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Genetic Variation: Importance, Sources, and Examples

Genetic Variation: Importance, Sources, and Examples

Genes are the molecular unit of heredity. In an organism, the gene sequence may occur in one or more forms, and these alternate forms of the same gene are called alleles. Any type of change that is observed in the sequence of these alleles or in the structure of the genes gives rise to genetic variation in the genome of that organism. This BiologyWise article explains the concept of genetic variation.
Komal B. Patil
The International HapMap project (2002-2009) was a collaborative effort by Canada, China, Japan, Nigeria, U.K., and the U.S. that aimed at establishing a haplotype map of the human genome, which would help in studying the patterns of human genetic variation.
In the case of any species of an organism, all individuals of that species are genetically similar, i.e., a majority of their genome is identical. Despite this, the term "similar" is used instead of "same" because within the species, there are genetic differences between the genomes of the individuals. These differences are significant enough to separate one individual from the other, but not significant enough for the individuals to form a different species altogether. This difference in the genome of this same-species organisms is called genetic variation. All organisms display this, and it can be witnessed not only at the cellular and organismal level, but also at the level of the population of that particular organism. The National Human Genome Research Institute defines it as, "diversity in gene frequencies".

This concept can be better understood using the example of humans. Despite the diverse range of physical statures, skin colors, facial features, hair colors, body types, etc., all humans share 99.9% of their DNA, i.e., if one were to compare the genome of any two random people, their genomic sequence would be 99.9% identical. Therefore, this implies that all the differences that are observed between people arise out of only 0.1% change in the DNA sequence. Hence, it can be stated that the human population displays 0.1% genetic variation within its population.

Sources of Genetic Variation

It is the change in nucleotide sequence of the DNA of an organism. The mutation may only affect the sequence of the gene or it may cause a change in the structure. They arise due to external damage to the DNA, internal errors in replication and repair, or due to transposable elements. Depending on the origin, they are classified accordingly into various types and subtypes of mutation. Despite their diversity, they all culminate in a common effect on the genome of an individual, i.e, they introduce variation in the gene sequence, which may further lead to desirable or undesirable effects in the individual. These mutations may either prove beneficial or harmful to the organism, and eventually the population, depending on the site of mutation. For example, if a mutation occurs in the LMNA gene (human) that codes for the protein that provides support to the nucleus in a cell, the protein thus formed malfunctions, thereby leading to the occurrence of a health disorder called progeria, in humans. On the other hand, if a mutation occurs in the MC1R gene in humans, it leads to the presence of red hair.

Meiotic Divisions
During the prophase I stage of meiosis, genetic recombination occurs via the crossing over of the arms of homologous chromosomes. Crossing over basically refers to the exchange of genetic material between these chromosomes, and it occurs when the arms of sister chromatids overlap during the process of synapsis (segregation of homologous chromosomes). This overlapping leads to the formation of a chiasmata, which allows the DNA on both arms to interact with each other, and trade places. This introduces variation in the genome since the change in location of the gene affects the way it is inherited in the offspring (law of recombination). For example, if we consider two genes A and B that are jointly inherited due to their close proximity and a crossing over event causes the gene A to be shifted to a new location on the sister chromatid, then these two genes will no longer be inherited together, instead they will be inherited independent of each other since they are spatially separated. This independent inheritance would lead to a variation in the genome of the organism.

Sexual Reproduction
All organisms that reproduce sexually, do so by the fusion of female and male gametes. Each gamete is a haploid cell with only half the number of chromosomes that a normal somatic cell possesses. In case of humans, since each somatic cell possesses 46 chromosomes, the haploid gamete would possess 23 chromosomes. Since each gene has two alleles, the various probable combinations of genetic alleles present in the gamete will be 223. This holds true for both gametes, hence both gametes present 223 possibilities, which roughly translates to 8 million possible chromosome combinations. When the male and female gamete undergo random fertilization, the possibilities of its probable chromosome content would multiply ( 223 X 223) and present approximately 70 trillion chromosomal possibilities. Hence, in this way, genetic variation is introduced and ensured in the progeny of any sexually reproductive organism. This variation would only increase if the individuals themselves were not closely related (larger gene pool).

