Inbreeding occurs when an individual has one or more common ancestors. A common ancestor is one that is present on both sides of the pedigree. I.e., in order for an individual to have a non-zero coefficient of inbreeding it is necessary for both father(sire) and mother(dam) to be descended from one or more common ancestors. The degree of inbreeding is commonly expressed using a formula first derived by the famous American geneticist Sewall Wright early in the 20th century and for this reason it is often known as Wrights Coefficient of Inbreeding. Traditionally the symbol F is used to signify the coefficient of inbreeding.