To determine whether in potatoes the tetraploid level is preferable to the diploid level, especially regarding tuber yield, four diploid (2n = 2 x = 24) Solanum phureja x dihaploid S . tuberosum hybrid parents and their vegetatively doubled, tetraploid (2n = 4 x = 48) counterparts were intermated, which resulted in two F, hybrid families at both levels of ploidy . The parents and clones of the F, families and their offspring were used in crosses in such a way that in addition Sib,, Sib2 , F, X Sib,, BC, and Sib, X Sib, families were produced . Of the first clonal generation of the 12 2 x families and their 12 counterpart 4 x families two tubers per clone were planted in three replications in a field experiment at Sturgeon Bay in 1969 ; of the parents six tubers were planted in each replication . Data were recorded on 16 characters, including plant height at four different times .The ANOVA's showed significant clone effects within each family for all characters . Computed from all family means as well as from the family means per ploidy level, differences due to family were also significant for all characters except one . As at the 2 x level and at the 4 x level of ploidy the mean phenotypic correlations between characters were of similar magnitude, it is concluded that they are independent of ploidy level . With the exception of eye depth, the mean coefficient of variation was greater at the 2 x than at the 4 x level of ploidy, indicating the greatest response to selection for those characters at the 2 x level . From the differences in family means between the 4 x and the 2 x level of ploidy it was apparent that the 4 x families generally had significantly taller plants, later maturity, fewer tubers, higher mean tuber weight, more tuber yield and more dry matter yield than their counterpart 2 x families . In contrast, the 4 x parents had on the average shorter plants, lower mean tuber weight, much lower tuber yield and lower dry matter yield than their 2 x counterparts . The phenotypic correlation and Spearman's rank correlation between the family means of the 2 X and the 4 x level of ploidy were positive for almost all characters and significant for nearly half of them .From the results it is concluded that 1 . in potatoes the 4 x level of ploidy is preferable to the 2 x level, and 2 . the performance of 4 x families is predictable from the performance of their counterpart 2 x families .Based on results mentioned in the literature and on the present results, a continued use of S . tuberosum dihaploids in potato breeding needs to be dissuaded .
