In previous sections, we discussed the genetic control of phenotypic traits at different levels including phenotypic level, biochemical level, and molecular level. This has helped us in developing an understanding of how an individual gene consisting of a DNA segment may function in giving rise to a phenotype mediated through the synthesis of a protein and how can the activity of such a gene be regulated. However, the co-ordinated activity of different genes and the regulation of this activity leading to different developmental patterns, starting from an egg or a zygote to the fully developed adult is a subject, which is most difficult to study due to the limitation, which our experimental designs impose on such a study. This subject is sometimes described as the study of differentiation and can be studied, again at various levels including morphology, biochemistry, molecular biology and genetics. Only a coordinated study involving these different approaches will allow us to develop a proper understanding of how and why a particular pattern of development is followed by an individual. As an example, we may consider thei development of an adult human (with about 1014 cells) from a single cell i.e. zygote derived due to fusion of an egg and a sperm. A popular question in this connection is : how does a ball of seemingly identical cells (the blastula), derived by mitotic divisions from single fertilized egg, becomes differentiated into an organism of recognizably different regions and tissues ?
It has now been shown in several cases that all cells of blastula, are actually not identical but consist of at least two populations, one responding to signals from the others (shown in some amphibians). From this initial difference in cells, during the process of development, anterio-posterior and ventro-dorsal distinctions will arise and a vast and amazing array of cell types will be found in the mature individual, so that even two adjacent cells may have different phenotypes and functions. The developmental pathway is so precise, that each step of development is dependent on the preceding stage. Any disruption in the normal sequence of events results in an abnormal phenotype, sometimes due to mutations. It is these abnormal phenotypes and the gene mutations responsible for them, which help in the proper understanding of the sequence of events and the basis for this sequence.
In any study of development, we know that every cell of an organism, at any stage of its development, contains the entire genome with all genes. Then, how these genes regulate their activity in a programmed manner to confer the complexity observed in the multicellular organism ? An analysis of this problem at the genetic level requires a simple system amenable to experimentation, in order to probe into the mechanism involved. In this section, an effort will be made, with the use of some examples, to demonstrate how in recent years some information on the genetics of development became available. We will also discuss, what kind of questions are likely to be answered in future, due to the powerful techniques of recombinant DNA developed during 1970s and 1980s and more particularly during 1980s and 1990s.