A SINGLE-BASE TRANSVERSION IN THE FLANKING REGION OF AN EQUINE MICROSATELLITE LOCUS AFFECTS AMPLIFICATION OF ONE ALLELE

The equine dinucleotide microsatellite HMS7 is part of a microsatellit e panel utilized in a parentage verification programme at the Veterina ry Genetics Laboratory (Davis, California, USA). Apparent non-Mendelia n inheritance was noted when a Quarter Horse mare was excluded as the parent of two offspring based on analysis of the HMS7 locus. The mare' s DNA type qualified her as a parent of the offspring at an additional 20 microsatellite loci. The three animals appeared homozygous for HMS 7 with each possessing an allele different from that of the other two animals. Polymerase chain reaction primers designed to bind outside th e published primer-binding sites amplified an additional shared allele in all three horses, which qualified the mare as the dam of the two o ffspring. Sequencing of this newly detected allele revealed a C to A t ransversion in one of the published primer-binding regions. Apparent n on-Mendelian inheritance at the HMS7 locus has been encountered in an additional 26 Quarter Horse parentage cases. In all instances, the lac k of amplification and resultant 'null' allele was shown to be caused by the same transversion.