Genotype to Phenotype: A Complex Problem

This is a great new study that just appeared in Science. If it is so complex in yeast, why would we expect humans to be simpler?

Dowell RD et al. Genotype to Phenotype: A Complex Problem. Science 328, 469 (2010) [PDF]

"Our genome-wide survey of conditionally essential genes demonstrates that in most cases a complex set of background-specific modifiers influence a mutation whose phenotype differs between individuals. These results raise the possibility that similar complex modifiers may largely explain the difficulty in identifying the genetic basis for individual phenotypes.