Population geneticists primarily study allele and genotype frequencies. They use quantitative methods to analyze the frequency of alleles. For example, a population geneticist may study the frequency of certain patterns on the fur of wild cats, then will revisit the same population several generations later to find how the frequency of patterns has changed from the initial measurement. This type of study would give scientists a good idea of what genetic changes are happening in a population.
Darwin’s theory of natural selection posits that only a certain percentage of offspring in any given generation will survive to reproduce. Whether an individual survives or not depends on the inheritance of alleles that will increase its ability to survive in its environment. Alleles that do not benefit an individual and increase its chances at survival will not be passed down to future generations, thus decreasing the frequency of that particular allele in a population. Population geneticists can study changes in allele frequencies from generation to generation in order to determine what mechanisms underlie the natural selection in a given population.
New genetic variations can arise through mutation, gene duplication, exon shuffling, and horizontal gene transfer. Alterations to existing genetic variation can occur by natural selection, random genetic drift, migration, and nonrandom mating.
Nonrandom mating does not change the allele frequencies in populations, unless other evolutionary forces are also present. The other ways of increasing genetic variation will affect the allele frequencies in populations because they tend to increase heterozygosity.
Inbreeding is one form of nonrandom mating. Inbreeding occurs when two genetically related individuals mate and produce offspring. Homozygotes are more likely to be found in populations that have a high degree of inbreeding, due to the decreased genetic variation between breeding pairs. Sometimes, populations that have too much inbreeding will experience what is called an inbreeding depression. An inbreeding depression occurs when homozygotes are less fit to survive in their environment, resulting in decreased reproductive success in the population. Biologists will often intervene to introduce new genetic variation into a population by introducing new individuals, and thus new genes, into the population.
To calculate allele frequencies in a population, divide the number of copies of a particular allele in a population by the the total number of all alleles for that gene in a population.
To calculate genotype frequencies in a population, divide the number of individuals with a particular genotype in a population by the total number of individuals in a population.
For example, consider the following population:
- 49 dark green frogs with the genotype DD
- 42 brown frogs with the genotype Dd
- 9 yellow frogs with the genotype dd
Our population of frogs is diploid, meaning that each frog inherits one allele for a gene from each parent, so each individual has two total alleles for a particular gene. Homozygotes have two copies of the same allele; heterozygotes have one copy of two different alleles. This means that when we calculate our allele frequencies, we have to account for the fact that each individual frog has TWO copies of an allele for each gene.
To calculate the frequency of the r allele, we need to add up how many total d’s we have in our population. Each frog with the Dd genotype has one d, each frog with the rr genotype has two d’s, and each frog with the DD genotype has zero d’s. Once we get the figure for the total number of d’s in a population, we need to divide that by total number of ALL the alleles. We have:
(Dd) + 2(dd) / 2(DD) + 2(Dd) + 2(dd)
Keep in mind that we are multiplying the genotypes by two because we are counting alleles, and each individual has two alleles for each gene. Plugging in our population numbers to the above formula gives us:
42 + (2) (9) / 2 (49) + 2 (42) + 2 (9) = 60 / 200 = 0.3 = 30%
The allele frequency for d is 30%. Since we have only two alleles and each frequency must add up to 100%, we know that our other frequency, D, is 70%.
We can also calculate genotype frequencies. For this exercise, we are counting the genotypes, NOT individual alleles, so we do not need to multiply by two as we did in the above example. If we want to calculate the frequency of dd, we need to find the number of dd individuals in the population, and divide by the total number of individuals in the population. So we have:
dd / DD + Dd + dd
9 / 49 + 42 + 9 = 9 / 100 = 0.09 = 9%
So we know that 9% of the individuals in our population have the dd genotype. If we calculate the frequency of one of the other genotypes, we can add them together and subtract from 100% to find the frequency of the third genotype.
Understanding how to calculate allele and genotype frequencies is vital to understanding the Hardy-Weinberg Equilibrium and being able to use the Hardy-Weinberg equation. Be sure you understand how to do these calculations before you move onto Hardy-Weinberg problems.
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