You hear people talk about the "gene pool" all of the time, both seriously and comically. On the serious side, animals nearing extinction can develop problems because of the shrinking gene pool for the species. On the funny side, you may hear people say things like "Get that guy out of the gene pool!"
In this edition of HowStuffWorks, we'll talk about what a gene pool is and how it can grow and shrink.
How Life Works: DNA and Enzymes
To understand a gene pool, you need to know a little bit about genes. If you have read How Cells Work, then you are familiar with the inner workings of the E. coli bacteria and can skip this section. Here's a quick summary to highlight the most important points in How Cells Work:
- A bacterium is a small, single-celled organism. In the case of E. coli, the bacteria are about one-hundredth the size of a typical human cell. You can think of the bacteria as a cell wall (think of the cell wall as a tiny plastic bag) filled with various proteins, enzymes and other molecules, plus a long strand of DNA, all floating in water.
- The DNA strand in E. coli contains about 4 million base pairs, and these base pairs are organized into about 1,000 genes. A gene is simply a template for a protein, and often these proteins are enzymes.
- An enzyme is a protein that speeds up a particular chemical reaction. For example, one of the 1,000 enzymes in an E. coli's DNA might know how to break a maltose molecule (a simple sugar) into its two glucose molecules. That is all that that particular enzyme can do, but that action is important when an E. coli is eating maltose. Once the maltose is broken into glucose, other enzymes act on the glucose molecules to turn them into energy for the cell to use.
- To make an enzyme that it needs, the chemical mechanisms inside an E. coli cell make a copy of a gene from the DNA strand and use this template to form the enzyme. The E. coli might have thousands of copies of some enzymes floating around inside it, and only a few copies of others. The collection of 1,000 or so different types of enzymes floating in the cell makes all of the cell's chemistry possible. This chemistry makes the cell "alive" -- it allows the E. coli to sense food, move around, eat and reproduce. See How Cells Work for more details.
You can see that, in any living cell, DNA helps create enzymes, and enzymes create the chemical reactions that are "life."
Bacteria reproduce asexually. This means that, when a bacteria cell splits, both halves of the split are identical -- they contain exactly the same DNA. The offspring is a clone of the parent.
As explained in How Human Reproduction Works, higher organisms like plants, insects and animals reproduce sexually, and this process makes the actions of evolution more interesting. Sexual reproduction can create a tremendous amount of variation within a species. For example, if two parents have multiple children, all of the children can be remarkably different. Two brothers can have different hair color, different heights, different blood types and so on. Here's why that happens:
Because of the random nature of gene selection, each child gets a different mix of genes from the DNA of the mother and father. This is why children from the same parents can have so many differences.
- Instead of a long loop of DNA like a bacterium, cells of plants and animals have chromosomes that hold the DNA strands. Humans have 23 pairs of chromosomes, for a total of 46 chromosomes. Fruit flies have five pairs. Dogs have 39 pairs, and some plants have as many as 100.
The human chromosomes hold the DNA of the human genome. Each parent contributes 23 chromosomes.
- Chromosomes come in pairs. Each chromosome is a tightly packed strand of DNA. There are two strands of DNA joined together at the centromere to form an X-shaped structure. One strand comes from the mother and one from the father.
- Because there are two strands of DNA, it means that animals have two copies of every gene, rather than one copy as in an E. coli cell.
Photo courtesy U.S. DOE, Human Genome Project
- When a female creates an egg or a male creates a sperm, the two strands of DNA must combine into a single strand. The sperm and egg from the mother and father each contribute one copy of each chromosome. They meet to give the new child two copies of each gene.
- To form the single strand in the sperm or egg, one or the other copy of each gene is randomly chosen. One or the other gene from the pair of genes in each chromosome gets passed on to the child.
A gene is nothing but a template for creating an enzyme. This means that, in any plant or animal, there are actually two templates for every enzyme. In some cases, the two templates are the same (homozygous), but in many cases the two templates are different (heterozygous).
Here is a well-known example from pea plants. Peas can be tall or short. The difference comes, according to Carol Deppe in the book "Breed your own Vegetable Varieties":
...in the synthesis of a plant hormone called gibberellin. The "tall" version of the gene is normally the form that is found in the wild. The "short" version, in many cases, has a less active form of one of the enzymes involved in the synthesis of the hormone, so the plants are shorter. We refer to two genes as alleles of each other when they are inherited as alternatives to each other. In molecular terms, alleles are different forms of the same gene. There can be more than two alleles of a gene in a population of organisms. But any given organism has only two alleles at the most. Shorter plants usually cannot compete with the taller forms in the wild. A short mutant in a patch of tall plants would be shaded out. That problem isn't relevant when a human plants a patch or field with nothing but short plants. And short plants may be earlier than tall ones, or less subject to lodging (falling over) in the rain or wind. They also may have a higher proportion of grain to the rest of the plant. So shorter plants can be advantageous as cultivated crops. Specific mutations or alleles are not good or bad in and of themselves, but only within a certain context. An allele that promotes better growth in hot weather may promote inferior growth in cold weather, for example.
One thing to notice in Deppe's quote is that a mutation in a single gene may have no effect on an organism, or its offspring, or its offspring's offspring. For example, imagine an animal that has two identical copies of a gene in one allele. A mutation changes one of the two genes in a harmful way. Assume that a child receives this mutant gene from the father. The mother contributes a normal gene, so it may have no effect on the child (as in the case of the "short" pea gene). The mutant gene might persist through many generations and never be noticed until, at some point, both parents of a child contribute a copy of the mutant gene. At that point, taking the example from Deppe's quote, you might get a short pea plant because the plant does not form the normal amount of gibberellin.
Another thing to notice is that many different forms of a gene can be floating around in a species.
Understanding the Gene Pool
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. 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:
The two most common places to see these effects is in animals nearing extinction and in animal breeds.
- Low fertility
- Genetic diseases
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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