Experimental Test of Predictions of Inbred Line Performance in Crosses

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Abstract
Heritability of 13-week individual body weight in rats was estimated to be 40–50 percent. The standard deviation for body weight in the individual outbred rat (female basis) was estimated to be about 20 gm. Litter size, litter sequence, sex-linked genes and maternal influence were found not to be important sources of variation in adult body weight. Differences between sire progenies within lines and within mating systems (inbreeding, topcrossing and linecrossing) were found to be generally lacking in significance. Inbred lines derived from the same outbred foundation stock were found to diverge significantly from each other. No general loss of vigor, as measured by body weight, was evident when the average body weight of these inbred lines was compared with outbred stock maintained from the same source material. Differences between inbred lines were reflected by similar differences between their progeny produced by crossing with the outbred stock and by crossing with a series of other inbred lines. Crossed progeny (topcross, bottomcross and linecross) were found to be significantly heavier on the average than the component inbred lines from which they were derived. They did not, on the average, exceed the body weight of the heavier parent line .(or outbred group) entering each specific cross. Linecross progeny body weights from the crossing of specific inbred lines were found to be predictable as indicated by significant regressions on the average body weight of the parent lines, on the body weight of the heavier lines, and on the body weight of the lighter lines entering each cross. Significant regressions of topcross and bottomcross body weights on these three estimates were also found where the outbred group served as one parent in each cross. These three methods were found to be of about equal value. The relative value of inbred lines for crossing with other inbred lines was found to be predictable from line's own performance. Line's performance in topcrossing and bottomcrossing and line's performance when testcrossed with highly inbred “tester” lines indicated average combining value and the amount of hybrid vigor likely to be obtained on the average. Comparisons of correlations of the observed body weights and the expected body weights for these methods showed no clear-cut evidence of differences in their effectiveness in prediction.Dominance and/or overdominance were apparently responsible for the hybrid vigor observed in cross-progenies.

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