What are “Dominant” and “Recessive” Traits? The Rundown of Heredity
Having six fingers is a dominant trait! This fun fact leaves some people puzzled — that means if you have it, your kids will have it, right? Shouldn’t it be more common in the population than having five fingers, then? Does this mean it’s somehow advantageous to have an extra finger?
I’ve heard all these asked, and the answers are no, no, and no. Clearly, while many of us have heard the terms “dominant” and “recessive,” there’s not much clarity about what they mean. This article serves as a review.
In classical genetics, traits —passed-down physical characteristics — are encoded by genes. For each gene, you get a copy from your mother and a copy from your father. (These copies stay separate; they don’t blend together.)
If you have only one copy of a gene’s dominant version, the trait will show up. It will appear the same whether you get it from one parent or both. The recessive version is the one that’s “masked” when a dominant version is present, so for its trait to be expressed, you need two copies, one from your father and one from your mother.
Returning to the “six fingers” example, let’s say someone has one copy of the gene variant leading to an extra finger from one parent and one copy of the ordinary finger development gene from the other parent. She will have six fingers — one copy is all she needs for this to happen. Then, when she has a child with someone with ordinary hands, the father can only pass on the ordinary gene variant, but she could pass on the ordinary or six-finger variant. If she passes on the dominant six-finger variant, the child will have six fingers. So there is a 50/50 chance the child will get this trait. (And the child who doesn’t get the six-finger variant has no way to pass it on to the grandchildren.)
This trait is uncommon just because most people don’t have this genetic variant. Having six fingers doesn’t increase or decrease someone’s odds of surviving and reproducing, so its low frequency in the population shouldn’t change much over the generations.
Just because a variant is dominant doesn’t mean it’s better for survival, but dominant gene variants that are harmful to survival tend to die out. If people with a harmful mutation die before they can have children, they can’t pass on that mutant gene variant — in other words, it can’t continue in the gene pool. (The classic example of a deadly disorder that’s dominant is Huntington’s disease; it typically starts to set in when people are 30–50 years old, after they’ve likely already had children.)
That’s why most serious genetic diseases are recessive. A person with two copies of the mutation leading to Tay-Sachs disease will usually die in childhood, but a person with only one copy will survive to reproductive age just like anyone else, able to pass it on and keep the mutation in the gene pool.
In those cases, I’m only talking about genes that aren’t on the sex chromosomes. The X chromosome has a lot of genes, important for both sexes, that the Y chromosome doesn’t have. So if you’re an XX female, you have two copies of these genes, and if you’re an XY male, you only have one copy.
Therefore, recessive X-linked traits are more common in males. One example of this is red-green colorblindness; the gene encoding the color photoreceptors in the eye is on the X chromosome. An XX female with one mutant copy can still perceive the full spectrum of color, thanks to the functional copy on her other X chromosome. But if an XY male gets that mutant copy on his X chromosome, that’s all he gets. He doesn’t have the functional copy, so he turns out colorblind.
(Clearly, if an XX mother is affected by an X-linked recessive trait, her sons must all have it — they can only get their X from her, after all. And if an XY father has it, his daughters must all be carriers, because he only has that X to give them.)
Other Forms of Dominance
In the examples above, there are two possible outcomes: a dominant trait or a recessive trait. A single dominant version of a gene is all it takes for the “dominant” trait to show up, and it doesn’t matter whether you get it from one parent or both, because it will appear the same.
Some genes, however, show incomplete dominance: the result of having one dominant copy and one recessive copy is something in between the two outcomes. For example, if a flower inherits one copy of the petal-color gene that codes for red petals, and a variant copy coding for white petals, its petals will be pink. (This isn’t due to blended genes; the pink flower will still pass on either red or white to its offspring.)
Harmful recessive gene versions can be beneficial with only one copy, a phenomenon called “heterozygote advantage.” Sickle-cell anemia, for example, is common in malaria-stricken regions because people with only one copy show greater malaria resistance. People with this mutation have better chances to survive and reproduce, but if two reproduce with each other, there is a chance their offspring will have sickle-cell anemia (which, if untreated, can cause premature death).
Codominance occurs when two variants of a gene can appear with a single copy each. A-type blood and B-type blood are both dominant over O-type blood; if one gets an O from one parent and an A from the other, the blood type will be A, the same as if one got the A type from both parents. The A variant and B variant are codominant: the combination of them creates distinct AB-type blood.
And not all genetic traits are determined by variants of the same gene. Very often, the effects of multiple genes contribute to a single trait.
A simple example is the two-gene interaction of coat color in Labrador retrievers. Whether a Lab is black, brown, or golden depends on both a gene for pigment production and a different gene for pigment incorporation. The dominant version of both leads to a black coat. The recessive mutation in the pigment production gene leads to a lighter brown coat, as long as the dog still has the functional gene to incorporate the pigment. But if the dog carries two copies of the recessive mutation in the pigment incorporation gene, the coat will be golden.
Many inherited traits — such as height, skin color, or intelligence — are known as complex traits, influenced by a variety of genes. And although some diseases, such as Huntington’s and Tay-Sachs, are pinpointed to a single gene, it’s more typical for a disease to be associated with different genes that increase the risk together, but do not simply cause the disease by themselves, as is the case with cancer, diabetes, and Alzheimer’s disease.
It’s clear that everyone is a unique mix of their mother and their father. Discoveries in genetics revealed how offspring get certain traits — namely, they get a copy of each gene from each parent. Some traits will show up due to just one gene copy, some require copies from both parents in order to show up, and some have a more complex expression.
And we figured this out even before the discovery of DNA! Now, researchers can study the chemical sequence of mutations and see how they lead to their resulting traits. As the science progresses, we’re getting closer to unraveling the mysteries of our differences.