INTRODUCTION

Human development is the fascinating process by which a single cell the size of a period on a page gives rise to a 3–4 kg baby and eventually an adult with ∼100 trillion (1014) cells and >200 distinct cell types. Genes not only determine inherited traits such as what colour eyes one will end up with, but also govern the developmental process through variable expression of genes in particular cells to give rise to functional tissues and organ systems in a highly orchestrated process. Each cell in the body continues to harbour the exact same genetic information, but the expression of only a subset of the genes in each type of cell makes that cell unique (e.g. a liver versus a skin cell). The cell's expression profile is determined by its interactions with neighbouring cells, signalling from outside and inside the cell, the interactions of genes with each other within the cells, and the physical location of the cell in the body.

Genes that drive development are important in establishing features that are conserved among many organisms, and as such the genes and their functions are highly similar across species. This similarity makes it possible to study gene function in less complex organisms such as Drosophila (the fruit fly) or the mouse in which detailed experiments can be performed across developmental stages. These studies have allowed us to understand the functions of many genes and gene families in humans. In particular the possibility of ‘knocking out’ single genes in these model animals has been instrumental in identifying genes that, when mutated, cause malformations and disease. These model systems also have revealed the complexity of gene interactions and the redundancies in function that occur in some developmental gene families.

This chapter presents families of genes that coordinate gene expression in a sequential and organised cascade to ensure appropriate gene expression and the development of a particular body shape and orientation. Most of the genes play a crucial role in activating and repressing groups of other genes, forming a complex and interdependent network of signals. Only key groups of genes and develop - mental processes are covered, as the topic is too vast to be explored in a single chapter.