Exclusion mapping strategy to refine the Mom2 locus. (A) Based on the observation that the Mom2R locus encodes a recessive embryonic lethal mutation, we devised a strategic intercross of (CAST × DBA.B6 Mom2R/+)F1 mice to limit the boundaries of the Mom2 locus. Since mice homozygous for the B6 Mom2R allele (B/B) should not survive, markers within the Mom2 region that were homozygous for B6 alleles would be excluded. DBA.B6 Mom2R/+ animals from the N7–N8 generations were mated to CAST mice. DBA chromosomes are shown in white; the B6 Mom2R region is shown in black; CAST chromosomes are represented by diagonal stripes and heterozygotes are shown in gray. The possible F2 progeny are shown in black boxes; all offspring were genotyped for proximal (D18Mit186) and distal (D18Mit213) markers flanking Mom2 (Silverman et al. 2002, 2003). (B) The number of resulting F2 progeny are shown below the boxes. Nonrecombinant F2 progeny are in the black box on the left; representative recombinant progeny are in the boxes to the right. F2 offspring that were B/B at either the proximal (D18Mit186) or distal (D18Mit213) marker and were B/C or C/C at the other, indicated the occurrence of a recombination within the Mom2 region. Nonrecombinant CAST DBA.B6 Mom2R animals were uninformative for linkage analysis. (C) Haplotype analysis of recombinant F2 animals from the [CAST × DBA.B6 Mom2R/+]F1 intercross. Mapped loci are listed on the left. Numbers below each column of boxes represent the number of F2 progeny that fall into each haplotype. F2 progeny were identified that were B/B at either the proximal or distal marker, and were B/C or C/C at the other, indicating an informative recombinant within the Mom2 region; these 22 mice were genotyped by PCR for polymorphic markers within the Mom2 region. This analysis positioned Mom2 between the D18Mit7 locus and the Slc14a2 gene.
