Pre-Mendelian ideas about heredity – Vapour and fluid theory, Magnetic power theory, Preformation theory, Lamarck’s theory, Darwin’s theory, Germplasm theory and Mutation theory.
Pre-Mendelian ideas about heredity:
In ancient times, there were speculations on the nature of heredity. Early philosophers and workers had forwarder various ideas or theories to explain the phenomenon of inheritance. They are briefly presented below.
Moist Vapour Theory:
Pythagoras (580-500 B.C.) believed that each organ of the body of male produced moist vapours during coitus which formed the body parts of the embryo.
Fluid Theory:
Empedocles (504-433 B.C.), proposed that each body part produced a fluid.
The fluid of different body [arts of the two parents mixes up and is used in the formation of embryo.
Any defect in the descent and mixing up of the fluids results in missing of characters of one parent or both the parents.
Reproductive Blood Theory:
Aristotle (384-322) thought that the males produce highly purified reproductive blood containing the nutrients from all body parts.
Females also produce reproductive blood but this in impure.
The two reproductive bloods coagulate in the body of the female and form the embryo.
Due to purity of reproductive blood, the contribution of characters by the male is more than the female.
Preformation Theory:
The theory was proposed by two Dutch biologists, Swammerdam and Bonnet (1720-1793). This theory states that a miniature human called ‘homunculus’ was already performed in the egg and sperm. The development of zygote resulted only in the growth of a miniature humans who was already present in the egg and sperm. However, this theory was rejected because this could not be proved scientifically.
Theory of acquired characters (Lamarck’s Theory):
This concept was proposed by Lamark (1744-1829), a French biologist. This theory states that a new character once acquired by an individual shall pass on to its progeny. This theory was disproved by Weismann. He cut the tail of mice for successive generations and always got the baby mice with tail. Thus, this theory was rejected.
Theory of Panngenes (Darwin’s Theory):
This theory was proposed by Charles Darwin (1809-1882), an English naturalist. According to him, each part of the animal body produces a minute copy of its own, called gemmule or pangene. The gemmules are collected in the reproductive organs. The gemmules were passed on to the gametes. The young one formed from the gametes would be having all the gemmules characteristics of the parents, and will represent a blending of the qualities of its two parents. Thus, theory of pangenesis is also known as the ‘theory of blending inheritance’.
Germplasm Theory:
This theory was advocated by August Weismann (1834-1914), a German biologist. According to this theory, organism’s body contains two types of cells namely somatic cells and reproductive cells. The somatic cells form the body and its various organ systems, while the reproductive cells form sperm and ova. The somatic cells contain the ‘somatoplasm’ and reproductive cells contain the ‘germplasm’.
The germplasm can from somatoplasm, but somatoplasm can not form germplasm. The cells of the somatoplasm become differentiated during the formation of the compels organs of body while cells of the germ cells remain undifferentiated and retain their power to generate life. The germplasm thus goes on in continuous stream from generation to generation. Changes in the somatic cells (somatoplasm), which were caused by the environment, cannot influence the germplasm and hence acquired characters are not inherited.
Mutation Theory:
Hugo de Vries (1901) put forward a theory of evolution, called mutation theory.
The theory states that evolution is a jerky process where new varieties and species are formed by mutations (discontinuous variations) that function as raw material of evolution.
Theory of Epigenesis:
Proposed by a German Biologist Wolff (1738-1794), the theory states that the egg and sperms are undifferentiated cells.
The differentiation into various organs/parts takes place only after fertilization in the zygote resulting in the development of adult tissues and organs.
This concept is universally accepted.
Magnetic power Theory:
It was proposed by William Harvey and its suggested that the uterus by the friction of coitus acquire some magnetic power to conceive an embryo.
Mutations or discontinuous variations are the raw material of evolution.
Mutations appear all of a sudden. They become operational immediately. All mutations are inheritable.
Useful mutations are selected by nature. Lethal mutations are eliminated. However, useless and less harmful ones can persist in the progeny.
Accumulation of variations produces new species. Sometimes a new species is produced from a single mutation.
Evolution is a jerky and discontinuous process.