In the very early embryo, a cell needs to ‘know’ where it lies along two main dimensions: fore and aft (anterior/posterior) and up-down (dorsal/ventral). What does ‘know’ mean? It initially means that a cell’s behaviour is determined by its position along chemical gradients in each of the two axes. Such gradients necessarily start in the egg itself, and are therefore under the control of the mother’s genes, not the egg’s own nuclear genes. For example, there is a gene called bicoid in the Drosophila mother’s genotype, which expresses itself in the ‘nurse’ cells that make her eggs. The protein
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