# Population Genetics Study Guide

Introduction

Rather than focusing on gene segregation within a cross or family, population genetics can be thought of as an extension of Mendelian genetics to the population level. While a single diploid individual may only have two alleles for any given gene, there can be many alleles at different frequencies in a population.

A population is a group of organisms of a single species whose members interact with one another somehow. As a result, a species is usually divided into several populations. The frequencies of different genes amd genotypes in populations can be described in population genetics, and the degrees of genetic diversity can be calculated.

### Gene and Genotype Frequencies

The proportion of distinct alleles of a gene in a random mating population is known as gene frequency, or genetic frequency. Gene frequencies are used to determine the genetic makeup of a population. See how gene frequencies can change between generations in the image below.

Source

Genotype frequency is is the ratio of different genotypes in a Mendelian population, a group of interbreeding organisms that share the same gene pool. Genomic frequency is also known as zygotic frequency.

### Hardy-Weinberg Principle

So how do we calculate genetic gene and genotype frequency? We use two mathematical equations based on the Hardy-Weinberg principle, which states that if there is no selection, mutation, migration, or random genetic drift, the frequency of genes and genotypes in a random mating population remains constant generation after generation.

The first of these two equations is used to estimate the frequency of certain alleles within a population. Its formula is shown below.

• (p + q = 1), where ‘p’ represents the frequency of a dominant allele in a population, and q represents the frequency of a recessive allele in a population.

The second equation is used to determine genotype frequency, and is as follows:

• (p² + 2pq + q² = 1), where ‘p2’ denotes the homozygous dominant genotype (AA), ‘2pq’ denotes the heterozygous genotype (Aa), and ‘q2’ denotes the homozygous recessive genotype (aa).

### Factors that Affect Gene Frequency

There are four main factors that affect the frequency of genes within a population: mutation, gene flow, genetic drift, and natural selection. Let’s go over them below.

Mutation

A mutation is a random change within an organism’s DNA sequence. Gene mutations are the primary source of novel alleles and, as a result, genetic diversity. However the likelihood of a mutation occuring is very low, and thus generally has little impact on gene frequency.

Gene Flow

Gene flow refers to the movement of genes into and out of a population, and is generally determined by the rate of migration. When new individuals migrate to a population, they bring their own unique genes with them. When they interbreed with individuals of the original population, they introduce these unique genes, and thus affect the gene frequency.

Genetic Drift

The term “random drift” or “genetic drift” refers to a shift in gene frequency that generally occurs within small populations. Genetic drift can be caused by a vast number of phenomena, such as sudden immigration or emigration, a population bottleneck, or when a small group of individuals leave the population and start their own colony (also known as the founder effect).

Natural Selection

Natural selection occurs when specific traits offer an advantage or disadvantage in survival and reproductive success. Traits that are beneficial to organisms are selected and passed on to the next generation, while detrimental ones are phased out of the population. Due to this selective pressure, the alleles linked with those characteristics will alter in frequency over time.

### Uses of Population Genetics

• Plant breeders can utilize gene and genotype frequency in a population to assess the competitive capacity of different genotypes in varietal combinations. Such research aids in the discovery of genotypes with high adaptive value that lead to better crops, higher yields, and thus economic growth.

• The relevance of many elements in natural evolution may also be shown by studying gene frequency in a population. The Hardy-Weinberg concept generates composite and synthetic varieties in cross-pollinated crops.

## ✅ Conclusion

• Population genetics is the study of the frequency of genes and genotypes in a Mendelian population.

• A population is a group of organisms of a single species whose members interact with one another somehow.

• The proportion of distinct alleles of a gene in a random mating population is gene frequency.

• The Hardy-Weinberg equations are used to determine gene and genotype frequency.

• Mutation, gene flow, genetic drift, and natural selection are factors that affect gene frequency.

### FAQs

1. What is the main purpose of population genetics?

Population genetics aims to figure out how and why allele and genotype frequencies fluctuate over time within and between populations. The branch of biology gives us the most in-depth and comprehensive understanding of how evolution works.

2. How do you calculate population genetics?

Population genetics are calculated by dividing the number of copies of the gene by the number of times the allele appears in the population. A population’s gene pool comprises all the copies of all the genes in that population.

3. What is a population?

A population is defined as a group of individuals of the same species that live in the same region and interbreed.

4. What Hardy-Weinberg equation is used to calculate gene frequency?

(p + q = 1), where ‘p’ represents the frequency of a dominant allele in a population, and q represents the frequency of a recessive allele in a population.

5. What Hardy-Weinberg equation is used to calculate genotype frequency?

(p² + 2pq + q² = 1), where ‘p2’ denotes the homozygous dominant genotype (AA), ‘2pq’ denotes the heterozygous genotype (Aa), and ‘q2’ denotes the homozygous recessive genotype (aa).

6. What do you call all the genes in a population?

A population’s gene pool is made up of all of the genes and the different alternative or allelic versions of those genes.

7. When factor that affects gene frequency occurs when an individual from one population is introduced to another population?

Gene flow refers to the movement of alleles from one group to another by individual migration.

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## Sources

1. Population Genetics https://flexbooks.ck12.org/cbook/ck-12-biology-flexbook-2.0/section/5.18/primary/lesson/population-genetics-bio/ Accessed on 7 Dec 2021.

2. Population genetics https://en.wikipedia.org/wiki/Population_genetics Accessed on 7 Dec 2021.

3. Population Genetics https://plato.stanford.edu/entries/population-genetics/ Accessed on 7 Dec 2021.

4. Population genetics https://www.britannica.com/science/population-genetics Accessed on 7 Dec 2021.

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