RESUMEN
Cancer and ageing are often said to be diseases of development. During the past fi fty years, the genetic components of cancer and ageing have been intensely investigated since development, itself, was seen to be an epiphenomenon of the genome. However, as we have learned more about the expression of the genome, we fi nd that differences in expression can be as important as differences in alleles. It is easier to inactivate a gene by methylation than by mutation, and given that appropriate methylation is essential for normal development, one can immediately see that diseases would result as a consequence of inappropriate epigenetic methylation. While fi rst proposed by Boris Vanyushin in 1973, recent studies have confi rmed that inappropriate methylation not only causes diseases, and it also may be the critical factor in ageing and cancers.
RESUMEN
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.