Gene expression is a dynamic process that is significantly influenced by environmental factors. This interaction underlies the complex nature of biological development and the phenotypic differences observed among individuals, even among those with identical genetic makeups. Factors such as radiation, temperature, behavior, nutrition, and stress play pivotal roles in determining how genes are expressed. The concept of the reaction range is central to understanding this interaction. It posits that while genetics establish the potential boundaries — such as a height range from 5'5" to 6'1" — environmental influences determine where within these boundaries an individual's traits will fall.

The reaction range theory effectively illustrates that genes set the limits within which environmental factors operate. For instance, if genetics determines a possible height range for an individual, environmental factors like nutrition, physical activity, and overall health during developmental years will influence where within that range the individual's actual height will fall. This theory highlights the environment's significant role in the expression of genetic potential.

This interplay is further elaborated through the genetic-environmental correlation, which underscores a bidirectional relationship. Genes can influence the environment's shaping of individual traits, and conversely, environmental conditions can affect gene expression. A classic example of this is seen in children with a genetic inclination towards reading, who are more likely to seek out books. This behavior can lead to enhanced environmental support, such as increased access to books and encouragement from parents and educators, which, in turn, promotes further skill development in reading.

In the field of epigenetics, the focus is on how identical genetic codes can result in vastly different outcomes. Epigenetics studies modifications that do not change the DNA sequence but still affect gene activity. Identical twins, for example, share the same DNA but can exhibit differences in gene expression profiles due to distinct environmental exposures and life experiences. These differences can lead to diverse health outcomes and physical traits between twins, highlighting the potent effect of environmental factors.

Another illustrative example involves musical abilities. A child with a genetic disposition for music raised in an environment rich with musical experiences is more likely to develop a higher proficiency than one without such opportunities. This scenario shows not only the potential of genetics but also the powerful role of nurturing in achieving and extending genetic possibilities.