The limits to artificial selection for body weight in the mouse II. The Genetic Nature of the Limits

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Abstract
1. The effects of long-continued selection for body weight in two lines of mice, one large and one small, are described.2. The large line showed a sharp increase in weight after remaining at an apparent limit for twenty generations. A rare combinational event is suggested as the most likely explanation.3. Reversed and relaxed selection from the large line at the limit failed to yield any response. This indicates that effectively, the additive genetic variance in this line had been exhausted.4. In contrast, the small line at the limit regressed slightly towards the base population when selection was relaxed. Reversed selection yielded a ready response until a new limit was apparently reached. Loci affecting body weight in this line had therefore not been fixed by selection.5. Natural selection, operating on viability between conception and the time when the selection was made, appears to explain best the lack of fixation in the small line.6. Attention is drawn to the necessity of more experimental work to elucidate the genetic nature of the limits to artificial selection.