An organism's phenotype (physical characteristics) is a result of the interaction between its genotype (genetic makeup) and the environment. A person with the genetic potential to be 7 feet tall won't achieve that height in the absence of proper diet, for example. A malnourished person will be stunted compared with a genetically identical individual who was well fed. The impact of the environment on the developing organism is called environmental effects, and one way that the environment can affect phenotype is by affecting development. (You can read more about what affects phenotype in Chapters 4 and 7.)
The following sections highlight examples of environmental effects. Here's the take-home message: Small changes in the regulation of development in genetically similar individuals can have major effects.
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