A gene pool is the totality of genetic information in a species. Gene pools vary in size and can grow and shrink. Warning bells sound when a gene pool diminishes too much, as it signals that a species is threatened with extinction.
We are all members of a gene pool, the human gene pool. You may have heard of the term, particularly in light-hearted conversation. The Darwin Awards are famously published every year to chronicle those that have done the most for our gene pool by removing themselves from it. These are the bizarre, stupid and frankly avoidable ways that some people lose the ability to breathe - i.e. die.
Dive into the Gene Pool
The human gene pool is the combination of all the versions of all the genes we possess. There are 25,000-30,000 individual genes; and at each location where a gene resides, there are two versions of that gene, known as alleles. The gene pool increases every time there's a genetic mutation and that new mutation survives to the next generation. The gene pool diminishes when an allele is lost, or dies out.
Let's break this down still further.
Take a chromosome, let's say chromosome three. Each gene sits at a particular location on that chromosome, called a locus. So a gene at a particular locus on chromosome three will be there in two different versions - alleles. Those alleles may be the same - homozygous - or they may be different - heterozygous. Now let's zoom out to the whole human population and there may be dozens of different alleles that occupy this particular location on chromosome three. The human gene pool is therefore the sum total of all the alleles at all the chromosomal locations, or loci. The bigger the gene pool, the greater the genetic diversity.
Changes to the Gene Pool
It's bad news for a species when a gene pool drastically diminishes. It leads to deformity, more genetic diseases and if the species doesn't recover, extinction. In fact, low genetic diversity is a reason Tasmanian Devils are considered an endangered species.
Taking our example of a gene at a particular locus on a chromosome this is how such a scenario would work. Let's say there are 50 allelic versions of a gene. A mutation in one allele will only cause a disease if two mutated copies are inherited. With so many alleles, the chances of two mutations being passed simultaneously to the next generation are reasonably slim. A large gene pool is a good defence against genetic mutations. However, if the gene pool was reduced, such that there were only 6 allelic versions, the chances of the disorder appearing in the offspring are very great indeed.