QTL analysis of F2 wan homozygotes. (A) Genome-wide scan of MCV, RBC count, Hgb, Hct, and MCHC. Significant QTL (minimum LOD score of 2.2, indicated by the top dashed line) were detected on Chr 6 and Chr 12 using MCV as the quantitative trait. No suggestive (LOD score of 2.0, indicated by the bottom dashed line) or significant LOD scores were obtained using RBC count, Hgb, or Hct as traits, but 2 suggestive QTL are seen for the MCHC. (B) Interval map of the Chr 12 MCV quantitative trait locus (solid line) showing the position (cM) of the markers. This quantitative trait locus was designated Hsm1. The peak LOD score for Hsm1 was obtained using a marker for the erythroid β-spectrin gene (Spnb1). The posterior probability density curve corresponding to the 95% confidence interval (bold dashed line) was computed by the method of Sen and Churchill.³¹  This analysis shows that the 95% confidence interval for Hsm1 is between 28 and 40 cM. (C) Allele effects of Hsm1 using the peak LOD score marker (Spnb1, β-spectrin) indicates that it acts as a dominant and lowers the MCV. C indicates CAST/Ei Spnb1 allele; H, C3H Spnb1 allele. ^*P < .01. Error bars indicate standard error. (D) Interval map of the Chr 6 MCV quantitative trait locus reveals multiple shoulder peaks, indicating that several QTL are likely present in the interval. The 95% confidence interval is between 40 and 80 cM. The peak LOD score was obtained with D6Mit15.