Physical and genetic gene maps

The physical map gives the position of a gene
locus and its distance from other genes on the
same chromosome in absolute values, expressed
in base pairs and related to given positions
along the chromosome. The genetic map
gives the relative position of gene loci according
to the frequency of recombination, expressed as
recombination units or centimorgans (cM). One
centimorgan corresponds to a recombination
frequency of 1%. Since recombination occurs almost
twice as often in oocytes as in spermatocytes,
the genetic map in females is about 40%
longer than in males. Each gene locus has an official
designation with a defined abbreviation
using the letter D (for DNA), the number of the
chromosome, and the number of the marker,
preceded by an S for single-copy DNA, e.g.,
D1S77.