The combination of all of the versions of all of the genes in a species is called the gene pool of the species.
Because the DNA of a fruit fly is understood very well, let's use the fruit fly as an example, specifically that of Drosophilia melanogaster. Here are some facts about fruit fly DNA:
- The DNA of a fruit fly is arranged on five chromosomes.
- There are about 250 million base pairs in this DNA.
- There are 13,601 individual genes (reference).
Each gene appears at a certain location on a certain chromosome, and there are two copies of the gene. The location of a particular gene is called the locus of the gene. Each of the two copies of the gene is called an allele.
Let's say we look at locus 1 on chromosome 1 on a particular fruit fly's DNA. There are two alleles at that location, and there are two possibilities for those alleles:
- The two alleles are the same, or homozygous.
- The two alleles are different, or heterozygous.
If we look across a population of 1,000 fruit flies living in a jar, we might identify a total of 20 different alleles that occupy locus 1 on chromosome 1. Those 20 alleles are the gene pool for that locus. The set of all alleles at all loci is the full gene pool for the species.
Over time, the size of a gene pool changes. The gene pool increases when a mutation changes a gene and the mutation survives (see How Evolution Works for details). The gene pool decreases when an allele dies out. For example, let's say that we took the 1,000 fruit flies described in the previous paragraph and selected five of them. These five fruit flies might possess a total of only three alleles at locus 1. If we then let those flies breed and reproduce to the point where the population is once again 1,000, the gene pool of this 1,000 flies is much smaller. At locus 1, there are only three alleles among the 1,000 flies instead of the original 20 alleles.
This is exactly what happens when a species faces extinction. The total population dwindles down to the point where there might be just 100 or 1,000 surviving members of the species. In the process, the number of alleles at each locus shrinks, and the gene pool of the species contracts significantly. If conservation efforts are successful and the species rebounds, then it does so with a much smaller pool of genes to work with than it had originally.
A small gene pool is generally bad for a species because it reduces variation. Let's go back to our fruit fly example. Let's say there are 20 alleles at locus 1, and one of those alleles causes a particular disease when a fly has two copies of that allele (homozygous). Because there are 20 total alleles, the probability of a fly getting two copies of that harmful allele is relatively small. If that harmful allele survives when the gene pool shrinks down to a total of only three alleles, then the probability of flies getting the disease from that allele becomes much larger. A large gene pool provides a good buffer against genetic diseases. Some of the common genetic problems that occur when the gene pool shrinks include:
- Low fertility
- Genetic diseases
The two most common places to see these effects is in animals nearing extinction and in animal breeds.
A lot of care must be taken when breeding animals in order to avoid genetic diseases. When breeding, it is sometimes helpful to outcross. In outcrossing, an animal outside the breed is allowed to mate with an animal inside the breed. The offspring from that mating increase the size of the gene pool, decreasing the probability of genetic diseases being passed on.
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