The Boy Who Gave Birth to His Twin

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Control genes

For the last 30 years, biologists have been struggling to identify the architects of embryonic development. In their search they have stumbled across a special class of genes whose sole function is to control suites of subordinate genes. These control genes work like molecular addresses, specifying the location of regions or structures within the developing embryo. For example, one gene might say 'eye'. In cells where this gene is active, a suite of subordinate genes swings into action as the cells become directed towards the formation of an eye. In practice, the developmental decision-making involves the hierarchical action of different sets of control genes. These combine in sequence to organise the progressive regionalisation of the body.

Developmental control genes were first found in flies, but they have since turned up in a whole host of animals, including ourselves. In fact, control genes seem to be remarkably similar across the animal kingdom. It's as if evolution found an effective way of building a body and then stuck firmly to it. These genes are so similar that you can remove a fruit fly control gene and replace it with the human equivalent and the fly will develop as normal.

To illustrate the importance of control genes in successful development, you only have to look at mutant fruit flies in which one or more of the genes have become damaged or defective. With the molecular address book thrown into confusion, the fruit fly body plan is literally turned upside down. Legs can end up on heads where the antennae are supposed to be, and eyes can sprout on the tips of legs. In one unfortunate mutant, the head disappears altogether, and is replaced by a second anus.

Though control genes are a major factor in the programmed pattern of embryonic development, they are clearly not the whole story. For at least in the first few days of its life, an embryo is still very much under its mother's control, relying on her chemical signals to see it through the first few rounds of cell division. Even though the embryo soon gains genetic autonomy, maternal influence extends throughout pregnancy. After all, a human embryo is nothing without its mother, and the placenta becomes a place of both sustenance and susceptibility.