Distribution of individual F₂ mouse ratios of spine BMDs measured at 6 and 4 months of age (each ± 7 days). F₂ progeny from an AKR/J × SAMP6 cross (Cross 1, 253 mice) were scanned by DEXA. Ratios of spinal-region values for individual mice, 6-month divided by 4-month BMDs (ΔsBMD), are plotted as a histogram over the best-fit Gaussian distribution for these data. Horizontal double arrows below the histogram indicate intervals spanning ± 1, 2 or 3 standard deviations from the mean, corresponding ∼70%, 96% and 99% confidence intervals. Nearly identical results were obtained for Cross 2. Smaller panels indicate distributions separated by gender.

Composite interval mapping for postmaturity change in spine BMD (ΔsBMD) in AKR/J × SAMP6 Cross #2. Details are as given in the legend to Figure 3. Additional threshold lines are shown for the X-chromosome plot only; these were generated by 1000 permutations considering only the map of this chromosome, although utilizing the 3 top-ranking background variables (from other chromosomal locations) in the regression.

Genes in the polymorphic region of the X-chromosome QTL peak. The 96% confidence interval indicated in Figure 5, is shown here expanded. SNPs and STRs are indicated as in the preceding figures; three of the STRs used for mapping (DXMit197, DXMit5 and DXMit31) are indicated by arrows () with numbers beneath. Approximate locations (5′ ends) of all identified gene are marked by thick arrows in the bottom row. Unknown genes are indicated as “U”; not all are shown due to space limitations.

Age-dependence of spinal BMD in the AKR/J and SAMP6 mouse strains. Bone mineral density (BMD) was determined for the spines of individual, transponder identified mice of each strain at the ages indicated (A). Mice were excluded if there was superposition of transponders with the spinal column; the remaining assay cohorts comprised 24 AKR mice and 23 SAMP6 mice. Error bars show standard deviations (SDs) for both age (x-axis) and spine BMD (y-axis). Significances are given for 2-tailed t-tests (Fisher-Behrens test for samples of unknown or unequal variance), comparing spine BMDs of strains at 4, 6 and 8 months. Survivals of the two cohorts are shown in the lower panel (B); at 8 months of age, 16 AKR and 21 SAMP6 mice remained.

Composite interval mapping for three traits in AKR/J × SAMP6 Cross #1. (A) Mapping of postmaturity change in spine BMD (ΔsBMD) of F₂ cross-progeny mice. Individual mice, identified by implanted transponders, were scanned for by DEXA at ages of 4 and 6 months. The ratio of 6-month divided by 4-month spinal BMD values was input as the trait for Composite Interval Mapping, using a window size of 10 with 3 background variables ranked by stepwise multivariate regression. Not all chromosomes are shown, due to the lack of informative markers on chromosome 15 (AKR/J = SAMP6 for all STR markers tested), and robust genotyping for only one marker each in the informative regions on chromosomes 1, 6, 8, 10, 18 and 19. None of these markers was significantly linked to ΔsBMD by simple linear regression. R²(where R is the correlation coefficient between marker allele and the trait) is plotted just below the ΔsBMD LOD scans, indicating the “fraction of variance explained” by a QTL posited at that position. (B) Mapping of postmaturity change in total skeletal BMD, excluding the skull, of F₂ cross-progeny mice. This trait was analyzed exactly as described for “A” above, substituting total BMD for spinal BMD. (C) Mapping of spine BMD for F₂ cross-progeny mice at 4 months of age. All panels:LOD scores, indicating likelihood of QTL location, are plotted by QTL Cartographer, v. 2.5, across the genetic map of each chromosome determined from data of this cross by Mapmaker QTL™. Horizontal dashed lines indicate empirical genome-wide significance thresholds [20] at p_e=0.05 and p_e=0.01, determined from 1000 permutations of phenotype against genotype. Additive effect, a, defined by multivariate regression as the average trait effect per AKR/J allele, is displayed below each QTL LOD-plot. All plots were generated by QTL Cartographer.

Distribution of single-nucleotide polymorphisms, distinguishing between mouse strains AKR/J and SAMP6, near the X-chromosome QTL peak. Informative SNPs are positioned with reference to the physical map of the murine X chromosome (NCBI Build 34.1), as are short-tandem-repeat (STR) polymorphisms used in interval mapping. The results of composite interval mapping in Cross 1 are depicted above these lines; selected functional-candidate genes are indicated for reference. The x-axis has been distorted slightly in aligning genetic and physical maps. A two-headed horizontal arrow below the curve marks the ∼96% confidence interval for QTL position.