SIX years after it was “officially” sequenced, the human genome continues to throw up surprises. The latest, reported in the scientific journal Nature by a large, multinational team of geneticists, is that individual human genomes are more varied than had previously been realised.

The original genome projects (there were two; one publicly funded and one private), worked with DNA from a small number of individuals in order to provide an agreed sequence that could act as a platform for further research. It was, therefore, never quite “the” human genome. Since then, one task for researchers has been to see how much variation there is from person to person.

To find out how much things vary between people, they had previously focused on places in the string of molecular “letters” of which DNA is made where one letter is replaced by another in a significant fraction of the population. (These are known as single-nucleotide polymorphisms.) But it has been known for a long time that there is another sort of variation around. During the process by which DNA is copied, entire blocks of DNA can be accidentally deleted or multiplied. (Following such multiplied and deleted regions between the generations was an important genetic technique before the invention of cheap and rapid gene-sequencing technology.)

What the team has shown in the latest report is that duplication and deletion are much more widespread than was previously realised. Also, the duplication and deletion often involve active genes as well as the so-called non-coding part of the DNA, which is not translated into the protein molecules that keep cells running.

Does all that matter? The absence of a gene has obvious implications. Unless it is covered up by the presence of that gene on a sister chromosome (for chromosomes come in pairs; one from the mother and one from the father), trouble is likely to ensue. Multiple copies of a gene may bring more subtle problems. Genes pass their orders to the rest of the cell via messenger molecules copied from their DNA. Too many copies of a gene might mean too many messengers and thus too much protein. That might, in turn, cause disease.

DNA duplication of the sort previously known about can cause illness. A number of brain diseases, most famously Huntington's chorea, are caused by a genetic “stutter” in which three genetic letters get repeated more often than they should be. One curious feature of such “triplet-repeat expansions” is that once established, they tend to get worse down the generations. For reasons not yet understood, the number of repeats grows each time new sex cells are generated.

Certain diseases are similarly associated with multiple genes. It has been known for decades that one group of blood diseases is linked to unusual numbers of copies of one of the genes involved in making haemoglobin, although there is no suggestion that these diseases, too, get worse down the generations.

However, the team has discovered several hundred genes whose copy number varies, and they suspect there may be more to come. If copy numbers do shift between the generations, perhaps because the genes in question are in places that the DNA-copying machinery finds difficult to handle, that could explain some odd disease patterns that seem to run in families, but do not obey the normal laws of genetic inheritance.

This is speculation, of course. But it gives medical geneticists a new lead. And generating new leads was the whole point of the human genome projects in the first place.