Principle of Inheritance and variation

Principle of  Inheritance and variation

Genetics

➢ Branch of biology that deals with the study of heredity and variations.

➢ The term genetics was given by William Bateson in 1906 who derived it 

from Greek word ‘genesis’ meaning to grow into or to become.

Heredity

➢ There is an old proverb that “like begets like”, that is all living organisms tend 

to produce young ones like themselves. 

➢ Heredity is the transmission of genetic characters from parents to their 

offspring.

Inheritance:

➢ It is actually the process by which characters or traits pass from one 

generation to the next. 

➢ An elephant always gives birth only to a baby elephant and not some other 

animal. 

➢ A mango seed forms only a mango plant and not any other plant.

➢ Thus the offsprings resemble their parents.

Variation

➢ It is the degree of differences in the progeny (offspring) and between the 

progeny and the parents. 

➢ The term variation is also used for a single difference in a trait.

Branches of genetics

1.transmission genetics:transmission of genes 

from one generation to 

the next.

2.molecular genetics:It deals with the structure 

and function of genes at a 

molecular level.

3.Population genetics:It deals with the application 

of Mendel’s laws and other 

principles of genetics to 

populations of organisms. 

Historical aspects

➢ Early agriculturists (8000–10,000 BC) knew that causes of variation are hidden 

in the process of sexual reproduction. 

➢ They successfully bred domesticated varieties from wild plants and animals 

through selective crossing and artificial selection.

➢ Chicken is the domesticated form of Wild Fowl.

➢ Sahiwal Cow of Punjab is domesticated form of an ancestral Wild Cow.

Premedian idea about Inheritance

➢ They are often called Theories of Blending Inheritance as they believed that 

characters of the parents blended or got mixed during their transmission to 

the offspring. 

Theories of blending Inheritance:

Moist vapour theory

➢ Pythagoras proposed that, during coitus (intercourse), moist vapours from all 

parts of a male’s body gave rise to a similar body in female’s womb. 

Fluid theory

➢ Aristotle suggested that male’s semen was highly purified blood and the 

female’s menstrual fluid was the female semen which is not as pure as male 

semen. 

➢ The two fluids combined during coitus. 

➢ Female semen provided ‘inert’ fluid for the formation of the embryo and male 

semen gave form and vitality to the embryo. 

Performation theory

➢ It says that the organism is already present, i.e., pre-formed in the sperm or 

egg in a miniature form called homunculus(little man). 

➢ Fertilization is required to stimulate its growth. 

➢ This theory was given by Swammerdam (1679) and advocated by Malpighi.

Particulate theory

➢ Maupertuis (1698 – 1757) considered that heredity is controlled by minute 

particles which come from all parts of the body to the reproductive organs. 

➢ An individual is formed when the particles from male and female combine. 

Kolreuter

➢ The first systematic studies of genetic crosses were carried out by Joseph Kölreuter

from 1761 to 1766.

➢ In crosses between different strains of tobacco plants, he found that the offspring 

were usually intermediate in appearance between the two parents. 

➢ This led Kölreuter to conclude that both parents make equal genetic contributions to 

their offspring.

Theory of pangenesis

➢ Darwin (1868) thought that every cell of the body produces a tiny particle 

called gemmule or pangene. 

➢ It contains both the parental characters and variations. 

➢ All the gemmules or pangenesis of the body cells collect in the gametes and 

are passed on to the zygote.

Theory of continuity of germplasm

➢ August Weismann (1892) proposed this theory. 

➢ According to this theory germplasm is ‘immortal’ and is passed from 

generation to generation. 

➢ Somatoplasm that forms the body is ‘mortal’ and perishes when the organism 

dies.

Gregor johann Mendel

➢ He grew up on a farm in Hynčice (formerly Heinzendorf) which was then a 

part of Austria and is now a part of the Czech Republic.

➢ As a young boy, he worked with his father grafting trees to improve the 

family orchard. 

➢ A rural upbringing taught him plant and animal husbandry and inspired an 

interest in nature.

➢ Mendel selected Garden pea (Edible Pea).

➢ Pisum sativum.

➢ Chromosome number 2n=14.