Since the late 1800s—well before advances in genetics and molecular biology—developmental biologists have been studying the mechanisms through which a single cell divides into multiple cell types and gets arranged into tissues, organs, and the organism’s overall form (Gilbert 2013). Genetic knowledge and technologies [provide] a productive strategy to work towards teasing open the complexities of development. (Extracted from p. 1-3 of Nature-Nurture? No).
The first complication of this reason to look for genes is really a clarification. The term “entry-point” is used to distinguish from ideas that one could start from genes as the “blueprint” or “information” from which all else in the development of the organism follows. To appreciate this consider the following (also fromNature-Nurture? No).
When it is said that a person resembles their parent, two aspects of heredity—the transmission of traits to offspring—are being raised: how does an offspring develop to have the trait in question at all, e.g., its eye color, and how does the outcome of the development at some point in the lifespan differ from that person to the rest of the family or population… Difference can… be studied without providing much insight about development. For example, the eyes of fruit flies, normally red, are sometimes white. Biologists identified the location on the chromosomes that corresponds to the white-eye mutation early in the 1900s and later investigated the pigment-formation metabolic pathway and the enzymes involved as fruit fly eyes develop the normal or mutant color. Such knowledge says little about how eyes develop. Even on the narrower question of how eyes get to have color, a lot has to be known already about the development of the eye as a whole to make sense of how and when during the fly’s development the enzyme produces color in the eye (p.2).
Nevertheless, “[b]iologists often study abnormalities in order to gain insight about typical processes of development” (p. 2), and this is where looking for mutant genes that alter development can be helpful. The second complication then is that abnormalities not tied directly to genes can be studied “in order to gain insight about typical processes of development” (p. 2).
In Swyer syndrome, for example, a child with XY chromosomes has female, not male, external genitalia. The influence of estrogen and progesterone on development is illuminated by the finding that, if these hormones are administered at puberty, breasts can develop and regular menstruation can occur (Michala et al. 2008).
Variation under a more or less normal range of conditions can also help biologists understand typical development. The age at which a baby comes to walk, for example, varies according to whether it sleeps on its back or stomach, has been swaddled or carried around in a sling, and so on, and this variation has been related to the timing and degree to which the baby uses its upper body muscles (Fausto-Sterling 2014)(p.2).
Fausto-Sterling, A. (2014). “Letting go of normal.” Boston Review (March/April), http://www.bostonreview.net/wonders/fausto-sterling-motor-development (viewed 21 June 2014).
Gilbert, S. (2013). Developmental Biology. Sunderland, MA: Sinauer.
Michala, L., D. Goswami, et al. (2008). “Swyer syndrome: Presentation and outcomes.” BJOG: An International Journal of Obstetrics & Gynaecology 115(6): 737-741.
(Introduction to this series of posts)