A strain is congenic when it differs from a particular inbred
strain at a single locus as a result of backcrossing a particular
allele into the inbred strain. Congenic strains may also differ at
multiple loci that are closely linked to this different locus. Most
congenic strains differ at loci that are immunologically defined,
particularly the RT1 complex in rats. Harlan produces
several such strains.
Congenic strains are used to investigate the biological
properties of the single different locus without the complications
that arise from differences at other loci. For example, the PVG and
DA strains differ at the RT1 complex and at many other
loci. A comparison of strain differences provides little
information on the biology of H-2. However, PVG and
PVG-RT1av1 are two strains that are
identical at about 99% of their genome, but differ at the
H-2 complex.
Differences in the biological properties of these strains, such
as response to an infection, may be studied by focusing on the
RT1 complex.