Human genetic variation refers to genetic differences both within and among population

Population

A population is all the organisms that both belong to the same group or species and live in the same geographical area. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals...

s. There may be multiple variants of any given gene in the human population (allele

Allele

An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...

s), leading to polymorphism

Polymorphism (biology)

Polymorphism in biology occurs when two or more clearly different phenotypes exist in the same population of a species — in other words, the occurrence of more than one form or morph...

. Many genes are not polymorphic, meaning that only a single allele is present in the population: that allele is then said to be fixed

Fixation (population genetics)

In population genetics, fixation is the change in a gene pool from a situation where there exist at least two variants of a particular gene to a situation where only one of the alleles remains...

.

No two humans are genetically identical. Even monozygotic twins, who develop from one zygote, have infrequent genetic differences due to mutations occurring during development and gene copy number variation has been observed.