Quantitative Gene Action in Tetraploid Alfalfa 1

Two different populations of tetraploid alfalfa, Medicago spp., were studied to determine the action of genes controlling three quantitative traits: plant height, dry weigh, and number of stems per plant. One population consisted of clones from four alfalfa varieties and their self, single, three‐way, and double‐cross hybrid progenies. The same mating system was used for a second population using clones from four species of alfalfa, all belonging to the M. sativa‐M, falcata complex. Evidence for interallelic and/or intra‐allelic interactions was obtained by comparing parental clones with their progenies and by comparing observed and theoretical means of the hybrids. In the first population, hybrids were taller and heavier and had more stems than their parents, indicating multiple allelic and/or epistatic interactions. Also, the majority of observed means of hybrids were larger than their theoretical means, indicating favorable interactions of genes. In the second population, observed means were larger than theoretical means in only half of the crosses. We believe this was due to unfavorable interactions of genes brought into the smaller hybrids by parents of very diverse origin. Theoretically, in an autotetraploid system, double crosses provide the maximum opportunity for obtaining favorable intra‐allelic and inter‐allelic interactions. In most instances in this study, the double‐cross hybrids outperformed other hybrids and parent clones in weight per plant.