Genetic Mapping

Genes occupy fixed positions, called loci, arranged in a strict linear order along a chromosome, and the diagrammatic representation of those positions and the distances between neighbouring loci is called a genetic map, or linkage map, because it is built entirely from linkage/recombination data rather than from directly visualising the DNA. Alfred Sturtevant, a student in Morgan's laboratory, first developed the technique in 1913, based on the simple insight that recombination frequency between two loci is directly proportional to the frequency of crossing over between them, which in turn tracks the physical distance separating them. The unit of distance on a genetic map is the map unit (m.u.), where one map unit is defined as one percent recombination frequency; a map unit is also called a centimorgan (cM) in honour of T.H. Morgan, and 100 centimorgans make up one full Morgan (M). Genetic maps can, in principle, be built from a series of simple two-point crosses, testing one pair of genes at a time, but this approach is inefficient because it systematically misses double crossover events, which quietly restore the parental combination and so are invisible to a two-point cross. The more efficient and more accurate alternative is the three-point test cross, in which a triple heterozygote (carrying three linked genes, each with a dominant and a recessive allele) is testcrossed to the triple recessive homozygote in a single experiment. Because all three loci are tracked together, a three-point cr …