Long before the existence of genes was theorized, Gregor Mendel[#](Mendel 1870) described what he called the "segregation of characters", resulting from independent assortmentMendelian principle in which alleles at different loci tend to assort into gametes independent of each other.. He noticed that several characters of pea plants varied among individuals, but that traits (like flower color) did not seem to blend: when plants with white flowers were mated with plants with purple flowers, the flowers of offspring were either purple or white - not somewhere in between. His experiments showed that, rather than blending, traits are passed on in discrete units (we now know them as genesA hereditary unit consisting of a sequence of DNA that occupies a specific location on a chromosome and determines a particular characteristic in an organism.).

Each cell contains a set of chromosomes A tightly-wound, rod-shaped DNA-containing structure, with many associated proteins. Chromosomes are located in the nucleus (in eukaryotic cells). (and all the genes in them) inherited from each parent. Traits are determined by the allelesFor a given location on a chromosome, one of two (or more)slightly different molecular forms of a gene. Alleles codefor different versions of the same trait. - different versions of a gene - inherited from each parent. Alleles may not be expressed in an individual, but they may still be passed on, unaltered, and expressed in offspring[#](Snustad & Simmons 2005).