Organisms within the same species have differences called variation. For example, some people are taller than others, some have blue eyes, and others have scars. There are two types of variation, genetic and environmental.
Genetic variation is caused by different genes in DNA. DNA is a very important molecule located within the nucleus in cells and can be arranged in structures called chromosomes.
Genes are short sections of DNA that code for a specific characteristic. There are usually 46 chromosomes in a human cell, 23 from each parent. This means that most genes are inherited. However, a small percentage of the genes are not inherited and are instead caused by mutations.
Environmental variation is caused by how an organism lives and grows. For example, if someone stays indoors while his identical twin brother stays outside and gets a suntan, variation is introduced even if they have no genetic variation.