142 INTELLIGENT DEVELOPMENTanterior and posterior (or a front and a rear). Other aspects of the envi-
ronment of the egg (e.g., the uterine lining, or the surface of the soil) may
also help defi ne polarity.
In addition, the mother has deposited specifi c mRNAs or TFs into the
egg cell that are unevenly distributed. When the zygote divides, some of
the daughter cells will contain more of those chemicals than others. Th eir
concentrations infl uence the decisions as to which genes are transcribed.
Th e diff er ent proteins so produced alter the structure and function of
cells in diff er ent ways. Cell fates, that is, are already unevenly distributed
in the egg.
In other words, we already see how a uniform ball of cells becomes
diff erentiated according to the structure of its environment— involving
genes, but not under instructions from them. Th e diff er ent cells must now
signal to one another about their relative positions, so that each further
“knows” what it should become.
Th ere is a long and fascinating history of ideas on how this happens. It
was long ago realized that some kind of positional information is needed,
as in the coordinate systems in today’s land maps and GPS systems. In a
paper in 1952, Alan Turing suggested that concentration gradients of
diff usible chemicals might serve that purpose. Th ese chemicals were duly
dubbed “morphogens,” and the theory was elaborated by Lewis Wolpert
and Francis Crick in the 1960s.
Th e idea is that morphogens are secreted by cells and diff use
through the tissues of an embryo during early development, setting up
concentration gradients. Each cell is given positional information by its
place in the gradient. Th e cell then switches genes on or off to take con-
trol and direct the assembly of components that follows. Cells far from
the source of the morphogen will receive it in low concentrations. Th ey
express only low- threshold target genes and associated products. In
contrast, cells close to the source of morphogen will receive it in higher
concentrations. Th ey will express both low- and high- threshold target
genes and products.
Th is was the idea superbly represented in Lewis Wolpert’s French fl ag
model. In this case, the morphogen targets genes that, meta phor ically
speaking, produce diff er ent colors according to concentration. High con-
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