Discontinuous genes

Exon—a coding region within a discontinuous gene. 

Aiuti A, Vai S, MorteUaro A, Casorati G, Ficara F, Andolfi G, Ferrari G, Tabucchi A, Carlucci F, Ochs HD, Notarangelo LD, Roncarolo MG, Bordignon C (2002b) Immune reconstitution in ADA-SCID after PBL gene therapy and discontinuation of enzyme replacement. Nat Med 8 423 25... 

The demonstration of genotoxic activity in a pharmaceutical molecule may not necessarily translate into discontinuation of development and ultimate licensing of the product to be marketed. A survey of the Physicians Desk Reference reveals many examples of pharmaceutical products that have been shown to elicit genotoxic activity in one or more gene-tox assays.41 This observation indicates that other factors (such as risk, benefit, seriousness of ailment, target patient population, dosage, and frequency of administration, among others) are taken into consideration. 

Example 1 the development of behaviour In some animals, behaviour is totally instinctive (or hard-wired), while in others it is more flexible (soft-wired), because it also depends upon some kind of learning. In the first case, the development of behaviour is a continuous process, in the sense that all its phases are determined by genes. In the second case, instead, behaviour develops in a discontinuous way, and arises in two distinct stages after an initial period where only genes control behaviour because learning is not yet possible, there comes a new phase where behaviour depends both on heredity and learning. 

Example 2 the development of sex Sex is determined by genes in most vertebrates, but normally what genes determine directly is only the sex of the gonads. In these cases, sexual development is clearly a discontinuous process in an initial phase, sex determination is entirely genetic but only the gonads are involved, while in a subsequent phase it is hormones that take control of sex determination for all remaining components of the reproductive system. 

It is likely, in other words, that the first animals had embryonic developments totally programmed by genes, and that only later did developmental strategies evolve that could also exploit the supracellular information of the body plan. This shift from a continuous (one-phased) development to a discontinuous (two-phased) one would have been a tranformation of enormous importance, and could well correspond to the Cambrian explosion. This is a new hypothesis, and it may be worthwhile to consider its predictions for what happened before, during and after the Cambrian explosion. 

We have therefore a new model which can be summarised in this way the Cambrian explosion was the transition from a primitive type of development that was totally controlled by genes to a discontinuous type of embryonic development that could also use, from a certain point onwards, the supracellular information of the body plan. 

Less commonly, variation is discontinuous when differences in response reveal a discrete proportion, large or small, who respond differently from the rest, e.g. poor drug oxidisers or fast and slow acetylators of isoniazid. Discontinuous variation most commonly occurs when response to a drug is controlled by a single gene. The term genetic polymorphism refers to the existence in a population of two or more discontinuous forms of a species which are subject to simple inheritance. By convention the frequency of each species is 1% or more. 

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