A putP null mutation is a simple loss of function mutation which would be recessive to the wild-type allele. Following chemical mutagenesis of Salmonella typhimurium cells, Lawes and Maloy isolated host mutants which are conditionally resistant to phage P22. The expectation is that only matC would be complemented because the three genes tested represent dicrete loci. The overexpressed gene may be one of the three mat genes or a fourth gene that we do not know about. Note that it is overexpression on a plasmid producing this effect because the mutant strains contain a single copy of each of these genes in their chromosome and that is inadequate to produce a suppression of the Mat phenotype.
Many antibiotics act by binding to bacterial ribosomes and inhibiting translation.
Resistance to such antibiotics is often due to mutations that produce specific nucleotide changes in one of the ribosomal subunits which prevent the antibiotic from binding to the ribosome. In such cases, the antibiotic resistant alllele is often recessive to the antibiotic sensitive allele. If the antibiotic stops the ribosomes in their tracks, it will prevent subsequent ribosomes from transcribing that message. Thus, the sensitive phenotype is dominant to the resistant phenotype. Briefly compare and contrast complementation vs recombination.