What determines the chromosome count of orga?

Answered by Dr John Welch, Department of Genets, University of Cambridge.

John:: Well, there are two quite different, but perhaps equally important why so much chromosome number can change. But first, sometimes there's a problem when the cell makes copies of itself and that ends in the complete genome doubling, we end up with two complete copies of the genome. This does seem to be an important process in the evolution. It has contributed to change in animals, in fungi, in single-celled protozoa, but it's hugely important in the evolution of plants. The second important way in which chromosome number can change is by two chromosomes fusing into one or one chromosome splitting apart into two. This is something that's happened in our own lineage for example, our own chromosome Two seems to be the result of the fusion between two chromosomes which were present in our most recent common ancestor with chimps and bonobos. The important thing about this type of change in chromosome number is on the whole genetic information that's present in the cell doesn't change very much. And this means that in this case, for the person with the unusual number of chromosomes or the organisms with an unusual number of chromosomes, there doesn't seem to be many consequences at all of changing the way that that information is distributed amongst chromosomes. But then there can be problems for the fertility for that individual when they go on to try and make sperm or eggs, in other words, becoming increasingly clear from experiments is that the reduction in fertility, the partial sterility seems to be much less than we predict. It seems to be only a small 4 or 5 per cent for example, reduction in fertility in individuals with a small number of chromosomes. This means that while the change in chromosome number might be subject, might be selected against natural selection might have to slightly reduce the number of odd numbered chromosomes in the population. This effect can be rather weak and so, just random sampling effects taking place over the generations can mean that the abnormal number of chromosomes reaches a high frequency in the population without weak natural selection being able to counteract this effect, and that's probably an important mechanism by which chromosome number changes in practice.