The transmission genetics, sometimes also described as classical genetics makes the major part of the first half this book, although reference has been made to molecule aspects, wherever considered necessary. These aspects include the following : (i) Mendelian genetics : It involves study of both qualitative and quantitative (polygenic) traits and the influence of environment on their expression; (ii) Morganian genetics : It includes recombination in all kinds of organisms, starting from higher plants and animals to fungi (Neurospora), bacteria, and viruses. Since recombination is one of the sources for releasing hereditary variation, its study at all levels particularly for preparation of linkage maps (also including molecular mechanism of recombination and preparation of molecular maps) has been undertaken in some detail; (iii) Non-Mendelian genetics : It involves a study of the role of cytoplasm and its organelles (particularly chloroplasts and mitochondria) in heredity. It has assumed special importance during the last three decades leading to study of characters located on these organellar genomes and preparation of chloroplast and mitochondrial maps; (iv) Mutations : These are another source of hereditary variation and have been studied at all levels—phenotypic level, biochemical level and molecular level. In a broad sense, these may Include both chromosomal changes (structural and numerical) and also gene mutations. Molecular genetics
During the last few decades, molecular genetics has developed at such a fast rate, that no aspect of genetics is complete without a discussion and explanation at the molecular level. In view of this, structure and function of gene and the regulation of its activity have been studied in considerable detail. Many recent achievements, including isolation and characterization of genes, have facilitated greatly the recent advances in the field of genetics. These techniques, which were regarded not feasible in 1950s, have now become routine procedures in many laboratories around the World. Isolation of genes also led to not only the identification and study of many DNA sequences regulating gene expression, but also to the production of transgenic
animals and plants to be used in industry and agriculture. These newer techniques even allowed a genetic dissection of a hereditary trait examined by Mendel in pea, where it was recently demonstrated that the wrinkled seeds in pea having homozygous recessive genotype (rr)
result due to insertion of a small DNA element in a gene for SBE I (starch branching enzyme).
This gene is responsible for the synthesis of an enzyme SBE I essential for producing round seeds. The insertion of a DNA sequence called transposon
in the gene leads to failure in the production of SBE I and hence in complex metabolic disturbances producing wrinkled seeds.
The techniques of gene isolation have been refined to the extent that, several genes with unknown protein products have been isolated using a variety of techniques. Utilizing recombinant DNA
techniques, genes for a number of human diseases are also being identified, mapped and isolated. This information, it is hoped, will be useful for amelioration of human sufferings due to hereditary diseases, because more than 30% of human diseases can be traced to genetic causes. In a recent programme called Human Genome,
the goal is to determine the sequence of nucleotides in DNA making the whole genome. Under this programme, the number of genes in humans is estimated to be 50,000 to 100,000, of which only 2,000 have been mapped so far (in 1991). Of these mapped genes, about 500 are responsible for different diseases, some of them already isolated and cloned. Many more genes will be mapped, isolated and cloned in future. The significance of these newer developments in the study of genetics of human diseases cannot be overemphasized, because these studies lead us to ask questions like the following: Are we prepared to shoulder this genetic burden? How much money are we willing to spend to keep these genetically handicapped human beings alive and enable them to lead as normal a life as possible? These questions have social implications, so that the science of genetics has to interact with social sciences also.
Similarly, the techniques of molecular mapping, DNA fingerprinting
and genetic imprinting
have already found use in different areas including forensic medicine involving identification of criminals, and/or doubtful parentage.
The significance of genetic studies in agriculture is also well known and need not be
emphasized here. After the green revolution
witnessed in 1960s, we now in 1990s look forward to a second green revolution
resulting from what is also described as gene revolution
by many. The production of herbicide, insect and virus resistant crop plants in recent years suggest that these newer developments in gene technology will make its own contribution to supplement the efforts of conventional plant breeding in increasing production of food, feed, fibre, etc. Similarly, animals are being improved for producing better milk and meat both in quality and quantity. Animals and plants are also being produced for molecular farming,
a phenomenon, where genes of industrial importance are transferred into animals and plants to be used in industry for production of chemicals.