Many people have trouble believing that chromosome number can change and stay changed in a species. Their first thought is often of Down syndrome or the other problems that usually come with missing or extra chromosomes. It can be hard to imagine how a living thing could end up with a new chromosome number without these problems.
And yet it happens all the time in creatures as varied as yeast, corn, butterflies, voles and even mice. And now it has been seen in people.In a recent report, a doctor in China has identified a man who has 44 chromosomes instead of the usual 46. Except for his different number of chromosomes, this man is perfectly normal in every measurable way.
His chromosomes are arranged in a stable way that could be passed on if he met a nice girl who had 44 chromosomes too. And this would certainly be possible in the future given his family history.
But why doesn't he have any problems? A loss of one let alone two chromosomes is almost always fatal because so many essential genes are lost. In this case, he has fewer chromosomes but is actually missing very few genes. Instead, he has two chromosomes stuck to two other chromosomes. More specifically, both his chromosome 14's are stuck to his chromosome 15's. So he has almost all the same genes as any other person. He just has them packaged a bit differently.
This is an important finding because it tells us about a key genetic event in human prehistory. All the evidence points to humans, like their relatives the chimpanzees, having 48 chromosomes a million or so years ago. Nowadays most humans have 46.
What happened to this 44 chromosome man shows one way that the first step in this sort of change might have happened in our past. Scientists could certainly predict something like this. But now there is proof that it can actually happen.