Transfer of details such as resemblances and variation of characters from one person to its offspring or next-generation, mainly through direct descendants of the gene, is known as heredity.
PRE-MENDELIAN OR EARLY THEORIES OF HEREDITY
Many theories were put forward regarding heredity before Gregor Mendel. These theories explained in what physical form the traits passed from the parents to their children. These theories tell that the characters of the children are a blend of the characters of the parents. So, they are known as the theories of blending inheritance.
- Proposed by the Greek philosopher, Pythagoras
- Proposed in 500 B.C.
- Tells that each organ of an animal emitted vapors and the new offspring is formed by the combination of vapors from different organs.
- Proposed by Greek philosopher and scientist, Aristotle
- Mainly had four theories
(i)Man’s semen is pure form of blood
(ii)Woman’s menstrual fluid is not much pure form of blood as semen or the female semen
(iii)The two fluids combine during intercourse
(iv)Male semen gave form and vitality to embryo and female semen provided inert substance for the formation of embryo.
1.By Anton von Leuwenhoek
Observed the human sperms and saw them wriggling in semen and called them animalcules.
He proposed each sperm is a potential human being and developed into a new person when introduced to woman’s womb.
Failed since cannot explain the inheritance of maternal characters by the offspring
2.Spermist and ovist theories
Swammerdam, Malpighi (1673), Hartsoeker (1694), Dalempatius (1694), and Roux (1888) and so on proposed that miniature animals were present in the gametes and that the development was unfolding of the miniatures. This miniature form was called a homunculus.
Some believed homunculus was present in the spermatozoon. They were called spermists.
Some believed homunculus was present in the eggs. They were called ovists.
Proposed by French biologist, Maupertius
Tells that each animal produced minute particles for reproduction, and a new individual was formed by the union of particles of these two parents.
Theory of Pangenesis
Proposed by famous English naturalist, Charles Darwin in 1868
He assumed a representative character called pangene or gemmule is formed in each part of the body and migrated via blood to the reproductive cells or gametes, and hence to the child to guide the formation of the respective part. The young one gets the blend of pangenes, hence gets a mixture of characters of both parents.
Disapproved by Weismann in 1892.
Theory of continuity of germplasm
Germplasm is immortal and is only passed from one generation to the next. He tells that somatoplasm perishes when the organism dies and has no role.
There were many theories proposed before Gregor Mendel’s theories. But none of them prevailed because of inadequate physical and chemical details
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