52 H. K. HAYES 



METHODS OF BREEDING FOR HETEROSIS 



In general there is a much closer relation between the characters of par- 

 ents and of their Fi crosses in self-pollinated plants than between the char- 

 acters of inbred lines of cross-pollinated plants and their Fi crosses. 



Characters of Parents and Fi Crosses in Self-pollinated Plants 

 A recent study by Carnahan (1947) in flax, which is normally self-polli- 

 nated, may be used for illustrative purposes. Four varieties of flax were se- 

 lected to represent desirable parental varieties. Each was crossed with four 

 other varieties, of different genetic origin from the first group, to be used as 

 testers. Sufficient seed for Fi and F2 progenies was produced so that all 



TABLE 3.2 



PARENT AND Fi CROSSES, YIELD 

 IN BUSHELS PER ACRE* 



Parent Tester Varieties 



Varieties 5 6 7 8 



16 14 17 13 



* Parent yields outside rectangle, Fi crosses 

 within. 



progenies could be planted in replicated, 8-foot rows at the rate of 200 seeds 

 per row. Combining ability was studied in Fi and F2 in comparison with the 

 parents for yield of seed, number of seeds per boll, number of bolls per plant, 

 weight of 1000 seeds, date of full bloom, and plant height. 



As shown in Table 3.2, each Fi cross yielded more than its highest yielding 

 parent, although for one cross the difference was only slightly in favor of 

 the Fi. For an average of all crosses, the Fi yielded 40 per cent more than the 

 average of the parents, and the Fo, 26 per cent more. The lowest yielding 

 cross, 3X8, was produced from a cross of the two lowest yielding parents. 

 The highest yielding cross, 1X5, however, could have been selected only 

 by actual trial. It was obtained by crossing the highest yielding selected 

 variety with the second highest yielding tester variety. 



There was excellent agreement, on the average, for each of the characters 

 studied between the average expression of the characters of the parents and 

 their Fi crosses. Carnahan concluded that for each character studied there 

 appeared to be a good relationship between the performance of the parents 

 and the average performance of their Fi crosses. The characters of the par- 

 ents in this study were as good or better indication of the combining ability 

 of a parental variety as that obtained from a study of average combining 

 ability in four crosses. 



