A model of overdominant gene action to explain heterosis for yield in the autotetraploid potato has been presented. Loci with multiple allcles and a maximum heterotic value for quadrigenic genotypic structures have been postulated. Various experimental results have been analyzed on the basis of such a model in contrast with a dominance situation. The analysis suggests a close positive correlation between heterozygosity and yield. The implication of the proposed overdominant model to potato breeding would be that substantial genetic advance in yield should be made upon increasing the genetic diversity of the parental clones. However, the alien sources of germplasm should undergo some previous selection for adaptation. A proper balance between heterozygosity and adaptation, mainly to photoperiod, should maximize the heterosis for yield.

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The objective of this research was to determine optimum plot size and number of replications to evaluate yield of sweetpotato (Ipomoea batatas Lam.) clones. The optimum plot size was estimated using the methods of maximum curvature and comparison of variances. The adequate number of replications was determined using the Hatheway method. Using the maximum curvature method, the estimated optimum plot size was 10 basic units (b.u. = six plants or 1.2 m2) for La Molina and San Ramon, and 5 b.u. for Tacna, Peru. Using the comparison of variances method, the optimum plot size was 15 b.u. for all locations tested. The adequate number of replications with a plot size of 15 b.u. was four.

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