… we attempt to understand normal causal pathways by examining the effects of major disturbances.
This is from The Triple Helix: Gene, Organism, and Environment by Richard Lewontin (2000, 1998):
… It is all very well to say that certain genes come to be transcribed in certain cells under the influence of the transcription of certain other genes, but the real question of the generation of form is how the cell “knows” where it is in the embryo. For this purpose we do not need a revolutionary insight into the laws of biology, but only a lot of hard work.
… much of the phenotypic variation that appears among individuals in a population of Drosophila or any other organism, variation in size, shape, physiology, and behavior, cannot be traced to any well-defined variation for a particular gene, if they are influenced by genes at all.
[line break added] Human skin color, for example, is clearly heritable, at least if we consider the differences between geographically defined populations, but the genes for human skin color have never been located. In contrast, the repeated natural experiment of human relocation and cultural absorption shows that the world variation in the phonemic structure of human language is not influenced at all by genetic differences.
… If we want to understand the causes of differences in shape, size, color, behavior, and physiology among individuals we must be prepared to work with genetic differences at many gene loci, each of small effect, and with interactions between gene and environment.
… Whether by means of mutations of large effect, or by chemical, electrical or surgical manipulation, or by the study of accidental traumatic damage to the physiology and anatomy of organisms, we attempt to understand normal causal pathways by examining the effects of major disturbances.
[line break added] The correspondence between cognitive function and brain anatomy is studied in split-brain experiments, or in lobotomies, or in people who have suffered severe destruction of parts of the brain. The acquisition of bird song is studied by deafening birds to see how much difference aural experience makes.
[line break added] All studies of the genetic mediation of development are carried out using gene mutations or methods of genetic engineering that block signaling between parts of the genome or prevent the production of particular gene transcripts.
The only way to find out whether such knockout experiments have identified the elements of normal function is to analyze the small variations that are normally encountered to see whether the same causal pathways are involved.
My most recent previous post from Lewontin’s book is here.
-Julie
Coloring 