So, here’s something I don’t understand. I have sickle-cell disease and I’m told it’s a genetic condition, but neither my dad nor my mom have it.
Well, genetic inheritance is a bit more complex than most people imagine it to be. There are different “modes” of inheritance. Sickle-cell disease, for example, is transmitted from parents to child in an “autosomal recessive” fashion.
An auto-whatnow? How are cars involved?
No, “autosomal”. It means the mutation that causes the disease is located on one of the 22 human chromosomes that are not sex chromosomes. These 22 chromosomes are known as “autosomes”.
Oh, I see. So you’re saying that I got this mutation from my dad and my mom, though, right?
So how come they don’t have the disease?
The most likely explanation is that both your parents are “carriers”. You see, we each have two copies of every chromosome: one copy from our dad, another copy from our mom. On one of these chromosomes, there is a gene that, when mutated, will give rise to sickle-cell disease. You need both copies of this gene to be mutated in order to develop the disease.
When you need both copies to be inactivated in this way, the disease is transmitted in a “recessive” manner. Your dad has a mutation on one chromosome but not on the other. Same for your mom. That’s why they don’t have the disease: they each have a working version of this gene and one is enough to prevent the development of symptoms.
Oh, right. So how come I have the disease but not my brother? The doctors told him he wasn’t even a carrier! That makes no sense!
It makes plenty of sense. Making babies is like playing the lottery. Sometimes you get lucky, sometimes you end up wasting your money. Ask your parents. You were conceived when a sperm cell from your dad fertilized one of your mom’s egg cells.
Hey, we’re all sexual beings, even your parents, otherwise you wouldn’t be here, talking to me about your sickle-cell disease, would you? Now, some of your dad’s sperm cells will carry the chromosome that has the mutation, and some of them will carry the other chromosome that isn’t mutated. Same goes for your mom’s egg cells. This means that, when sperm cell #336,778 ends up randomly fertilizing egg cell #249, the resulting embryo could have two, non-mutated copies of this chromosome. That’s your brother. He won the lottery.
Ah, I get it. So I got the bad sperm and the bad egg.
In a manner of speaking, yes. The sperm that created you had the sickle-cell mutation, and so did the egg cell. Your gene does not function properly, which means you express the disease. If your parents had more babies, these younger siblings could very likely be carriers of the disease if either cell carried the mutation but not the other. They would be like your parents: one copy of the gene mutated, one copy healthy, and thus none of the symptoms.
I see. Cool. But what about asthma?
Oh boy. Let’s do that another day. For now, just remember: if a disease is transmitted in an autosomal recessive fashion, both parents need to be carriers and they have a one-in-four chance of producing a child who is affected with the disease.
(I am trying out a new article format to boost my readership and reach the ADHD generation. Thoughts?)
(Feature picture by Ed Uthman)