Schematic representation of the two step fast forward genetics approach. The mutant ecotype is crossed to a mapping ecotype background. The resulting F1 progeny are subsequently selfed to generate a F2 population in which the mutant phenotype segregates according to Mendelian rules (25% mutant, 75% wild type). Equal numbers of 50 to 200 mutant and wild type individuals are pooled (bulk segregant pools) and used in the procedure. First, 'light sequencing (1 to 10 × genome coverage) is used to map the mutation to a genomic region, which is revealed by a strong decrease of mapping ecotype alleles in the mutant pool and a mild increase in the wild type pool. Next, a capture array is designed to capture DNA from the linked region in the mutant pool, followed by deep sequencing. This results in the identification of low frequency SNP alleles from the mapping ecotype, which allow for fine-mapping as well as abundant (homozygotic) novel non-reference alleles (red dot). These variants are prime candidates for the mutation causing the phenotype since it is the only common variant in all the individuals in the mutant pool.