Genetics is the study of the mode and mechanism of the transmission of heritable information. Heredity is the passing of a trait from one generation to the next. The heritable information of an organism is contained in its DNA, and the DNA an organism has is called its genome. DNA passes from cell to cell by cell division and from parent to offspring by reproduction.

The actual unit of inheritance is the gene, a region of DNA that codes for one trait. The sequence of DNA makes up the genotype of an individual. A genotype can be for one single gene, for the entire genome of an individual, or anywhere in between. The physical location of a gene on a chromosome is called a locus. The particular copy of a gene at each locus is called an allele. For example, the gene for eye color occurs at one...