Developmental geneticists use model organisms as the basis for studying how a fertilized egg becomes a multicellular adult. Each model has special attributes that give it value as an experimental system. The accumulation of knowledge, mutants, and techniques from long years of study enhances the utility of model organisms.
The evolutionary relatedness of all organisms, revealed by the conservation of genes, structures, and functions often makes it possible to extrapolate from model organisms to all living forms.
Genetics simplifies the study of development. A key to the genetic dissection of development is the isolation of a comprehensive set of mutations affecting a particular process. Multiple alleles of any one gene make it possible to analyze the many roles a protein might play in development. Saturation mutagenesis and screens for suppressors or enhancers of a phenotype can help identify many of the genes participating in a developmental pathway.
To understand a gene's role in development, researchers analyze mutant phenotypes. They also determine where the gene is expressed through RNA in situ hybridization experiments or by following the protein product with fluorescently tagged antibodies or with GFP labels. The construction of genetic mosaics can help determine which cells need to express the gene so that the organism can develop normally. Careful examination of individuals mutant for two or more genes can help reveal how proteins cooperate in developmental pathways.
Cellular differentiation requires progressive changes in gene expression. These changes usually result from decisions concerning the transcription of batteries of genes. Some developmental decisions, however, involve posttranscriptional gene regulation. The earliest stages of development require control of gene expression in both the maternal and zygotic genomes.
Differentiation requires either that cells have intrinsic differences at the biochemical level, or that they are exposed to different information in their environment. Asymmetries in early embryonic development, or the asymmetric distribution of molecules during cell division, can generate intrinsic differences. Cell-to-cell communication, effected by the binding of ligands to receptors and mediated by signal transduction pathways, supplies cells with complex information about their relative position in the organism.
