How do dominant and recessive genes work?

Chris Smith put this to geneticist Kat Arney...

Kat - So, this is a really great question because quite often when we learn about genetics in school, we learn about the idea that there are dominant genes and there are recessive genes. Actually in real life, it's very, very complicated. A lot of us learn about genetics through things like Mendel's pea plant. So you have red flowers, or purple flowers, and white flowers. If you cross a red plant with a white plant, you tend to get red plants because the red gene is dominant. That's because the red version of the pigment gene is making a pigment. It's the red pigment that colours the flowers and the white version of the gene is actually broken. It's not making any pigment. So since you have a working gene that makes pigment - the red one - it will be dominant.

Chris - So, it's like a blank canvass. The white one gives you a blank canvass. The red is flicking paint at the canvass.

Kat - Exactly. So usually like a recessive gene or a recessive version of a gene is one that's not working or not working properly, or not working very well. But actually, I've been doing a lot of research lately talking to a lot of geneticists, it's a very oversimplified - this idea of dominant and recessive. You can see it when you look at things like hair colour or eye colour in your family, you might think, "Oh well, this colour should be dominant over that" but it's a bit all complicated than that. So yes, there are some traits where if a gene is active and making something like a pigment or a molecule, or even making a faulty and overactive molecule, then that will be dominant. If something is recessive, then it is not making something very well. But yeah, actually, there's lots and lots of genes contributing to lots of traits in things like height, intelligence, even many, many diseases. So, the idea of something being very simply dominant or very simply recessive is not a helpful idea for many, many traits in diseases.

Chris - The definition I saw when I had my first genetics textbook was that a recessive gene is one which is clinically manifest only in the homozygous state. Let's put that into plain English. You need to have two copies of that gene present in order to see its effect because if there's another gene there that isn't recessive, it will sort of trump the genetic one and you'll see the manifestation of that gene.

Kat - Exactly. Usually, recessive genes is something that's not working properly. So a great example is cystic fibrosis. If someone inherits two faulty copies of a gene called CFTR then it means they don't make this molecule that shuttles salt in and out of their cells, in their lungs, in their guts, and so they have all these problems? But actually, what's really interesting is that now, we're starting to look at more and more people's genomes, do more and more genetic sequencing. We're discovering and that lots of people are walking around with two copies of "broken genes" - for want of a better word - but they're okay. So really, this concept of dominant and recessive, I think we need to really rethink it the more we discover about our genomes.