Effects on abdominal bristle number were determined for three mutations,scabrous-like, dark hairy margin andscabrous, and for some second chromosome recessive lethal genes prevalent in various irradiated and unirradiated selection lines derived from an outbred population. The relationship between the effect of a lethal gene on a quantitative character and the equilibrium frequency of the lethal in a line selected for that character was examined.
Replicate lines, each initially with one hundred pairs of parents selected at 50% intensity, were derived from the Canberra strain. In later generations population size was reduced and selection intensity increased. Three lines were selected without irradiation and five with 1000 r X-rays per generation for thirty generations. Selection was continued until generation 66. Long-term responses were similar in unirradiated and irradiated lines, and there was evidence that genes with large effects influenced response patterns in both groups of lines.
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