Each gene may comes in more than one form. For example, pea plants have a gene which specifies the color of their flowers. Flowers can be white or red. These different colors are determined by variations, or alleles, of the same gene. The physical location of a gene on a chromosome is the gene's locus.
Many organisms have two parents, and so receive two sets of DNA. Such organisms are called diploid. For example, humans have 46 chromosomes. Each human receives twenty-three chromosomes from each parent. The pairs of chromosomes are numbered one through twenty-two, roughly in order of decreasing size. These chromosomes are the autosomes. The final two chromosomes are the sex chromosomes X and Y. Each chromosome is distinct, and a given autosome or sex chromosome always carries the same information. For example, each chromosome 4 carries the same genes as every other chromosome 4, but not necessarily the same alleles of these genes. Therefore, the organisms that receive DNA from two parents, have two copies of each autosome, and either two X's or an X and a Y for the sex chromosomes.