Genetic engineering is widely used in plants and animals to promote rapid growth and create resistance to common diseases. One genetic modification that has achieved prominence in fish is the insertion of growth hormone transgenes, which produce dramatically larger sizes and rapid growth rates. However, there is concern that escaped genetically modified fish might breed with their wild counterparts, passing on the genetic modification and changing the wild population. The effects of the transgene on key fitness traits, such as growth in the wild, are very difficult to predict. This is because growth is encoded by many genes that interact with each other and with the environment to produce individuals of different sizes. In a new study, a unique experimental design in Coho salmon was used to explore the effects of an inserted transgene on the genetic architecture explaining growth rates. An experimental family was created, where half of the offspring contained the genetic modification and half did not. Besides this change, these offspring shared the same wild-type genetic background. After 6 months, the offspring with the transgene were noticeably larger than those that did not contain the modified genes. The most significant result was that the insertion of the transgene disrupted the genetic architecture of growth in the transgenic offspring by interacting with a different set of genes than those expressed in the unmodified offspring. There are strong implications for the ecological risks involved, should transgenic fish escape captivity and interbreed with their wild counterparts. The insertion of the transgene can affect the way that natural selection might act on growth related traits, and may occur in different directions in wild versus transgenic fish. As a consequence, different genetic variation for growth-related genes may arise. The results show that risk assessments also have to consider the stability of genetic variation underlying growth related traits over time. The new work by former SAFS PhD student Miyako Kodama, SAFS professor Kerry Naish, and Robert Devlin of Fisheries and Oceans Canada, appears in the journal Evolutionary Applications.