A<unk>  <unk>  is a DNA sequence variation occurring when a<unk><unk>  — A, T, C or G — in the genome (or other shared sequence) differs between members of a biological species or paired chromosomes in a human. For example, two sequenced DNA fragments from different individuals, AAGCCTA to AAGCTTA, contain a difference in a<unk><unk> . In this case we say that there are two alleles. Almost all common<unk> s have only two alleles. The genomic distribution of<unk> s is not homogenous;<unk> s usually occur in non-coding regions more frequently than in coding regions or, in general, where natural selection is acting and fixating the allele of the<unk>  that constitutes the most favorable genetic adaptation. Other factors, like genetic recombination and mutation rate, can also determine<unk>  density.<unk>  density can be predicted by the presence of microsatellites: AT microsatellites in particular are potent predictors of<unk>  density, with long (AT)(n) repeat tracts tending to be found in regions of significantly reduced<unk>  density and low GC content.
Within a population,<unk> s can be assigned a minor allele frequency — the lowest allele frequency at a locus that is observed in a particular population. This is simply the lesser of the two allele frequencies for<unk> s. There are variations between human populations, so a<unk>  allele that is common in one geographical or ethnic group may be much rarer in another.
These genetic variations between individuals (particularly in non-coding parts of the genome) are exploited in DNA fingerprinting, which is used in forensic science . Also, these genetic variations underlie differences in our susceptibility to disease. The severity of illness and the way our body responds to treatments are also manifestations of genetic variations. For example, a<unk>  base mutation in the APOE (apolipoprotein E) gene is associated with a higher risk for Alzheimer disease.
Single nucleotide polymorphism
Conformational isomerism
Polyploidy
Homology 
Gene knockdown
Aneuploidy
Non-mendelian inheritance
Abnormality 