22andHim

Missing two chromosomes but doing fine. A partial karyotype of a man with 44 chromosomes.

A doctor from China contacted me through this blog with some exciting news. He had found a patient with 44 chromosomes instead of the usual 46. And the patient was perfectly normal as far as anyone could tell.

The doctor contacted me because the story of how this patient ended up with 44 chromosomes mirrored my story of how humans may have gone from 48 to 46 chromosomes a million or so years ago. The idea that human chromosome reduction could happen this way was theoretical when I wrote about it. Now we have living proof that it can and does happen.

Sticking Two Chromosomes Together

At first it might seem weird that losing a couple of chromosomes had no real effect on the patient since losing even one is usually fatal. But his case is different because he didn’t really lose two chromosomes (and all of their essential genes). Instead the chromosomes ended up stuck to two other chromosomes. So he has the same genes…they are just packaged differently.

When this happens with a single chromosome, it is called a balanced translocation. These are more common that you might think with about 1 in 1000 people having one.

The way to end up with 44 chromosomes like our patient requires that both parents have the same balanced translocation. The only way this is at all probable is if the parents are closely related. In this case, they are cousins.

I won’t go into the details (click here to learn more) but these parents had a 1 in 36 chance of having a child with a double balanced translocation. And this is our patient.

From 48 to 46 to 44?

As I said before, a big reason why this is all so interesting is because it provides confirmation of one way that humans may have gone from 48 to 46 chromosomes so many years ago. The first step might have been similar to what happened to our patient. Two closely related parents with the same translocation have a child together that has fewer chromosomes.

Back then, chromosomes 12 and 13 fused together to create what we now call human chromosome 2. The fused chromosome then slowly spread through the community. And then, for some reason, the group of humans with 46 chromosomes eventually supplanted the group with 48.

We can’t know for sure, but this may have happened through some random event where the 48 chromosome humans were mostly wiped out and the humans with 46 chromosomes were spared.  Humanity has nearly been wiped our before with the most recent case being a volcanic eruption 75,000 years ago.

If something similar happens in the future, I wonder if people will be questioning our close relationship to chimpanzees. “How could chimpanzees be our closest relatives,” these future folks might ask, “when we have four fewer chromosomes than they do?”  This assumes, of course, that the number of chromosomes has not changed in chimpanzees by then…

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