The environment can play a significant role in the production of phenotypes. However, the developmental
mechanisms by which the environmental agents effect normal development are just becoming known. At least
three paths have been found through which the environment can modify gene activity. The first is the neuroendocrine route. Here, the nervous system monitors the environment and transfers signals to the endocrine system.
The endocrine hormones can then alter gene expression. The second route involves environmental factors that
change the methylation pattern of genes, thereby altering their transcriptional capabilities. The third route involves the direct induction of gene expression in the host by microbial symbionts. The normal regulation of
phenotype production by the environment should be considered a normal component of development and developmental biology