Errors in Chromosome Segregation
During cell division, after the genetic material has been replicated, the chromosomes may not segregate equally, and hence may not be equally distributed in the daughter cells. This unequal distribution gives rise to the presence of abnormal number of chromosomes in the daughter cell, i.e., the ploidy (number of chromosomes) level is changed in a cell. Depending on the loss or gain of chromosomes, it is ascribed different terms. For example, if there are three copies of a chromosome, then it is a trisomy or triploidy. If there is loss of chromosomes it is called aneuploidy. For example, the presentation of Down's syndrome in a human is due to the trisomy of chromosome 21.

Horizontal Acquisition of Genes
Genes or the alleles of genes may be acquired via various viral and bacterial infections in an organism. In such a scenario, the infection causes the exposure of foreign DNA to the organism. While most of this DNA is destroyed by way of the organism's immunological response, sometimes, a part of the foreign DNA is incorporated into the DNA of the host cell, causing the host to acquire new and unique DNA sequences that increases its genetic variation. This concept can be better understood by the help of an example of the infection of the human papilloma virus (HPV). When this virus infects the human host, it incorporates its DNA into the host's DNA (horizontal gene transfer) and uses the host cell machinery to carry out synthesis and replication of its own molecules. This eventually leads to the occurrence of cervical cancer in humans. Although this condition can be treated, a part of the viral genome is retained in the host cell, and that sequence is passed onto the future progeny.

Factors Affecting Genetic Variation

Natural Selection
It implies the survival of the most adapted and reproductively successful individual in any given population. This adaptability arises due to the various genetic changes that spontaneously occur in the organism's genome. But not all genetic changes are beneficial, and hence the genetic variation in a population may increase or decrease depending on the type of natural selection that occurs. If a population undergoes balancing selection, genetic variation is introduced and maintained in the population. This can be seen in the case of variability in shell pattern of the mussel Donax variabilis. However, if a population undergoes stabilizing selection, genetic variation is reduced to allow the propagation of only a particular type of phenotype. This can be seen in the change of body color of peppered moths, where the new color allows it to blend in with its surroundings, efficiently.

Genetic Drift
When individuals from a species migrate to a different habitat or are separated due to geographical changes, they change and adapt to their new environment in order to survive. This phenomenon is called genetic drift. In some cases, this adaptation causes the introduction of variation in the gene sequences of these organisms, and makes the overall population more diverse with respect to exhibited characteristics. It manifests itself in many forms such as geographical isolation, ecological isolation, reproductive isolation, etc. However, in few cases, due to certain geographical constraints or migratory behavior of the organisms, the genetic variation of the population is reduced as a consequence of the loss of a part of the population of an organism. This is seen in cases that undergo the bottleneck effect or the founder effect.

Importance of Genetic Variation

The presence of high genetic variation is vital to the adaptability, survival, and evolution of any given population. This is so, because a population is able to evolve only if its members are able to adapt to changing surroundings, survive, and then successfully reproduce. When a population that exhibits a high genetic variation faces environmental changes, or if that population migrates to a new habitat, diverse genomic combinations within the organism will result in the production of offspring that will possess genetic variants that may prove beneficial to them in the new environment. The offspring that are able to adapt will survive and proliferate, whereas those which fail to adapt will barely survive. Such genetic changes that lead to adaptability of the organisms are favorable, and hence will accumulate in the population, whereas those which serve no useful purpose will decrease in frequency. However, if a population that exhibits low genetic variation faces similar circumstances, there are fewer possible combinations that can occur during the production of offspring since the variants are fewer in number. Hence, logically their chances of adapting and surviving are drastically reduced. Since these organisms do not adapt or survive successfully, they are unable to reproduce and their population eventually becomes extinct.

The estimation of the genetic variation of a particular trait present in any given population is called the genetic variability of the population. It is caused and affected by the same mechanisms that cause and affect genetic variation.