There may be multiple variants of any given gene in the human population (genes), leading to polymorphism. Many genes are not polymorphic, meaning that only a single allele is present in the population: the gene is then said to be fixed.[1] On average, in terms of DNA sequence all humans are 99.5% similar to any other humans.[2][3]
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.[4] Differences between individuals, even closely related individuals, are the key to techniques such as genetic fingerprinting. Alleles occur at diff
