Genetics of Body Size and Related Characters. I. Selecting Small and Large Races of the Laboratory Mouse

Abstract

A large population was made up by hybridization of several common laboratory strains of mice which were in no way remarkable as to their body size. From one segregating foundation stock 8 successive generations of selections for body size have produced: (1) a race of small body size; and (2) an exceptionally large race, whose individuals average 1/4 larger at birth and twice the wt. of the small race at 60 days of age. Size was judged from the 60-day wt., and selections were based on progeny tests, sib likeness and individual phenotype. The races diverge each generation, but at a lessening rate. Their size distributions show little or no overlap, and the present difference between the races exceeds the mean wt. of the small race. Less grams of body wt. were lost by the minus selections than were gained by equal effort at plus selection; and less grams were gained or lost by [female][female] than by [male][male]. When measurements are transformed to a percentage or a logarithmic scale the wt. gains and losses become approximately equal in both sexes and races. These data agree with the view that size genes or modifiers multiply each other''s effects (that is, act geometrically), and do not simply add to or subtract from the body wt. some definite and constant number of grams.