The Questions of Developmental Biology

One way to search for the chemicals predicted by Turing's model is to find genetic
mutations in which the ordered structure of a pattern has been altered. The wild-type alleles of
these genes may be responsible for generating the normal pattern. Such a candidate is the leopard
gene of zebrafish (Asai et al. 1999). Zebrafish usually have five parallel stripes along their flanks.
However, in the different mutations, the stripes are broken into spots of different sizes and
densities. Figure 1.22 shows fish homozygous for four different alleles of the leopard gene. If the
leopard gene encodes an enzyme that catalyzes one of the reactions of the reaction-diffusion
system, the different mutations of this gene may change the kinetics of synthesis or degradation.
Indeed, all the mutant patterns (and those of their heterozygotes) can be computer-generated by
changing a single
parameter in the reactiondiffusion
equation.
The cloning of this gene
should enable further
cooperation between
theoretical biology and
developmental anatomy.
*If the angle were 90°, the shell would form a circle rather than a spiral, and growth would cease.
If the angle were 60°, however, the next whorl would be 4 feet on that radius, and if the angle
were 17°, the next whorl would occupy a distance of some 15,000 miles!
Principles of Development: Developmental Anatomy
1. Organisms must function as they form their organs. They have to use one set of structures
while constructing others.
2. The main question of development is, How does the egg becomes an adult? This question can
be broken down into the component problems of differentiation (How do cells become different
from one another and from their precursors?), morphogenesis (How is ordered form is
generated?), growth (How is size regulated?), reproduction (How does one generation create
another generation?), and evolution (How do changes in developmental processes create new
anatomical structures?).
3. Epigenesis happens. New organisms are created de novo each generation from the relatively
disordered cytoplasm of the egg.
4. Preformation is not in the anatomical structures, but in the instructions to form them. The
inheritance of the fertilized egg includes the genetic potentials of the organism.
5. The preformed nuclear instructions include the ability to respond to environmental stimuli in
specific ways.