Chromosomagnon man: what's a chromosome or two among friends?

Natural History, June, 2008 by Olivia Judson

In 1921, the American zoologist Theophilus S. Painter reported that humans have forty-eight chromosomes. That was a big upwards revision: at the time, most people thought the correct number was twenty-four, and previous counts had been as low as sixteen (the same as onions). Painter was the first to analyze material taken from flesh human cells and prepared before deterioration and necrosis could cause the chromosomes to clump. The cells came from the testicles of castrated inmates of the Texas State Insane Asylum (that was at a time when medical ethics were in their infancy). Painter's number was rapidly accepted as reliable. In fact, it wasn't: humans have forty-six chromosomes. Yet for more than thirty years, everybody who examined human chromosomes again said they saw forty-eight, or figured they had counted wrong.

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When I first heard the story, I thought it ludicrous. How could everyone have miscounted for so long? Was it the power of suggestion--you expect to see forty-eight, so you do? But then I looked at a photograph of the early data, and I understood the reason for the mistake: the chromosomes were piled one on top of the other, and it was hard to see where one ended and another began. The technology simply wasn't good enough to ensure an accurate count until the mid-1950s.

Nonetheless, the way we suddenly "lost" two chromosomes neatly mirrors an evolutionary tale. For chromosome numbers evolve, and at some point after the human lineage split off from the one that gave rise to chimpanzees, our chromosome number dropped from forty-eight to forty-six. Chimpanzees, and our more distant cousins the gorillas, still have forty-eight.

Chromosomes are essentially long strings of DNA. They contain protein-coding genes, regulatory genes, so-called junk DNA, and special structures such as telomeres (which prevent the ends of the chromosomes from unraveling) and centromeres (handles for moving chromosomes around during cell division). But why do we have chromosomes at all? Why not string all our DNA together into one gigantic chain? That's what lots of bacteria do: they have a single, circular chromosome, which lacks many of the features that ours have. But eukaryotes--organisms such as animals, fungi, algae, and plants--keep their DNA in a cell nucleus and break it up into multiple, linear chromosomes.

There are at least a couple of reasons for this. First, a bacterium has only one parent; in order to reproduce, it just splits into two. Eukaryotes generally have two parents, so the minimum chromosome number would, in any case, be two. I've only heard of two animals with only two--a roundworm called Parascaris equorum that lives in the guts of horses, and an Australian ant, Myrmecia pilosula. Second, eukaryotic genomes tend to be much bigger than bacterial genomes. It's probably easier for the cellular machinery to manipulate sets of smaller chromosomes than one colossal one, which would likely be prone to breaking.

Human chromosomes normally come in pairs, one in each pair inherited from the mother, the other from the father. They are numbered 1 through 22 if they come from a matching pair; and then there are the mismatched "sex" chromosomes, labeled X and Y. Many other species have a comparable arrangement, but not all. In some spiders, for instance, females have three different pairs of X chromosomes and males have three X's but no Y chromosomes at all--so females end up with three more chromosomes than males do.

You might imagine that it matters in some obvious way how many chromosomes a species has--that the more complex the organism or the bigger its genome, for example, the greater the tally of chromosomes. Yet birds have tiny genomes strung out over many more chromosomes than ours, and a species of fern with a relatively simple genetic array has hundreds of chromosomes. Moreover, while some groups show little evolution in chromosome numbers, others vary immensely. Of the hundreds of dragonfly species that have had their chromosomes counted, for instance, almost all have twenty-six; yet species in one genus of blue butterflies, Agrodiaetus, can have as few as twenty or as many as 268. And doubling of all or part of the genome, leading to a sudden large jump in chromosome numbers, has been a common occurrence in plant evolution.

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If you were to select 20,000 people at random and count their chromosomes, you'd find that most--but not all--of them have forty-six. Owing to an accident of cell division, for instance, people with Down syndrome usually have forty-seven chromosomes, having inherited three copies of chromosome 21. Similarly, people with Turner syndrome typically have forty-five, because they are missing one of the sex chromosomes.

Others have an unusual number for a different reason--two of their chromosomes have gotten stuck together, end-to-end. In those cases, however, individuals typically have the right total amount of genetic information, so they look and act just like everyone else.