Heredity, Variation and Genetics | Pre - Mendelian Ideas about Inheritance

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Heredity

• Heredity (L. hereditas- heirship or inheritance) There is an old proverb that " like begets like ”, that is all living organisms tend to produce young ones like themselves.
• 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 offspring resemble their parents.
• Heredity is the transmission of genetic characters from parents to their offspring.
• Heredity is also called inheritance is actually the process by which characters or traits pass from one generation to the next.

Variation

• Variation 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.
• In that case the various differences in the traits are termed as variations (plural of variation).
• Early agriculturists (8000–10,000 BC) knew that causes of variation are hidden in the process of sexual reproduction.
• Because of it, they successfully bred domesticated varieties from wild plants and animals through selective crossing and artificial selection. Chicken is the domesticated form of Wild Fowl.
• Indian Cow (e.g. , Sahiwal of Punjab) is domesticated form of an ancestral Wild Cow.
• However, our ancestors had no idea scientific basis of inheritance and variation.

Genetics

• The branch of biology that deals with the study of heredity and variations is known as genetics.
• The term genetics was given by Willian Bateson in 1906 who derived it from Greek word ' genesis ' meaning to grow into or to become.

Branches of Genetics

• There are three main branches of genetics : Transmission genetics , Molecular genetics and Population genetics.
• Transmission Genetics ( Also called classical or Mendelian genetics ). It deals with the transmission of genes from one generation to the next. It is dealt in this Chapter
• Molecular Genetics. It deals with the structure and function of genes at a molecular level . It is dealt in the next Chapter.
• Population Genetics. It deals with the application of Mendel's laws and other principles of genetics to entire populations of organisms.

Pre - Mendelian Ideas about Inheritance

• A number of view points were put forward prior to Mendel to explain the transmission of characters from parents to offspring.
• They are often called theories of blending inher itance as they believed that characters of the parents blended or got mixed during their transmission to the offspring
• Moist Vapour Theory. Pythagoras (580-500 B.C.) 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 (384-322 B.C.) suggested that male's semen was highly purified blood and the female's menstural 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.
• Preformation Theory. The theory of preformation believes that the organism is already present , i.e. , preformed in the sperm or egg in a miniature form called homunculus.
• Fertilization is required to stimulate its growth.
• Sperms were obeserved for the first time by Leeuwenhoek, in 1672.
• Preformation theory was given by Swammerdam (1679 and advocated by Malpighi (1673).
• It was believed by a num ber of workers of that period like Hartsoeker (1694) and Dalempatius (1694).
• It was supported by Roux as late as 1888 but discarded by Wolff who suggested that organs are formed step by step (theory of epigenesis)
• Particulate Theory. Maupertius (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.
• 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 pangenes of the body cells collect in the gametes and are passed on to the zygote where they guide the growth of different parts of the embryo to form an offspring.
• Theory of Continuity of Germplasm. Weismann (1892) proposed this theory.
• According to this theory germplasm ( protoplasm of germ cells ) is 'immortal' and is passed from generation to generation.
• Somatoplasm (protoplasm of somatic cells) that forms the body is ' mortal and perishes when the organism dies.

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