Biomedical Engineering Reference
In-Depth Information
Top-Down Versus Bottom-Up
Although times were simpler when Mendel was tending his garden of
Pisum sativum
in the 1860s, the
scientific method used by the father of genetics is virtually identical to that used by a contemporary
molecular biologist. There is the formation of a hypothesis followed by observation and controlled,
documented experiments that produce results. These data are in turn interpreted to determine
whether they support or refute the hypothesis. Regardless of whether the experiments focus on the
color of pea plant petals in a monastery garden or on the results of a microarray assay performed on
a murder suspect's DNA, data are generated, and they must be accurately recorded for interpretation
and future reference.
Mendel recorded experimental findings with paper and pen, and he shared his hypothesis and
experimental methods and results with the scientists of the time through traditional journal
publishing mechanisms. His opus, "Experiments in Plant Hybridization," appeared in the obscure
Transactions of the Brunn Natural History Society
, where it was promptly ignored by scientists in the
hundred or so organizations that subscribed to the journal.
In part because his journal article describing his experiment with peas was very limited in
distribution, and in part because the relevance of his work was at best unclear, Mendel failed to stir
the imaginations of the thinkers of the time. After all, many of his observations were practiced by the
horticulturists of the day; quantifying the breeding practice wasn't viewed as a contribution to
science. As a result, he lost interest in genetics and turned to administration. When he died in
obscurity in 1884, Mendel was an unknown in the world of science.
Mendel's work illustrates the tension between the top-down and bottom-up approaches to
experimental and applied biology. For example, consider how a clinician's top-down view of the
human condition contrasts with that of molecular biologist regarding what contributes to health. As
shown in
Figure 1-8
, heredity, as defined by an individual's genetic structure, is only one of many
variables that a clinician evaluates in assessing overall health. Environmental factors, including
radiation and a variety of chemical and biological agents can modify an individual's genetic structure,
resulting in cancer or genetic changes that may be passed on to offspring. In addition, clinicians
focus on the normal aging process, trauma from accidents, parasitic infestation, and infection by
bacteria and viruses. Lifestyle factors, including smoking, drug use, diet, exercise, and factors that
contribute to stress are also considered.
Figure 1-8. Observations and Heuristics. Top-down approaches are often
based on heuristics and observations, whereas bottom-up approaches are
usually based on first principles.
