1.2 The ‘genes and behaviour’ problem
Amidst the progressive change to the brain and nervous system that occurs during development, there is one constant, one fixed element; the set of deoxyribonucleic acid, DNA, molecules found in each cell. These molecules are the genetic material, and they store the information for the construction of the organism. The same set of DNA molecules is found in every cell of an organism (its genome). (There are some exceptions to this such as sperm, unfertilised egg and red blood cells, but they need not concern us at present.) A different set of DNA molecules is found from one organism to another. Each person has a slightly different set of DNA molecules from each other person, i.e. there are genetic differences between people.
The chain of events that links the genome to protein production has been worked out in some detail, and the mechanism is universally accepted and very similar across all organisms. What has proved problematic is extending that chain of events from the genome to behaviour; how the genome, a set of molecules that guide protein production, affects behaviour, several levels of organisation away from molecules, is not easy to visualise. Take, for example, the garden spider (Araneus diadematus). It hatches from an egg sac sometime in the spring, wanders about for a while and then constructs an orb (disc-shaped) web; a web characteristic of the species (Figure 2a). A spider of a different species, say the triangle spider (Hyptiotesparadoxus), builds a triangular web (Figure 2b), whilst the wolf spider (Pardosaproxima), builds no web at all. All these spiders produce silk, yet what they do with it varies from species to species, in a species-specific way. Spiders receive no outside instruction in silk use (i.e. they do not learn from other spiders or books!). What is puzzling is the fact that the different species use their silk in different ways. Why doesn't the wolf spider build an orb web? Why doesn't the garden spider build a triangular web?
The only information every spider can be guaranteed to have is in its genome. The genome varies from species to species, and it follows that the genome must be sufficient to ensure that the spiders use their silk in a way that is characteristic of the species. So, how does the genome affect web design, such that an orb web, a triangular web or no web is constructed? That the genome influences the structures and musculature of the spider by producing or not producing certain proteins at particular times is just about possible to imagine, though by no means easy. It is also possible to argue that the size, shape and strength of the spider's legs and body have a direct effect on the distances between threads and tension of a web. This is fine if the only problem is differences between orb webs. But that is not the only problem. The real problem is how genes can affect web design, to produce an orb or a triangle, and how genes can affect whether silk is used to construct webs at all. The real problem is how differences in the genome cause the differences in behaviour between the three species of spider. At the moment there is no satisfactory resolution to this problem.
Now think about people and the range of behaviours they exhibit. How do genes influence human behaviour? The problem is even more intractable than with the simple spider example. Note that this discussion has accepted that genes affect behaviour; logically, in the case of the spiders, they must do. The difficulty is in explaining how genes affect behaviour. It is very difficult to conceptualise. So in the following discussion, when differences in genes are linked to differences in behaviour, remember that there is not a neat, sequential, causal relationship between a gene and a behaviour; it's more complex than that.
The course returns to this discussion of genes and what they do, in Sections 8 and 9. The next section describes the changes in the nervous system during the early months of life.