Complete Dominance Definition
Complete dominance occurs when one allele – or “version” – of a gene completely masks another. The trait that is expressed is described as being “dominant” over the trait that is not expressed.
Most organisms are diploid – that is, they get two copies of each gene, one from each of their parents. Having two copies of each gene protects against the harmful effects of mutations – in many cases, an organism might have a gene that has experienced a harmful mutation, but still be able to function because it also has a healthy copy of the gene.
Because most organisms – including you – have two copies of each gene they possess, scientists talk about “dominant” and “recessive” genes to express which one is expressed in the form of a trait.
Brown eyes, for example, is a trait that exhibits complete dominance: someone with a copy of the gene for brown eyes will always have brown eyes. Blue eyes, on the other hand, are recessive: if a copy of the gene for brown eyes is present, the blue-eyed gene will be completely masked.
Other dominant traits in humans include dark hair, dimples, double-jointedness, and immunity to poison ivy. Surprisingly, some rare traits can be dominant. If there are not many copies of a gene in a population, the trait will remain rare, even if the gene for it is dominant.
Examples of Complete Dominance
Eye color is one of the most commonly cited examples of dominant traits. Although eye color is actually influenced by several genes, and eyes can come in many shades of color, a simple dominant/recessive inheritance pattern can be found in blue vs. brown eyes.
This was one of the mysteries that inspired early scientists who thought about how the inheritance of traits works. Why could parents who both had brown eyes give birth to a blue-eyed child? Why did some children of brown-eyed parents have blue eyes, while most did not?
Brown eyes are caused by the production of a brown pigment, melanin, in the iris. People with blue eyes actually have a mutation, where melanin in the retina is not successfully produced.
As a result, people with just one copy of a working melanin-producing gene will be able to produce melanin and will have brown eyes. This also means that brown-eyed people can be “carriers” of the blue-eyed gene, and may have blue-eyed children if the child receives a recessive blue-eyed gene from each parent.
The graphic below illustrates how recessive traits can appear in the children of parents who are both “carriers” of the recessive trait. This illustration using red and white flowers can be equally applied to eye color or other traits with complete dominant/recessive inheritance patterns:
The blue-eyed mutation may have proven useful in northern climates that receive less sunlight than equatorial climates. In Africa, for example, many animals produce dark pigment around their eyes to absorb some light, reducing glare from the sun and allowing more acute vision. But in Europe, many adaptations to enhance low-light vision are found, because Europe receives less direct sunlight even during midday than equatorial Africa.
Because it was useful, the blue-eyed mutation may have spread throughout northern European populations, becoming a common trait even though it was recessive.
Scientists are not yet completely sure that that’s why blue eyes are common in Europe, despite being a recessive mutation, but this theory fits with findings that European Neanderthals may also have developed mutations to assist in low-light vision when they moved north.
It might surprise you to hear that the gene for the most common type of dwarfism – a relatively rare condition in which the bones of the arms and legs are very short – is dominant.
A person with just one copy of the gene for dwarfism will have dwarfism. This means that children of a parent with dwarfism have at least a 50/50 chance of having dwarfism themselves – but it also means that two parents with dwarfism can have a child without dwarfism if both are carriers of the gene for normal limb growth.
This principle of dominant/recessive inheritance can be seen in the Roloff family, the stars of the reality show Little People, Big World. The Roloff parents, both of whom have dwarfism, have two children who have dwarfism, and two children who do not.
Dwarfism may have remained rare despite being a dominant trait because it can confer health problems. In ancient times, for example, people with dwarfism may not have been able to hunt or farm effectively because of their short limbs.
Contrast this to the case of blue eyes, were a recessive trait may have spread to become common because it was helpful to survival.
Gregor Mendel, one of the first people to study inheritance in a scientific way, originated the idea of dominant vs. recessive traits.
He bred pea plants together and observed what kind of offspring different pairings could produce. Some traits, he found, were “dominant” – they were much more likely to be expressed than “recessive” traits. But “recessive” traits could skip generations – two pea plants with the “dominant” trait could have offspring with the “recessive” trait.
Mendel eventually did the math to determine exactly what was going on. He realized that he would see exactly the patterns he was seeing if each pea plant received a copy of a trait from each parent – and “dominant” traits masked the presence of “recessive” copies.
Traits that Mendel identified as dominant in pea plants included:
- Smooth skin
- Yellow peas
- Purple flowers
- Inflated seed pods
- Green pod color
- Axial position of flowers
- Tall stems
Mendel’s systematic breeding of peas finally began to answer the questions people had had about inheritance for a long time. The same principles, applied to humans, could explain why children inherited different traits from their parents, and why some children had traits that their parents had not expressed at all!