Abstract
The mitochondrial DNA (mtDNA) molecules of some rats obtained from the SASCO colony contain six EcoRI-sensitive sites while the mtDNA molecules of other rats obtained from the same source contain only four EcoRI sites. By mapping the positions of the EcoRI sites on the molecules relative to the D-loop it was determined that mtDNA molecules from all rats have four EcoRI sites in common, but all the mtDNA molecules of some rats have two extra EcoRI sites. EcoRI sensitivity of mtDNA molecules was shown to be maternally inherited. Construction of heteroduplex molecules failed to reveal evidence of gross nucleotide sequence changes associated with differences in EcoRI sensitivity.