T.H. Morgan, a fruitfly geneticist, furnished several lines of evidence in The Theory of the Gene to illustrate the nature of genes as important factors to define their actual biological function in subsequent generations. He pointed out several facts which are now unanimously accepted by geneticists worldwide. Some of his outstanding views are laid out here. The genes, proposed as “elements of heredity” that Gregor Mendel (Father of Genetics) mentioned as pure theoretical units are actually located in chromosomes as “chemical bodies” of higher order according to Morgan. Also, the separation of hereditary elements in germ cells takes place in such a way that each ripe germ cell comes to contain only one of a particular kind of element. Although there has been a lot of argument as to whether a gene is a material unit or a fictitious unit, that, it has a definite location in the chromosome, is an important criterion to be noted in terms of genetics.
A sequential unfolding of properties of genes basically takes place during embryonic development. An illustration of simple molecular constitution of the gene can help relate or picture the structure of gene as an aggregate structure and its mode of division. But since by infinite sub-divisions, the genes do not reduce in size or alter in their properties, they must essentially compensate by growing between successive divisions. There is a great deal of evidence for the stability of genes as stated by Morgan and other workers, but on rare occasions, this may tend to change, a phenomenon termed as “mutation”, following De Vries’ terminology. In terms of the frequency of mutations, there are a good number of proofs to illustrate the fact that, some genes mutate more often than others and in a few cases, the phenomenon is not infrequent, both in germ cells as well as somatic tissues. However, the constancy of genes with respect to that of others in linear order of chromosomes is not just accidental and very much deducible, both from the point of view of genetic evidence and from cytological observations. Except some of the mutational events like dislocation and reattachment of fragments of chromosomes, prove that it is an accident rather that a mutual interaction that may also determine the location of a gene. Thus, the effects produced by the genes are not consequential of their location and moreover, genes in different chromosomes may produce almost identical effects on the same organs. This aspect may directly be significant from the point of view of physiology of development.
Also, accumulation of certain facts indicated that one gene may be responsible for the expression of multiple traits in an individual (now known as pleiotropism), although it was first an unaccepted phenomenon. The concomitant effects may be of vital importance in terms of economy of an individual, consequently affecting its vitality, longetivity and fertility.
Cytologically speaking, when the chromosomes come together and separate during meiosis, it is the genes rather than the whole chromosomes that come to lie side by side, which is beautifully illustrated by Morgan in this piece of work. The research on salivary gland chromosomes of Drosophila melanogaster (fruitfly), which are very large in nature, served as an elegant landscape for the detection of genes and their corresponding bands. When certain alterations of the order of genes takes place, there is a corresponding change noticed in the sequence of these bands too. Various phenomena like linkage and crossing over could be easily related and understood from the studies on fruitfly reconfirming Mendel’s laws of genetics, which he earlier derived from his work on Pisum sativum (sweet peas). In physiological terms, all genes are active at all times. But, not all cells have the same fate, as genes are subject to developmental changes in the embryo. It is therefore conceivable that different sets/subsets of genes come into play as the embryonic deveelopment takes place. Thus, the genes become active or inactive depending on the microenvironment in which they lie. The genetic programme may and does vary from the early initiation of embryogenesis through differentiation and organogenesis in an adult. Genes were therefore considered by Morgan as great helping hands for several applications like that of understanding developmental programmes, medicine, diagnostics etc. Truly so, the power of genetics has proven to be amongst the most reliable and strongest tools in not only basic sciences but also fields like biotechnology, bioengineering and medicine.