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Genetic variation in humans, as for the purposes of this section, will refer to any variance in phenotype which results from heritable allele expression, mutations, and epigenetic changes. While human phenotypes may seem diverse, individuals actually differ by only 1 in every 1,000 genes and is primarily the result of inherited genetic differences. Pure consideration of alleles is often referred to as Mendelian Genetics, or more properly Classical Genetics, and involves the assessment of whether a given trait is dominant or recessive and thus, at what rates it will be inherited.  The color of one’s eyes was long believed to occur with a pattern of brown-eye dominance, with blue eyes being a recessive characteristic resulting from a past mutation. However, it is now understood that eye color is controlled by various genes, and thus, may not follow as distinct a pattern as previously believed. The trait is still the result of variance in genetic sequence between individuals as a result of inheritance from their parents. Common traits which may be linked to genetic patterns are earlobe attachment, hair color, and hair growth patterns.

Evolutionarily speaking, genetic mutations are the origins of differences in alleles between individuals. However, mutations may also occur within a person's life-time and be passed down from parent to offspring. In some cases, mutations may result in genetic diseases, such as Cystic Fibrosis, which is the result of a a mutation to the CFTR gene which is recessively inherited from both parents. In other cases, mutations may be harmless or phenotypically unnoticeable. We are able to treat biological traits as manifestations of either a single loci or multiple loci, labeling said biological traits as either monogenic or polygenic, respectively. Concerning polygenic traits it may be essential to be mindful of inter-genetic interactions or epistasis. Although epistasis is a significant genetic source of biological variation, it is only additive interactions that are heritable as other epistatic interactions involve recondite inter-genetic relationships. Epistatic interactions in of themselves vary further with their dependency on the results of the mechanisms of recombination and crossing over.

The ability of genes to be expressed may also be a source of variation between individuals and result in changes to phenotype. This may the result of epigenetics, which are founded upon an organism’s phenotypic plasticity, with such a plasticity even being heritable. Epigenetics may result from methylation of gene sequences leading to the blocking of expression or changes to histone protein structuring as a result of environmental or biological cues. Such alterations influence how genetic material is handled by the cell and to what extent certain DNA sections are expressed and compose the epigenome. The division between what can be considered as a genetic source of biological variation and not becomes immensely arbitrary as we approach aspects of biological variation such as epigenetics. Indeed, gene specific gene expression and inheritance may be reliant on environmental  influences.