The other face of pattern

The Drosophila characteristics that allowed the discovery of the first developmental genes are the so-called homeotic mutations which 

William Bateson first described in 1895. These mutations do not arrest development but transform one part of the body into another. Antennapedia mutations, for example, transform antennae into legs, which gives rise to an insect with two legs sprouting from its head, whereas bithorax mutations transform the third thorax into a second one, giving the insect an extra pair of wings. 

The great attention which has been given to the spatial characteristics of pattern appears, however, to have overshadowed its temporal properties. Pattern genes are expressed in a very early period of development, but their effects last for the whole life of an organism. Pattern genes, in other words, are genes that contribute to cell memory. They are expressed very early, and only for a very short period of time, but pattern characteristics survive indefinitely, and somehow, therefore, a cell keeps a memory of what they did. We could even say that, after the brief expression of its genes, pattern only lives in the memory space of a cell. 

Pattern does not account for the whole of cell memory, because there are many other characteristics of determination that have long-lasting effects, but it is certainly an important part of that memory. The discovery of the pattern genes, in conclusion, can also be regarded as the discovery of the first genes that contribute to cell memory. 

---