Table 2.

Median survival time of the backcross mice is dependent on genes located outside the sst1 locus

Cross	sst1 genotype	Estimated % of B6 genome	Number of mice per group	MST, days	SD
(BC4H)IC	S/S	3.12	14	28	2.5
(BC4H)IC	R/S	3.12	13	65	15.6
(BC4H)IC	R/R	3.12	12	69	16.8
BC3B	R/S	93.75	15	165	54.3
BC3B	R/R	93.75	18	172	56.5

The median survival times (MST) after intravenous infection with 2.5 × 10⁵ cfu of M. tuberculosis Erdman were compared in five groups of the backcross mice differing in their genetic composition. The first three groups were the progeny of intercross between backcross 4 generation onto C3H background (BC4H IC) heterozygous at the sst1 locus. Each mouse in that population carried 3.12% of the B6 genome on average. Another two groups were obtained after three backcrosses onto the B6 background (BC3B), in which transfer of the C3H-derived susceptible allele of the sst1 was ensured by genotyping the breeder males in each backcross generation with the chromosome 1 markers flanking the sst1-containing interval. Because of the segregation at the sst1 locus in this backcross, half of the mice possessed two resistant alleles of the sst1 (R/R) and another half was heterozygous at that locus (R/S). Each mouse in that population was heterozygous at approximately 12.5% of their genomes on average, whereas the rest of their loci (87.5%) were homozygous for the B6-derived alleles. The difference between the groups with the same sst1 genotype but differing in proportion of the B6 genome was highly significant (P < 0.0001).