4 - Constitutive Epigenetics

Summary

Developmental biologists and geneticists usually focus on different aspects of genes (translation versus transmission). The geneticist uses a particular view of genes as units of heredity (i.e. transmission to the next generation) and may neglect the role of genes in development. Consequently, the developmental biologist may ask whether the distinction between genotype and phenotype advances genetics by leaving out development. Does evolutionary genetics provide a sufficient theory of morphological evolution? The mapping function from genotype to phenotype is not one-to-one. A gene may affect multiple structures (pleiotropy) and traits are often affected by many genes (polygeny). Furthermore, the mapping of gene effects on phenotype may be nonlinear. Because gene action during development is a cyclic series of gene-cell interactions, genes are just one element in the developmental process. Thus the nature of interactions is the primary issue in development.

–S. J. Arnold et al. (1989)

Let us briefly take stock. In Chapter 1, I suggested that the problem of development should not be reduced to the problem of gene action and activation. In Chapter 2, I provided three examples to orient the discussion throughout the rest of the book, the first two of which have been implicated in the argument so far. In Chapter 3, I introduced and explored the nature of the modern consensus on development, according to which development is construed as a matter of the epigenetic activation of preformed genetic information.