Fig. 2.

Examples of inter-strain variation in the time course of the ERG. Note that the c-wave is more prominent for the C57BL/6J strain than for either 129S1/SvImJ or A/J. The A/J strain is also albino, and consistent with this some stimuli evoke more transient responses in A/J mice than in pigmented strains. Inter-strain variability is quantified in Supplemental Tables I and II It should be pointed out that variation in the ERG or any other property of vision among strains can be used to help pin-point the genes that are responsible for the variation. The presently-used inbred laboratory strains used today derive from very few wild-caught mice (and in a few cases, wild mice) and thus their genomes are mosaics of the chromosomes of these founder mice in which the haplotype blocks are very small (<1 Mb) [20, 21]. The genetic diversity of the founder mice allows the DNA derived from them to be identified by characterizing their polymorphisms. The polymorphisms between many standard laboratory strains have been characterized [22] at over 150,000 locations in the genome. These polymorphisms have been used to identify known genes that affect vision. What is needed on the part of the investigator is to measure the phenotype in each of many strains and then apply these data to a mapping program designed for this purpose. The program establishes associations between the DNA segments of each strain (from the small number of founder mice) and the phenotype. A program for this purpose is available on the web (http://snpster.gnf.org/cgi-bin/snpster_ext.cgi). Thus, naturally-occurring variation among strains of mice provides a resource for vision researchers that can be exploited readily at the present time.