Tandem gene arrays: a challenge for functional genomics.

In sequenced plant genomes, 15% or more of the identified genes are members of tandem-arrayed gene families. Because mutating only one gene in a duplicated pair often produces no measurable phenotype, this poses a particular challenge for functional analysis. To generate phenotypic knockouts, it is necessary to create deletions that affect multiple genes, select for rare meiotic recombination between tightly linked loci, or perform sequential mutant screens in the same plant line. Successfully implemented strategies include PCR-based screening for fast neutron-induced deletions, selection for recombination between herbicide resistance markers, and localized transposon mutagenesis. Here, we review the relative merits of current genetic approaches and discuss the prospect of site-directed mutagenesis for generating elusive knockouts of tandem-arrayed gene families.