What is Genetic variation?

Genetic variation refers to the variation in the genome sequences between individual organisms of a species. Individual differences or population differences can both be referred to as genetic variations. It is primarily caused by mutation, but other factors such as genetic drift and sexual reproduction also play a major role.

Genetic variation in organisms

Individuals within the same species share many features but are not always identical due to genetic variation. It is caused by changes in our deoxyribonucleic acid (DNA). Genetic variation causes genes to take in diverse forms, or alleles. An allele is a term used to describe each variant in a gene. For example, people with blue eyes have one allele of the eye color gene, whereas those with brown eyes have a different allele. Genetic variation permits natural selection to raise or reduce the allele frequencies already present in the population. Therefore, genetic diversity in our genome is considered a powerful factor in evolution. A community benefits from genetic diversity because it allows individuals to adapt to their surroundings.

The causes of genetic variation

• Mutation results in the formation of new alleles within a population
• Non-random mating
• Natural selection
• Gene flow
• Crossing over
• Recombination among homologous chromosomes during meiosis
• Genetic drift
• The bottleneck effect
• The founder effect

The importance of mating patterns in genetic variation

When a population interbreeds, mating may occur non-randomly due to one organism's preference to mate with another depending on particular qualities. Inbreeding and outbreeding are two types of non-random mating. Inbreeding appears when people with nearly identical genotypes mate with each other rather than with others with diverse genotypes. Individuals of one genotype mate with those of another genotype in outbreeding. Outbreeding can result in increased genetic diversity, while inbreeding can lead to decreased genome diversity.

The effect of the bottleneck effect and genetic drift on genetic variations

Genetic drift refers to changes in allele frequency that occur over time and can rise or decrease by chance. Drift can occur in any non-infinite population, but it has a greater impact on smaller populations. As a result of genetic drift, rare alleles can become extinct, and it also allows a new population to be genetically distinct from its parent population, leading to the theory that genetic drift plays a role in the development of a new species. Genetic drift is common after a population bottleneck has been experienced in a population.      

A population bottleneck emerges when a large number of people in a population die or are prevented from reproducing, resulting in a significant reduction in population size. Small populations lose genetic diversity within their genomes faster than large populations due to genetic drift. This is because random chance can cause some variants of a gene to be lost, which is more likely to occur in small populations. 

Effect of founder effect on genetic variations        

The founder effect occurs when a small subset of a larger population starts a new colony, resulting in a reduction in genetic variation. The new population may differ greatly from the original mother population in terms of phenotypes and genetics. In some cases, the founder effect may have a role in the development of new species.

The founder effect is the phenomenon of a small group of people leaving a larger population to establish themselves in a new area. They also retain a portion of genetic information within their genome that was present in the larger population. As a result, by carrying the subset, they are reducing the genetic variation in a new population.

Effect of mutations on genetic variations

A mutation is an alteration in the genetic code in the genome that affects the physical traits or phenotype of the organism. The majority of mutations do not affect the phenotypes. Mutations include even minor changes, such as the substitution of a single nucleotide base with another nucleotide base. Larger mutations, on the other hand, can have an impact on multiple genes on the same chromosome. Mutations can also occur as a result of DNA sequence deletions, insertions, or duplications, as well as substitutions.

Mutations can introduce newer alleles into a population of organisms, increasing genetic variation within the population. Most mutations do not affect phenotypes because they occur in the DNA sequence of a non-coding region of the DNA. Natural selection based on the consequence of a mutation can be a potent evolutionary mechanism. Mutations can also emerge as a result of structural variation in an organism's chromosome.

Effect of gene flow on genetic variation

Gene flow, also known as gene migration, is the movement of genetic material from one species population to another by interbreeding, modifying the gene pool composition of the receiving population. The emergence of new alleles as a result of gene flow promotes population diversity and results in novel trait combinations. Although gene flow does not affect the frequency of alleles throughout a species, it can affect the frequency of alleles in individual populations. 

Effect of recombination

Each organism is made up of a combination of its parents' genetic material. The genetic materials from the parents are combined when homologous DNA strands align and cross during recombination. Recombination effectively shuffles maternal and paternal DNA in daughter germ cells, resulting in unique combinations of variants.  

Genetic variation in humans

The human genome is made up of around 3×10⁹ base pairs of DNA. Except for identical twins, no two people have ever been or will ever be genetically identical. The genetic variance between any two people is approximately.1%. This indicates that between any two people, one out of every 1,000 base pairs will be different. 

The most prevalent polymorphisms or genetic variation in the human genome are single base-pair changes. These differences are known as single-nucleotide polymorphisms (SNPs). When two haploid genomes are compared, SNPs occur at a rate of about once every 1,000 bases on average. Less common types of polymorphism include copy number changes, rearrangements, insertions, deletions, and duplications caused by mutations. 

Single nucleotide polymorphism

An SNP is defined as a change in a single nucleotide between individuals of the same species that impacts at least 1% of the population. Among the 2,504 people investigated for the 1000 Genomes Project, 84.7 million SNPs were discovered. SNPs are the most common type of sequence variation, accounting for 90% of all changes.

Copy-number variation

Copy-number variation refers to a variation in genetic makeup induced by deleting or duplicating substantial regions of DNA on a chromosome. Estimates suggest that 0.4 percent of unrelated human genomes have different copy numbers. Although copy number differences are inherited, they can also occur during development.           

Context and Applications

This topic is significant in genetics, medicine, and the exams at school, graduate, and post-graduate levels, especially for bachelors in biology and biotechnology and masters in biology and biotechnology.

Practice Problems

Question 1: Which of the following is not a cause of evolution?

1. Mutation
2. Gene flow
3. Natural selection
4. None of the above

Answer: Option iv is correct.

Explanation: Mutation, gene flow, and natural selection are the causes of genetic variation, which, thereby, causes evolution. 

Question 2: Each variation in a gene is referred to as ______.

1. chromosome
2. allele
3. locus
4. mutation

Answer: Option ii is correct.

Explanation: Each variation in a gene is referred to as an allele.

Question 3: Genetic drift is more likely to occur in ____.

1. larger populations
2. smaller populations
3. both larger and smaller
4. None

Answer: Option ii is correct.

Explanation: After a group has experienced a population bottleneck, genetic drift is common. A population bottleneck emerges when a large number of people in a population die or are prevented from reproducing, resulting in a significant reduction in population size.

Question 4: When animals transfer from one population to another population, what enhances genetic variation?

1. Genetic drift
2. Mutation
3. Gene flow
4. Founder effect

Answer: Option iii is correct.

Explanation: Gene flow is the movement of genetic material from one species population to another by interbreeding, modifying the gene pool composition of the receiving population. The emergence of new alleles as a result of gene flow promotes population diversity and results in novel trait combinations.

Question 5: Outbreeding leads to ______ genetic variation.

1. increase
2. decrease
3. no effect

Answer: Option i is correct.

Explanation: Outbreeding, can lead to an increase in genetic diversity, while inbreeding can lead to a decrease.