Molecular genetic studies depend critically on genetic markers, sites in the human genome where the underlying DNA sequence varies between individuals. Genetic markers include sites where different numbers of repeated bases distinguish individuals and the allele is actually of different length (e.g., ACACAC versus ACACACAC), single bases which differ between individuals (e.g., a C on some chromosomes and a T on others), among many kinds of variation. Each different version of sequence at these sites is called an allele, and what molecular genetics generally seeks to do is to identify sites where alleles (and hence positions in the genome) co-occur with disease (or other trait measurement).
