Practice question

Minus in complementation matrix denotes failure to restore wild-type phenotype indicating persistent mutant trait in trans heterozygote despite diploidy. Both chromosomes carry defective alleles at same locus so no functional gene product available for pathway. Such negative result classifies mutants as allelic belonging to same complementation group and same gene. Consistent scoring requires recessive mutations and controlled conditions where wild-type growth easily distinguished from auxotrophy. Accurate plus/minus assignment underlies construction of cistron map and determination of gene number from mutant collections in formal genetics.