Genetic analysis has long been restricted to a few fungi, especially those that could be easily grown on simple media in the laboratory. The characteristics of the genome in terms of the number of chromosomes and the microscopic morphology of each of these are called the karyotype. In conventional gel electrophoresis DNA molecules smaller than 50 kb can be separated by size. Larger DNA molecules have a minimal velocity that is proportional to their length and therefore separated by size. The patterns produced are highly polymorphic, allowing discrimination between isolates of one species if sufficient primers are screened. The internal transcribed spacer region as well as the intergenic spacer of the nuclear rDNA repeat evolve much faster, and sequence differences in these regions occur between species within one genus or even between different populations of one species.