Genetic variation


Genetic variation is the difference in DNA among individuals. There are multiple sources of genetic variation, including mutation and genetic recombination.

Among individuals within a population

Genetic variation can be identified at many levels. It is possible to identify genetic variation from observations of phenotypic variation in either quantitative traits or discrete traits.
Genetic variation can also be identified by examining variation at the level of enzymes using the process of protein electrophoresis. Polymorphic genes have more than one allele at each locus. Half of the genes that code for enzymes in insects and plants may be polymorphic, whereas polymorphisms are less common among vertebrates.
Ultimately, genetic variation is caused by variation in the order of bases in the nucleotides in genes. New technology now allows scientists to directly sequence DNA which has identified even more genetic variation than was previously detected by protein electrophoresis. Examination of DNA has shown genetic variation in both coding regions and in the non-coding intron region of genes.
Genetic variation will result in phenotypic variation if variation in the order of nucleotides in the DNA sequence results in a difference in the order of amino acids in proteins coded by that DNA sequence, and if the resultant differences in amino acid sequence influence the shape, and thus the function of the enzyme.

Between populations

Geographic variation means genetic differences in populations from different locations. This is caused by natural selection or genetic drift.

Measurement

Genetic variation within a population is commonly measured as the percentage of gene loci that are polymorphic or the percentage of gene loci in individuals that are heterozygous.

Forms

Genetic variation can be divided into different forms according to the size and type of genomic variation underpinning genetic change.
Small-scale sequence variation includes base-pair substitution and indels.
Large-scale structural variation can be either copy number variation, or chromosomal rearrangement. Genetic variation and recombination by transposable elements and endogenous retroviruses sometimes is supplemented by a variety of persistent viruses and their defectives which generate genetic novelty in host genomes.
Numerical variation in whole chromosomes or genomes can be either polyploidy or aneuploidy.

Maintenance in populations

A variety of factors maintain genetic variation in populations. Potentially harmful recessive alleles can be hidden from selection in the heterozygous individuals in populations of diploid organisms. Natural selection can also maintain genetic variation in balanced polymorphisms. Balanced polymorphisms may occur when heterozygotes are favored or when selection is frequency dependent.