Heredity (the adjective is hereditary) is the transfer of characteristics from parent to offspring through their genes, or the transfer of a title, style or social status through the social convention known as inheritance (for example, a Hereditary Title may be passed down according to relevant customs and/or laws).
It was apparent to ancient humans that offsprings resembled their parents. For example, Genesis 30-46 tells how Jacob and Labans split their sheep into white and speckled varieties so they could distinguish the two to ensure none were later stolen. Although it was clear that traits were hereditary, the precise mechanism of heredity was not clear.
Ancient Concepts of Heredity
The Greek philosophers had a variety of ideas about heredity: Theophrastus proposed that male flowers caused female flowers to ripen; Hippocrates speculated that "seeds" were produced by various body parts and transmitted to offspring at the time of conception, and Aristotle thought that male and female semen mixed at conception. Aeschylus, in 458 BC, proposed the male as the parent, with the female as a "nurse for the young life sown within her".
Various hereditary mechanisms were envisaged without being properly tested or quantified. These included blending inheritance and the inheritance of acquired traits. Nevertheless, people were able to develop domestic breeds of animals as well as crops through artificial selection. The inheritance of acquired traits also formed a part of early Lamarckian ideas on evolution.
During the 1700s, Dutch microscopist Antoine van Leeuwenhoek (1632-1723) discovered "animalcules" in the sperm of humans and other animals. Some scientists speculated they saw a "little man" (homunculus) inside each sperm. These scientists formed a school of thought known as the "spermists". They contended the only contributions of the female to the next generation were the womb in which the homunculus grew, and prenatal influences of the womb. An opposing school of thought, the ovists, believed that the future human was in the egg, and that sperm merely stimulated the growth of the egg. Ovists thought women carried eggs containing boy and girl children, and that the gender of the offspring was determined well before conception.
Pangenesis was an idea that males and females formed "pangenes" in every organ. These pangenes subsequently moved through their blood to the genitals and then to the children. The concept originated with the ancient Greeks and influenced biology until little over 100 years ago. The terms "blood relative", "full-blooded", and "royal blood" are relicts of pangenesis. Francis Galton, Charles Darwin's cousin, experimentally tested and disproved pangenesis during the 1870s.
Charles Darwin: Theory of Evolution
Charles Darwin proposed a theory of evolution in 1859 and one of its major problems was the lack of an underlying mechanism for heredity. Darwin believed in a mix of blending inheritance and the inheritance of acquired traits (pangenesis). Blending inheritance would lead to uniformity across populations in only a few generations and thus would remove variation from a population on which natural selection could act. This led to Darwin adopting some Lamarckian ideas in later editions of The Origin and his later biological works. Darwin's primary approach to heredity was to outline how it appeared to work (noticing that traits could be inherited which were not expressed explicitly in the parent at the time of reproduction, that certain traits could be sex-linked, etc.) rather than suggesting mechanisms.
Darwin's initial model of heredity was adopted by, and then heavily modified by, his cousin Francis Galton, who laid the framework for the biometric school of heredity. Galton rejected the aspects of Darwin's pangenesis model which relied on acquired traits.
The inheritance of acquired traits was shown to have little basis in the 1880s when August Weismann cut the tails off many generations of mice to find that their offspring did continue to develop tails.
Gregor Mendel: Father of Modern Genetics
The idea of particulate inheritance of genes can be attributed to the Moravian monk Gregor Mendel who published his work on pea plants in 1865. However, his work was not widely known and was rediscovered in 1901. It was initially assumed the Mendelian inheritance only accounted for large (qualitative) differences, such as those seen by Mendel in his pea plants — and the idea of additive effect of (quantitative) genes was not realised until R.A. Fisher's (1918) paper on The Correlation Between Relatives on the Supposition of Mendelian Inheritance. For the subsequent history of genetics, see history of genetics.
Modern Development of Genetics and Heredity
In the 1930s, work by Fisher and others resulted in a combination of Mendelian and biometric schools into the modern synthesis of evolution.
Trofim Lysenko however caused a backlash of what is now called Lysenkoism in the Soviet Union when he emphasised Lamarckian ideas on the inheritance of acquired traits. This movement affected agricultural research and led to food shortages in the 1960s and seriously affected the USSR.
A few countries still award places in their legislatures according to the hereditary principle, including the small African kingdom of Lesotho (Senate includes traditional chiefs) and the United Kingdom (House of Lords includes 92 hereditary lords). But on 7 March 2007, Members of the elected British House of Commons finally voted to abolish the herditary principle altogether and move to a wholly elected second chamber. This will take some years to implement.
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- Stanford Encyclopedia of Philosophy entry on Heredity and Heritability
- Encyclopaedia Britannica, Heredity