Figure. Distribution of mesiodistal and buccolingual crown diameters (MD and BL, respectively) of the upper and lower first molars (M¹ and M₁, respectively) in 21 of the SMXA RI strains and the 2 parental strains (SM/J and A/J). (a) MD of M¹; (b) BL of M¹; (c) MD of M₁; and (d) BL of M₁. Data are expressed as means ± SD. Numbers in parentheses represent the numbers of the sex-pooled population used.