Crossing scheme for mapping a complementation group. The mapping crosses follow the typical design for mapping one allele (A) of a complementation group on the basis of its failure to complement a second allele (B). The first cross generates females that are heterozygous for the FRT chromosome carrying allele A and the marker chromosome, so that meiotic recombination can occur between them (cross 1). These females are crossed to males carrying a dominantly marked (Pr) marker chromosome (cross 2) so that all of the recessive markers can be scored in the recombinant progeny to determine in which intervals recombination took place. A small number of recombinant males (10 in each interval) are then individually crossed to females carrying the second allele (B) to determine whether the recombinant chromosomes carry allele A or not (cross3). This allows the mapping of the gene between two visible markers, in this example, between cu and sr. The progeny of cross 3, carrying both the recombinant chromosome and TM3 are then tested with SNP-RFLPs to narrow down the region containing the mutation. More males recombinant between cu and sr are then selected from the rest of the progeny of cross 2, that have been stored at 18°C. These males are then individually crossed to females carrying the allele B (day 40, cross 4). Progeny of this cross are used to determine whether the recombinant chromosomes carry allele A, and to genotype the recombinants with SNPs. The recombinants with a cross-over in the same small interval as the mutation are kept as stocks over TM3, for more precise mapping with further SNPs, once these have been discovered. For convenience, only visible markers used in the mapping are indicated in this figure.