Genes, Brains, and Human Potential The Science and Ideology of Intelligence

INTELLIGENT DEVELOPMENT 143

centrations activate a blue gene, lower concentrations activate a white
gene, with red serving as the default state in cells below the necessary
concentration threshold (fi gure 5.3).
Th e fi rst direct evidence for the theory came in 1982 with the discov-
ery of such a morphogen, called “bicoid,” in the early embryo of the fruit
fl y. It turned out to be the product (in fact, mRNA), not of the cell’s genes
but of a maternal gene. It is produced by the mother and deposited in
the egg before laying such as to form a concentration gradient across
the egg. Bicoid is translated in the egg as a classic TF that then regulates
transcription of genes in cells according to their location in the gradient.
Knockout studies— chemically inhibiting bicoid ’s action— showed that it
is critical in the formation of the embryonic head and body axis. Th e axis
in turn creates a crucial spatiotemporal framework for the action of other
morphogens in the unfolding of the body plan (organs, limbs, and so on).
To Wolpert, the coordination of the patterning rested with the
genes. Th e information is just a simple cue, and the cell itself an obedient
respondent. However, in his 1989 paper “Positional Information Revisited,”
Wolpert was already warning that “patterning by positional informa-
tion provides a relatively simple mechanism for making a wide variety
of patterns. Alas, compared to 21 years ago, that simplicity now seems
more like simple- mindedness. Th ings seem, at this stage, much more
complicated.”^3

morphogen gradient

blue genes active white genes active red genes active

FIGURE 5.3

French fl ag model of pattern formation in development.

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