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Sometimes a genetic marker is an allele of a gene producing a protein. It is called a functional marker that affects directly the function of the protein. However, in most cases the genetic marker is a piece of DNA from which we do not know the function. It is situated on a chromosome close to a gene and therefore related to one of the alleles of this gene. From the start of the molecular genetic work in animals, animal geneticists have done a lot of work to find genetic markers. Before that start, they only knew that a sire and a dam passed one of their two alleles to their offspring, but they did not known which ones. With genetic markers, it is possible to trace which allele of a sire and which allele of a dam is passed to the offspring.