Genetic variation refers to variation (change or difference) of the gene among the members of the same species or individuals.
Genotype is the total sum of genetic determinants carried by a cell that is transmitted from generation to generation. Genotype is determined by the genetic information contained in the entire DNA content of the genome in the chromosome.
Genes carry instructions that are used for building protein. Differences in DNA sequence or gene between individuals are also known as genetic variations, and each variation of a gene is called an allele.
Genetic variation occurs in entire species, known as genetic diversity. Genetic variation is essential for the natural selection process because natural selection can only decrease or increase the allelic frequency already present in the population.
Genetic variation plays an advantageous role in the population because it favors some individuals to adapt to the environment. Mutation, gene flow, sexual reproduction, random mating between organisms, random fertilization, and crossing over are the main cause of genetic variation. Genetic variation along with environmental variation causes phenotypic variation in a population. An example of phenotypic variation is the height of a plant.
Main source or cause of genetic variation
- A mutation is a heritable change in the sequence of nucleotides in an individual’s genome.
- Entire chromosomes of single nucleotides can be altered by mutation, and altered base sequences are a source of new alleles.
- A strain of any cell carrying a change in base sequences is called a mutation.
- A mutant differs from its mother strain in its genotype, the nucleotide sequence of the genome.
- New alleles are formed by mutation; without diversity, evolution is impossible.
- The reproductive rate is directly proportional to the rate of mutation.
- Chemical agents (5-bromouracil, 2-aminopurine), biological agents, and physical agents (ionizing radiation or ultraviolet, non-ionizing radiation) are can cause mutation.
In general, a mutation occurs in two types.
- They are those that are due to agents in the environment.
- Induced mutation occurs from exposure to natural radiation (cosmic rays and so on) that changes the sequence of a base in the DNA.
- Different chemicals, including oxygen radicals, can chemically modify DNA.
- They are those that occur from occasional errors in the pairing bases during DNA replication.
- Point mutation: a mutation that changes only one base pair is called a point mutation.
- Transition: it comprises the substitution of one pyrimidine by another. (Example: GC pair replace with AT pair and vice versa)
- Transversion: a purine is replaced by a pyrimidine or vice versa. (Example: AT pair replaced by TA or CG pair)
- Insertion or deletion (frameshift mutation): any deletion or insertion of a single base pair results in a shift in the reading frame.
- Migration of genes is known as gene flow, resulting introduce a new gene (changing the composition of a gene) into a population from one species to another.
- Gene can be changed by the transfer of genetic material from one species to another. In such instances, the transferred DNA either recombines with a genome of the recipient species or is in a plasmid capable of replication in the recipient without recombination.
- The availability of new alleles in the gene pole makes new gene combinations possible.
- Moving out of organisms from a population (emigration) also alters the gene frequency.
- New organisms immigrating into a population may play an important role in favoring organisms to adapt to changing environmental conditions.
- The migration results in a lack of genetic diversity.
- Gene flow may be good for the new population, as genetic diversity tends to help species survive.
- Allopatric speciation tends to occur when gene flow is interrupted by physical barriers.
- Sexual reproduction leads to the formation of a new combination of genes.
- Species with a rapid reproduction rate will likely have high genetic variation by producing different gene combinations.
- In sexual reproduction, gametes or sex cells are produced by meiosis.
- Genetic variation occurs when alleles in the sex cells (gamete) are separated and haphazardly mated upon fertilization.
- In meiosis, the crossing over of homologous chromosomes also results in the genetic recombination of a gene.
- Genetic drift is a mechanism of producing new characters by random alternation in the frequency of an allele in a population.
- It has a drastic effect on a small population.
- The effect of genetic drifting is the increase in differentiation among a population where the same species may have different alleles due to genetic drift.
- The converse of natural selection is genetic drift.
Other causes of genetic variation are:
- Random mating: hereditary and environmental interaction does not influence the unity between two individuals.
- Crossing over: the exchange of genes between homologous chromosomes, resulting in a mixture of mother characteristics in daughter individuals.
- Non-random fertilization: if one species(individuals) non-randomly mates with the other within a population. That means mates are selected by them; selection can drive evolution in a population.
- Random fertilization: random fertilization of male and female gamete in sexual reproduction.
- Environmental variance: some harmful environmental factors such as ultraviolet rays can change DNA nucleoid sequences.
Types of genetic variation
Generally, genetic variations are three types:
a. Single nucleotide variation
- It is the most common type of variation where one nucleotide sequence is replaced by another.
- Transition (purine to purine and pyrimidine to pyrimidine) and transversion (purine to pyrimidine or vice versa) are types of single nucleotide variation.
- Single nucleotide variation in the coding region may affect the protein’s functional and structural aspects.
- If the frequency of single nucleotide variation is greater than or equal to 1%, then it is called single nucleotide polymorphism. It may occur with the non-coding or coding region of genes.
b. Insertion or deletion
- DNA mutation involves the deletion or insertion of one or more nucleotide base pairs.
- These can have devastating effects on the gene because either amino acids are deleted, or the translation of the gene is frame shifted. For example, the most common cause of cystic fibrosis is the deletion of 3base pair in the codding region of the Cystic fibrosis transmembrane conductance regulator (CFTR).
c. Structural variation
- Large-scale structural change in the DNA is called structural variation and originated mainly due to chromosomal rearrangements such as deletion, insertion, duplication, or inversion.
- They include chromosomal rearrangement, copy number variation, and a variable number of tandem repeats.
- Copy number variation occurs due to complex gain or loss of homologous sequence at various sites in the genome. Several copies of a particular segment are altered in two or more different genomes as compared to the reference genome.
- A variable number of tandem repeats variation involves a change in the number of repeated DNA sequences arranged in a tandem array.
Examples of genetic variation
- Person blood types, skin color, eye color, hair color, dimples, and freckles are examples of genetic variations that can occur in the human population.
- The Colour of the human eye depends on a person’s gene. Many of the genes linked with eye color are involved in the production, storage, and transport of pigment, known as melanin.
- Mutation, sexual reproduction, and gene flow cause genetic variation, variation genes produce different amounts of melanin which determine eye color as people with brown eyes have a higher amount of melanin in the iris, but people with blue eyes have less amount of melanin pigment.
- The development of flowers that resemble insects to lure plant pollination and modified leaves of carnivorous plants are an example of genetic variation of plants.
- A snake that flies, albinism, animal that play dead, cheetahs with stripes, and animals that mimic leaves are an example of genetic variation in animals.
Why genetic variation is important?
- Alteration of DNA of species offers a better adaptation to the constantly changing environment; a diverse gene pole is good for long-time survival of species since the environment is always changing.
- A population cannot evolve in response to changing environmental agents without genetic variation and, as a result, may face an increased risk of extinction.
- For natural selection, genetic variation is important because it changes a protein’s gene activity or function and can introduce a different trait in an organism. if these traits are beneficial and help the individuals reproduce and survive, the genetic variation is more likely to be passed on to the next generation.
- Verma, P. S., & Agrawal, V. K. (2006). Cell Biology, Genetics, Molecular Biology, Evolution & Ecology (1 ed.). S . Chand and company Ltd.
- Lewin B (2007), Genes IX, Oxford University Press, and Cell Press.
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How random mating and crossing over cause genetic variation.