Early in the twentieth century, T. H. Morgan’s studies of cell structure identified Mendel’s factors as ‘genes’ borne on the elongated bodies, ‘chromosomes’, contained in the nucleus of almost every cell in the body (where the contributions of studies of the fruit fly Drosophila melanogaster are discussed). All organisms develop from the division of cells which previously formed part of one or (where reproduction is sexual) two parent organisms. August Weismann first recognised that the ‘germplasm’ that gives rise to gametes is distinct from the rest of the body, the ‘soma’. Somatic cells divide by ‘mitosis’, the longitudinal splitting of each chromosome with self-replication of each gene so that each half chromosome has exactly the same genes as its parent: all the somatic cells in an individual are genetically identical. Gamete-forming cells first multiply by the same process of mitosis but then divide by ‘meiosis’, a process in which the number of chromosomes is halved and (usually) the two genes in each allelomorphic pair are separated as Mendel postulated. The fusion of gametes combines half the genes of each parent to make a new individual. Gametes are described as ‘haploid’ since they contain half the number of chromosomes of the ‘diploid’ zygote and adult individual.
Only the gametes carry genes to the next generation. Changes often occur in the somatic cells, caused by use or disuse or by direct effects of the environment, but such changes cannot be transmitted to the offspring. Jean-Baptiste Lamarck is rather unfairly remembered mainly for his erroneous belief in the inheritance of acquired characters. Lamarckism has been typified by the idea that if giraffes stretched their necks to reach more food their offspring would be born with longer necks. A change to the body such as an elongated neck cannot directly affect subsequent generations: they can be changed only by the selection of individuals with genes promoting the growth of long necks. The ‘phenotype’, that is the organism defined by the characters made manifest, must be distinguished from the ‘genotype’ or genetic constitution, which alone can transmit changes to the offspring.
Note that the word ‘develop’ was originally used to describe two different consequences of gene action: the sequence of changes in an individual as the egg gives rise to the adult form, called ‘ontogeny’, and (on an enormously greater time scale) the process of evolutionary change, called ‘phylogeny’. We now reserve the term ‘development’ for ontogeny.