The genetic differences among individuals. There are trillions of possible GENE combinations within the human GENOME. Except for identical twins, no two people share exactly the same GENOTYPE (genetic constitution). Though any two individuals may have 99.9 percent of the same DNA sequences and gene pairings, the 0.1 percent of pairings that differ accounts for the endless details that make each individual unique.
The same genotype can have multiple expressions (phenotypes) among individuals. The genotype for EYE color, for example, can express itself as blue eyes in one person and brown eyes in another. Such variability exists for every gene pairing, with more or less obvious results. VARIATION also occurs through MUTATION, in which DNA sequences change during replication. Polymorphisms and mutations may have positive, neutral, or negative effects, which differ among individuals based on circumstance, lifestyle, and other factors.