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

144 INTELLIGENT DEVELOPMENT

Needing More Than Simple Cues
What has been discovered since is that each cell is not simply the passive
target of isolated morphogenic cues. It is being literally bombarded with
such morphogens in cross- cutting gradients in diff er ent sequences, in
diff er ent places, at diff er ent times. Like the fading in and out of notes in
music, it is the precise spatiotemporal intersection of these gradients of
morphogens that matters for harmonious messages.
So the simple bicoid model for Drosophila was an impor tant start.
But a vast variety of other factors have since come to light. For example,
other maternal gene products are likewise unevenly distributed (see
fi gure 5.4) and involved in head, thorax, and tail formation. Th ese, too,
are TFs for genes that produce proteins required for segment formation;
for diff erentiation of head, thorax, and tail; and also for dorsal- ventral (up-
down) diff erentiation. Overall, it is the cross- cutting, spatiotemporal pat-
tern of these factors that determines which genes are transcribed in the
egg and where.^4
Numerous morphogens involved in very early development are now
known. Th ey are given exotic names like Hox, Hedgehog, Notch, and Wnt
(a reduction of “wingless” and “integrated”) and BMP (bone morpho-

position in early embryo

Proteins

bicoid nanos

hunchback caudal

low

high

anterior posterior

concentration

FIGURE 5.4

Distribution of four maternal gene products in the oocyte determining early cell dif-

ferentiation. (From Wikipedia.)

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