Inheritance

The obvious similarities between children and their parents, or sometimes their grandparents, have long been recognized. Despite many attempts to explain this phenomenon, it is only in recent years that our knowledge of the process of heredity has enabled us to understand the mechanism fully.

The fact that in some species both male and female are needed to produce offspring, was realized from early times. The role each sex played was, however, a matter of argument. When Anton van Leeuwenhoek observed sperm in human semen, the idea arose that these contained a miniature human. So it was stated that the female's role was nothing more than a convenient incubator. Around this period, Regnier de Graaf discovered in ovaries what later would be called the Graafian follicle. This was thought to contain the miniature human, so the male’s sperm was considered to be just a stimulus for its development.

The problem with both these beliefs was that it could easily be observed that any offspring tended to show characteristics of both parents rather than just one. This led, in the last century, to the idea that both parents contributed hereditary characteristics and the offspring was merely an intermediate blend of both. While closer to present thinking, it also was not quite true. Logically the children of a tall father and a short mother should be of medium height. But it is not always like this. The offspring has two sets of genetic information - one from the mother and one from the father. For any individual characteristic, e.g. height or eye colour, only one of the two factors expresses itself. A few characters do show an intermediate state between two contrasting factors, but this is relatively rare.

Alongside these changes in the last century, another development took place. It was originally believed that new species would arise spontaneously in some manner. By the end of the century it was more or less accepted that they were formed by adaptation of existing forms. Natural selection is considered to be the mechanism by which these changes arise. Without this variety and consequent selection of the types best suited to the present conditions, species could not adapt and evolve to meet the changing demands of the environment.

If this theory of evolution is accepted, then the process of inheritance must permit variety to occur. At the same time, if the offspring are to be supplied with the same genetic information as the parents, the genetic material must be extremely stable. This stability is especially important to ensure that favourable characteristics are passed on from one generation to the next. This then is the paradox of inheritance - how to reconcile the genetic stability needed to preserve useful characteristics with the genetic variability necessary for evolution. To satisfy both requirements it is necessary to have hereditary units which are in themselves stable, but which can be reasserted in an infinite variety of ways. A summary of the historical events which contributed to our current understanding of heredity is given in Table:

Historical review of the main events leading to present-day knowledge of reproduction and heredity

Name	Date	Belief/ Observation/ discovery
Aristotle	384-322 BC	Mixing of male and female semen is like blending two sets of ingredients which gives ‘life’
General scientific belief	Up to 17th century	Simple organisms arise spontaneously out of non-living material
van Leeuwenhoek	1677	Discovered sperm – it was generally believed that these contained miniature organisms which only developed when introduced into a female
de Graaf	1670s	Described the ovarian follicle (later called Graafian follicle)
Lamarck	1809	Proposed theory of evolution based on inheritance of acquired characterisrics
Darwin	1859	On the Origin of Species by Means of Natural Selection was published
Pasteur	1864	Experimentally disproved the theory of spontaneous generation
Sutton	1902	Observed pairing of homologous chromosomes during meiosis and suggested these carried genetic information
Garrod	1908	Postulated mutations as sources of certain hereditary diseases
Johannsen	1909	Coined term ‘gene’ as hereditary unit
Oparin	1923	Suggested theory of origin of life
Avery, McCarty and McCleod	1944	Showed nucleic acid to be the chemical which carried genetic information
Hershey and Chase	1952	Showed DNA to be the hereditary material
Watson and Crick	1953	Formulated the detailed structure of DNA

Grammar 1. Неличные формы глагола. Инфинитив.