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

John W. MacArthur
The American Naturalist
Vol. 78, No. 775 (Mar. - Apr., 1944), pp. 142-157
Stable URL: http://www.jstor.org/stable/2457976
Page Count: 16
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Genetics of Body Size and Related Characters. I. Selecting Small and Large Races of the Laboratory Mouse
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Abstract

A large population was made up by hybridizations of several common laboratory strains of mice which were in no way remarkable as to their body size. From one segregating foundation stock eight 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 a fourth larger at birth and twice the weight of the small race at 60 days of age. Size was judged from the 60-day weight, 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 weight of the small race. Less grams of body weight were lost by the minus selections than were gained by equal effort at plus selection; and less grams were gained or lost by females than by males. When measurements are transformed to a percentage or a logarithmic scale the weight 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 weight some definite and constant number of grams.

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