On the Choice of Tester Parent for the Breeding of Synthetic Varieties of Maize (Zea mays L.)1

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Abstract
The selection of genotypes (plants or first generation inbred lines) from an open‐pollinated variety to combine into a synthetic variety involves the choice of a suitable tester parent. A theoretical examination of a single locus shows that the recessive homozygote is the best tester for maximizing the expected mean yield of the synthetic variety.In practice the gene frequency and the amount and direction of dominance will be unknown. With no dominance the mean yield of the synthetic variety is independent of the tester. With partial dominance any tester will give some increase in mean yield. With overdominance the choice of the ‘wrong’ homozygote as tester could cause a reduction in mean yield but if the parental variety is used as tester the yield will at least not decline. The parental variety may therefore be a safer choice as tester than other varieties with contrasting gene frequencies.Criteria are derived for deciding which loci are most important in choosing a tester. It is argued that at these loci the parental variety is more likely than other available varieties to be near homozygous recessive. If positive dominance is important, a good tester may be produced by selecting for low yield within the parent variety.Much of the theory and many of the conclusions presented in this paper are not new. The aim of the paper is to give a unified mathematical account of the known theory and to generalize and interpret it.

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