Cell organisation
Genes are expressed through master genes. Such as in fruit flies once the main body segment have been determined, the cells in each segment become specialised for the appropriate structure, (Wings Antennae) The master genes control the development of each segment. These genes were discovered by looking at mutations. The master gene produces mRNA which is translated into signal proteins. These proteins switch on the genes responsible for producing the proteins needed for the specialisation of the cell in each segment.