Example: The Bithorax Mutation
Some alleles are only expressed in a certain environment, or are expressed differently depending upon the environment.
This is sometimes called a gene by environment interaction
Example: In Drosophila melanogaster, there is an allele that codes for a drastic morphological transformation.
Normal flies have three thoracic segments, and ONE set of wings (on the second segment), a pair of reduced halteres is on the third segment.