What does it mean to be a carrier?

One of the things that my biology students often struggle with is what exactly does it mean to be a carrier for a particular trait. This is a difficult concept because it seems odd that we might have the information in our DNA for a trait such as a disease but we don’t actually have the disease. This happens because we have versions of these traits (genes) that are dominant or recessive.

If you have ever been in a room with a dominant personality you have probably seen what can happen. The dominant person speaks up, everyone knows what that individual’s opinion is and probably what they think about anyone who disagrees. There is no place for any other ideas in the room. But that doesn’t mean that those other ideas don’t exist in the room; they are present but nobody can hear them. This is similar to the relationship between dominant and recessive genes. We have two copies of each gene. This is because we have two copies of each chromosome, except for genetic males who have a single x and a single y chromosome (alongside the other 22 pairs of chromosomes). Our chromosomes contain DNA along with some proteins. We get one copy of each of the 23 chromosomes from our genetic dad and one from our genetic mum. This is kind of like getting notes from two different teachers about the same topic. They all have basically the same information but they are different versions.

If you have a dominant version of one of the genes on one of your chromosomes it will show up. Everyone will know what that version is “thinking”. You might have two copies of the dominant version, or you might have one copy of the dominant version and one copy of the recessive version. In either case, you will only ever hear from the dominant one. The recessive version, just like the quiet person in the room, never gets to have its say.

This is a carrier, someone who has one copy of a recessive trait so it doesn’t show up anywhere but it can be passed on to your kids. Again, you have two copies so to show a recessive trait both copies need to be the recessive version, just like if you only have quiet people in a room you can hear their opinions because they aren’t overshadowed by a dominant personality. This means for a kid to show the recessive trait they would need to inherit two recessive versions of the trait, one from each genetic parent.