Natural variations in the length or identity of DNA sequences occur at millions of locations throughout most genomes.
DNA polymorphisms are often neutral, but because of linkage may be used as molecular markers to identify regions of genomes that contain genes of interest.
Molecular markers are useful because of their neutrality, co-dominance, density, allele frequencies, ease of detection, and expression in all tissues.
Molecular markers can be used for any application in which the identity of two DNA samples is to be compared, or when a particular region of a chromosome is to be correlated with inheritance of a trait.
Many important traits show continuous, rather than discrete variation.These are also called quantitative traits.
Many quantitative traits are influenced by a combination of environment and genetics.
The heritable component of quantitative traits can best be studied under controlled conditions, with pure-breeding parents that are polymorphic for both a quantitative trait and a large number of molecular markers.
Molecular markers can be identified for which specific alleles are tightly correlated with the quantitative value of a particular phenotype.The genes that are linked to these markers can be identified through subsequent research.
