Introduction to Heredity and Variation

Every living organism resembles its parents more than it resembles an unrelated member of its own species — children look like their parents, seedlings grown from a particular variety of pea breed true to that variety, and yet no two siblings, human or plant, are ever perfectly identical either. Heredity is the biological process by which characters (traits) are transmitted from parents to their offspring across generations, while variation is the existence of differences, however small, among individuals belonging to the same species, including differences between parents and their own offspring. These two phenomena are not opposites but two faces of the same coin: it is precisely BECAUSE heredity faithfully transmits characters from one generation to the next that any new variation, once it arises, can itself be carried forward and preserved in later generations, rather than being lost the moment it appears. The scientific study of both heredity and variation together is called genetics, and this chapter builds the whole of modern genetics on a single, remarkably productive body of experimental work: the pea-breeding experiments that the Austrian monk and naturalist Gregor Johann Mendel carried out in the monastery garden at Brno (in present-day Czech Republic) over roughly seven years starting in 1856, work he published in 1865 as "Experiments on Plant Hybridisation" — a paper the scientific world of his own time almost completely ignored, and whose significance was only recognised more than three decades later, in 1900, when three separate researchers (Hugo de Vries, Carl Correns and Erich von Tschermak) independently rediscovered the same laws.

This chapter follows a deliberate logical sequence, moving outward from Mendel's own carefully controlled experiments to the fuller, messier picture of inheritance that later research revealed. It begins with Mendel's monohybrid cross and the three laws he derived from it — Dominance, Segregation and Independent Assortment — the essential grammar of classical genetics that every later topic in the chapter modifies or extends rather than overturns. It then works through the many genuine exceptions to simple dominance that subsequent geneticists discovered once they studied organisms and characters beyond Mendel's seven pea traits: incomplete dominance, co-dominance and multiple alleles (illustrated by the human ABO blood group system), polygenic inheritance (human skin colour) and pleiotropy (a single gene affecting many characters, as in phenylketonuria). It then moves to the physical, chromosomal basis of inheritance established by Sutton, Boveri and Morgan, and uses that chromosomal framework to explain how sex itself is genetically determined in different organisms — the XX-XY system of humans, the ZW system of birds, and the unusual haplodiploid system of honeybees — as well as the phenomenon of linkage and crossing over between genes that lie on the same chromosome. Finally, the chapter turns to human medical genetics: sex-linked inheritance (haemophilia, colour blindness), Mendelian single-gene disorders (thalassemia, of particular regional importance in West Bengal and eastern India), the practical technique of pedigree analysis used to trace such disorders through a family tree, and chromosomal disorders (Down's, Turner's and Klinefelter's syndromes) that arise not from any single mutant gene but from an abnormal NUMBER of chromosomes altogether.