The progeny of organisms that reproduce sexually are not exact duplicates of their parents and vary in many ways amongst themselves. This variation is the spice of life. It is what allows a species to adapt and evolve, and also provides plant and animal breeders with a wide array of choices. It is important to understand some of the basics of genetics in order to comprehend much of what is said later. The carrier of the genetic message is DNA, a large molecule which contains the code responsible for the structure and function of any living organism.
Genes are specific lengths of the DNA molecule, the base units of inheritance.
Variations of the same gene are called alleles. There may be several different alleles of a gene. Every sexually reproducing species inherits two alleles, one from each parent. These interact with varying degrees on each other. This number is known as the diploid number of chromosomes.