Figure WN14.3. Location of a QTL using the complementation method. (A) Doebley and colleagues (Doebley et al. 1995) crossed teosinte and maize to produce an F₁ (B). This was backcrossed to maize; offspring were chosen that carried maize markers except in the region surrounding a QTL on chromosome 1. Ultimately, a cross between two offspring from the backcross generated a homozygous nearly isogenic line, which carried a small region of teosinte genome around the QTL. Heterozygotes for this region had the maize phenotype (D and E, middle), whereas homozygotes looked like teosinte (E, right). In other words, the teosinte allele of the QTL is recessive. The complementation test is made by crossing heterozygotes for the two alternative QTL alleles with a homozygote for the tbl1 mutation, which had arisen spontaneously in maize (F). Half of the offspring carried the recessive tbl1 mutant allele together with the maize allele of QTL and had a maize phenotype (G, right). The other half carried the teosinte QTL allele along with the tbl1 mutant (G, left). Crucially, these looked like tbl1 mutant homozygotes, with a teosinte-like phenotype. This shows that the different alleles fail to complement and so are likely to be at precisely the same genetic locus.

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