Heredity in biology refers to the passing of genetic factors from parents to offspring or from one generation to the next. It is the sum of all biological processes by which particular characteristics are transmitted from parents to their offspring. Heredity encompasses two seemingly paradoxical observations about organisms: the constancy of a species from generation to generation and the variation among individuals within a species. Constancy and variation are actually two sides of the same coin, as becomes clear in the study of genetics.

Genes are the functional units of heritable material that are found within all living cells. Every member of a species has a set of genes specific to that species. It is this set of genes that provides the constancy of the species. Among individuals within a species, however, variations can occur in the form each gene takes, providing the genetic basis for the fact that no two individuals (except identical twins) have exactly the same traits.

Heredity is responsible for the transmission of traits from one generation to the next. These traits can be physical, such as eye color, blood type, or a disease, or behavioral. Hereditary traits are determined by genes, and a single gene can have several variants called alleles. There are two copies of each gene in our cells (with the exception of genes located on sex chromosomes). One of the copies comes from the sperm, the other from the egg. In an individual, these two copies (or alleles) are not necessarily identical. If the two copies of a gene are identical, we say that the individual is homozygous for that gene. If the two copies are different, the gene is heterozygous. The alleles of the same gene can have a dominant or recessive relationship with one another. If both alleles are different (heterozygous) and at least one of these two alleles is dominant, it is the dominant one that will be expressed (i.e., that we will observe as a trait in an individual). Conversely, a recessive allele (non-dominant) will not be expressed in an individual if both parents pass down the same allele (homozygote) .