Heredity variation and genetics | Pre - Mendelian Ideas about Inheritance | Evidences Against Blending Inheritance

👉 NEET Biology Examination 

👉 NEET Biology Tips

👉 NEET Biology Study Material

👉 UGC Net Study Material

Heredity Variation and Genetic

• 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 varia tion). 
• Early agriculturists (8000–10,000 BC) knew that the 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 about the 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 the 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, ie , 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 ) consid ered 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.

Evidences Against Blending Inheritance 

• Unisexual Trait. The trait of sex does not blend itself in unisexual organisms. Such an organism can either be male or female
• Skin Colour. Children of dark and fair coloured parents should be of intermediate colour if blending inheritance is true.
• This is not the case. The children are often of different colours, some fair - coloured, some dark coloured and others of intermediate colour.
• Atavistic Traits. Many individuals show ancestral characters not found in immediate parents. The phenomenon is called atavism (L. atavus- great grandfather , grandfather or forefather), reversion or throw - back. For example, a short tail may be found in some babies. Some persons are able to move pinna or external ear.
• Particulate Nature. Kolreuter (1760), a German botanist obtained fertile interspecific hybrids in Tobacco.
• The hybrids did not resemble either of the parents. Hybrids were self pollinated.
• Some offspring resembled the hybrids while remaining resembled one or the other grand parent in different characters.
• Thus both smooth and hairiness occurred on the leaves of one generation only to separate in the subsequent generation.
• This proved that the traits have particulate nature and remain discrete (separate)
• Non - Expression of a Trait. John Goss (1822) crossed yellow and green seeded pea varieties.
• The hybrids were all yellow seeded.
• The trait of green colour remained hidden because it appeared in next generation ( in the form of separate particles)
• Offspring of Hybrids. Naudin (1862) did not observe parental characters in crosses involving hybrids. The offspring showed traits of grand parents more than their parents , concluded that on repeated crossing of hybrids, their parental types appear in the offspring showing that hybrids contain traits of both the parents though they may not be visible externally.

========================================

Mail- indiabiologymanishmevada@gmail.com

Connect with this mail for more knowledge of Biology

This Site helpful for UGC Net Examination 

It is also helpful for MCAT (Medical College Admission Test) united States, Australia, Canada and caribbean islands.

For Join With Me Mail Me 

Manish Mevada

M.Sc, M.Phil, B.Ed

THANK YOU.