, the mutant protein actively interferes with the normal protein produced by the healthy copy. This is known as a dominant negative effect: the defective protein essentially sabotages its normal partner. This is particularly common with proteins that need to pair up to function. If one member of the pair is defective, the whole unit fails, leaving you with less than 50% of normal function even though only one of your two gene copies is mutated.
Why Dominant Doesn’t Mean Common
One of the most persistent misunderstandings about dominance is the assumption that dominant traits are more common in a population. They aren’t, necessarily. Dominance describes how alleles interact within a single individual’s cells. It says nothing about how frequently an allele appears across a population.
Polydactyly (extra fingers or toes) is caused by a dominant allele, yet the vast majority of people have five fingers per hand. The dominant allele is simply rare. Meanwhile, having five fingers is the “recessive” outcome, and it’s nearly universal. Allele frequency in a population is shaped by evolutionary pressures, random drift, and mutation rates, not by whether an allele is dominant or recessive.
There is, however, an interesting relationship between how harmful a mutation is and whether it tends to be dominant or recessive. Strongly damaging mutations are more likely to be recessive. This makes intuitive sense: if a devastating mutation were dominant, it would immediately affect every carrier and be quickly weeded out by natural selection. Recessive harmful mutations can hide in heterozygous carriers for many generations, persisting in the population at low frequencies much longer than a dominant equivalent would.
Wild Type and Dominance
In genetics, the most common version of a gene found in natural populations is traditionally called the wild type. Wild-type alleles are often (but not always) dominant over mutant versions. This is because the wild-type allele typically produces a fully functional protein, and one functional copy is usually enough to get the job done. Mutant alleles that produce reduced or nonfunctional proteins tend to be recessive because the wild-type copy compensates.
This isn’t a rule, though. Some mutations create proteins with new or enhanced functions that override the wild type. Others produce proteins that are always “switched on” regardless of normal cellular signals. These gain-of-function mutations act dominantly because the abnormal protein does something the normal version can’t counteract, not because it’s producing more of the same thing. This is why dominance isn’t a fixed property of an allele in isolation. It depends on what the other allele is doing, what protein is involved, and how the cell uses that protein.