12th Class Biology Principle Of Inheritance And Variation Heredity

Heredity is the study of transmission of genetic characters and variations from one generation to the next. Heredity involves the transfer of chromosomes from parents to offspring or one individual to another. Therefore, chromosome is the base of heredity. The physical basis of heredity are genes while chemical basis of heredity is DNA.

Pre-Mendelian view points

Vapour theory (Pythogoras) : Different body parts produce minute particles.

Fluid theory : Empedocles, proposed that each body part produced a fluid. The fluid of different body parts of the two parents mixes up and is used in the formation of embryo.

Preformation theory : Malpighi believed that homunculus or miniature individual is present in sperm or egg. Antony Von Leeuwenhoek was first to observe human sperm.

Particulate theory : Maupertuis proposed that the body of each parent gives rise to minute particles. These particles unite together to form the daughter individual.

Encasement theory : Charles Bonnet and his supporters presumed that every female contains within her body miniature prototypes of all the creatures which would descend from her, one generation within the other, somewhat like a series of chinese boxes. This was named as encasement theory.

Theory of epigenesis : Wolff proposed that the germ cells contain definite but undifferentiated substances, which after fertilization, become organised into various complex body organs that form the adult. This idea was referred to as epigenesis.

Pangenesis theory : Proposed by Charles Darwin (1868) according to this theory every cell, tissue and organ of animal body produces minute invisible bodies, called gemmules or pangenes. They can produce offsprings.

Weismann theory of germplasm : August Weismann (1889) suggested the theory of continuity of germplasm. He described reproductive cells as germplasm and rest of the body as somatoplasm.

Pre-Mendelian theories of inheritance are also called theories of blending inheritance.

Evidences against blending theory

Under this concept, the progeny of a black and white animal would be uniformly grey. The further progeny from crossing the hybrids among themselves would be grey, for the black and white hereditary material, once blended, could never be separated again. Pattern of inheritance shown by atavism also speaks against blending theory. The traits of sex do not blend in unisexual organisms.

Basic features of inheritance

(i) Traits have two alternative forms.

(ii) Traits are represented in the individual by distinct particles which do not blend or change.

(iii) Traits may remain unexpected for one or more generations and reappear later unchanged.

(iv) Traits may remain together in one generation and separate in a later generation.

(v) One alternative of a trait may express more often than the other.