HanJ. C. asked:
I recently tuned in your naked genetics podcast. Great job! Thank you!
Got a "burning" question: Dominant/recessive alleles of a gene was studied long before gene's molecular structure was known. What is the current understanding of molecular/physical basis for the dominance/recessiveness of the alleles of the same gene. Are the molecular structures of the alleles different? Moreover, how does the gene instruct the cellular apparatus to use the dominant allele (w) instead of the recessive one ( ) to produce proteins and whatnot?
Looking forward to hearing your enlightenment.
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, itís very, very complicated. A lot of us learn about genetics through things like Mendelís 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. Thatís because the red version of the pigment gene is making a pigment. Itís the red pigment that colours the flowers and the white version of the gene is actually broken. Itís not making any pigment. So since you have a working gene that makes pigment Ė the red one Ė it will be dominant.
Chris - So, itís 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 thatís 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, itís a very oversimplified Ė 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, ďOh well, this colour should be dominant over thatĒ but itís 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. Letís 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 thatís 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 donít 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, weíre starting to look at more and more peopleís genomes, do more and more genetic sequencing. Weíre discovering and that lots of people are walking around with two copies of ďbroken genesĒ Ė for want of a better word Ė 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.