heredity In popular parlance, the word ‘heredity’ is used to explain the observation that every living organism gives rise, through reproduction, to a look-alike organism. In biology and medicine, it is a term refering to the biological information that is transmitted from parents to offspring in every generation. Nowadays, the field of genetics is responsible for the scientific study of heredity and its mechanisms, and the main focus of genetic research is the examination of the
gene as carrier of information on the structure, function, and biological attributes of the organism, and its transmission to subsequent generations.
The term ‘heredity’ was introduced into the English language in the 1860s from the French
hérédité, as a noun referring to the properties and characters considered as hereditary. The term ‘heredity’ was preferred over the existing term ‘inheritance’ by biologists of the time, because it was not loaded with the Lamarckian overtones of the latter. Borrowed from landed gentry and used to refer to old family property as well as to that acquired during a particular lifetime, the term ‘inheritance’ was associated with notions of acquired characteristics. Francis Galton, an active spokesman for the importance of heredity in the human make-up, and founder of the science of
eugenics, claims in his autobiography to have been the first to use the term ‘heredity’ in the 1860s. However, other biologists, such as Charles Darwin, had started using the term some years earlier.
In 1900, Gregor Mendel's 1866 paper on the study of hybrids of the edible pea was independently ‘rediscovered’ in Europe. Although Mendel's experiments were part of his interests on the origin of new species by hybridization (rather than by variation), and were thus not directly concerned with the elucidation of the laws of heredity, they were interpreted in 1900 as the first systematic study unravelling the mechanisms of heredity.
Gregor Mendel (1822–1884), an Augustinian monk at Brno, Moravia (now part of the Czech Republic), performed his classic experiments using varieties of the edible pea (
Pisum sativum) grown in the monastery garden. By artificial fertilization, he crossed two pure varieties of peas and followed the inheritance of seven pairs of character differences (yellow or green seeds; round or angular seeds; white or grey-brown seed coats; green or yellow pods; smooth or ridge pods; tallness or shortness; axillary or terminal flowers). He reported that, in the first hybrid generation (F1), only one character in each pair of character differences would be manifested. He used the word ‘factor’ to refer to the determing agent responsible for each character, and described their effects as either dominant or recessive. Through self-fertilization, he crossed the F1 to produce the second hybrid generation (F2) and reported the reappearance of the recessive characters in a 1:3 ratio. Mendel explained his results by describing the characters studied as distinct, stable factors, which were passed on independently and unchanged from parent to offspring. Although the recessive characters would be masked in the F1, their independent transmission from parent to offspring could be confirmed by observing their reappearance in the F2. The reappearance of hidden recessive characters in the F2 disagreed with prevailing notions on ‘blending’ inheritance, postulating the blending and dilution of parental traits in the offspring. Mendel also carried out the self-fertilization of the F2, from which he confirmed the existence in the F2 of three types of plants: two pure parental types and one hybrid type.
Mendel's hybridization experiments are theoretically formulated in the figure. As example, this shows the cross between two varieties of peas displaying seed colour as character difference.
In 1900, with the international recognition of Gregor Mendel's work as the basis for a new science of heredity, a new wave of experimentation with hybrid formation began that appealed to the breeding interests of botanists and zoologists. In 1906, the Cambridge zoologist William Bateson introduced the word ‘genetics’ to refer to the expanding new field of research. Bateson became a vocal defender of the validity of Mendel's conclusions as the scientific foundation for the new discipline. He encouraged the use of Mendelian principles not only for the study of the plant and animal world, but also for the examination of heredity in humans. On February 1, 1906, he addressed the Neurological Society of London on the topic of Mendelian heredity and its application to man. In this lecture, Bateson presented to an audience of physicians a new picture of human heredity in which human physical traits were treated as Mendelian segregating characters, and he reformulated human hereditary disease as being caused by single genetic factors obeying Mendelian principles. He explained brachydactyly, congenital cataract, albinism, alcaptonuria, haemophilia, and colour blindness as being caused by Mendelian factors (dominant or recessive) of heredity.
Bateson spoke extensively about the behaviour of Mendelian factors, but was unable to provide a material mechanism guiding their operation. He refused to accept ideas associating the gene with a particular stretch of chromosomal material. However, between 1910 and 1915, Thomas Hunt Morgan and his students, working at Columbia University, New York, gathered enough data to support successfully the chromosomal theory of the gene, which firmly established the Mendelian genetic factors as material unities, or ‘genes’, embedded in the chromosome. The use of the chromosomal theory of the gene gave rise to a very productive area of experimentation, now known as ‘classical genetics’ which produced the first genetic maps, showing the relative positions of genes on the chromosome, and a gave clear notion of the nature of mutations.
Outside the laboratory, the concept of heredity occupied a crucial role in debates on the importance of nature over nurture and on the possibilities of using biological norms to guide social reform during the end of the nineteenth century and the first decades of the twentieth. Hereditarian theories, considering heredity as the central factor determining human character, were used by biologists, physicians, and social activists to explain human temperament, family pathology, and the structure of society. Francis Galton, a strong believer in the hereditarian position, founded the discipline of
eugenics, which sought to improve the quality of human heredity by manipulating human reproduction. The field of eugenics developed into a breeding programme proposing a series of measures to prevent the reproduction of those labelled as ‘unfit’ or ‘feebleminded’. As a counterpart, such programmes sought to promote the reproduction of those harbouring in their heredity ‘superior’ human qualities. Eugenic thought became highly influential during the first decades of the twentieth century in the US and in Britain, Germany, and other parts of Europe. It started losing its pre-eminence in the 1930s and 1940s, when it was highly criticized by scientists and the public for its scientific inaccuracy, for its class and race bias, and for the excesses to which it could lead, as exemplified by the horrors taking place during the implementation of state controlled reproductive policies in Nazi Germany.
Silvia Frenk
See also
eugenics;
gene;
genetics, human.
