A population of cells mutant in a gene whose wild type form can easily be screened or selected for, such as the LEU-2 biosynthetic gene in baker's yeast, is obtained. The population is transformed with a large pool of DNA fragments or cDNAs cloned in a shuttle vector. A few colonies grow on a plate lacking leucine. Shuttle DNA recovered from the colonies, when used as a hybridization probe of an ordered library , recognizes only a few contiguous clones. Alternatively, when a restriction map of rare cutting sites exists, the recovered transforming DNA can probe Southern blots of AFGE separations of DNA restriction fragments.
Complementation is limited to genes that can easily be scored or selected for in cell culture.
The limitation is due to the large numbers of transformants that must be generated. Complementation should be very useful in testing a cloned DNA to see whether it contains the gene it is thought to contain. An example of this approach is the isolation of human genes involved in cell development by complementation of yeast mutants. Another way of connecting points on a genetic map with points on a molecular map is hybridization of genetic molecular markers to ordered contigs.