A Study of Heterosis in Swine1

Abstract

A study was made of the results obtained in crossing inbred lines of swine which were developed by following a flexible system of inbreeding and rigid selection for characters of economic importance. Average inbreeding of the lines, which included nine inbred lines of Poland China and two lines based on crossbred foundations (Minnesota No. 1 and Minnesota No. 2) ranged from 22 percent to approximately 75 percent. An overall estimate of hybrid vigor was determined by averaging the advantage, in percent, of three measures of vigor, (1) weaning weight, (2) rate of gain and, (3) efficiency of food utilization, of the crossbred pigs over the average of the parental lines. Crosses of inbred lines within the Poland China breed showed less heterosis than crosses of Minnesota No. 1 and Minnesota No. 2 or crosses of the Poland China lines and either Minnesota No. 1 or Minnesota No. 2. These results are interpreted as an indication of the importance of genetic diversity in relation to heterosis. Similarity of genie complexes in two inbred lines of Poland Chinas is indicated by the performance of crosses involving them. On the basis of their origins, it would be expected that these two lines should differ about as much as could be expected of two lines within the Poland China breed. The performance of these two lines in crosses also gives some indication of the relationship between genetic purity and hybrid vigor. As measured by the coefficient of inbreeding, one line possesses approximately two times as much genetic purity as the other line. However, the two lines perform essentially the same in crosses. A comparison of the performance of the better combining inbred line crosses with that of hogs bred by conventional methods (data composited from the literature) showed an advantage of approximately 15 percent in favor of the inbred line crosses. This indicates that the methods employed in developing the inbred lines have been effective in bringing about a certain degree of purification for the more desirable genes. Copyright © . .