Heredity and Variation

• They are variations which affect the somatic or body cells of the organisms. They are also called modifications or acquired characters because they are got by an individual during its life time. 
• They are caused by three factors: 

• Environment: The environmental factors are medium, light, temperature, nutrition, wind, water supply, etc. The environmental factors bring about changes in phe¬notype of the individual. 
• Use and disuse of organs: The organ which is put to continuous use develops more while the organ less used develops little.
• Conscious efforts: Modifications due to conscious efforts are observed only in those animals which have intelligence. Receiving education, training of some pets, slim bodies, boring of pinna, long neck, small feet, mutilations in pets, bonsai, etc. are some of the examples of conscious efforts.

• They are produced in the germ cells of an organism and are inheritable. They may be already present in ancestors or may be formed suddenly.
• Accordingly, the germinal variations are of two types: 

• Continuous variations: Con¬tinuous variations are typical of quantitative characteristics. They show differences from the average which are connected with it through small intermediate forms. They are produced by chance separation or segregation of chromosomes, crossing over or chance combination of chromo¬somes during fertilization.
• Discontinuous variations: They are sudden unpredictable inheritable departures from the normal without any interme¬diate stage. Discontinuous variations are caused by chromosomal aberrations like deletion, duplication, inversion and translocation or change in chromosome number through aneuploidy and polyploidy, or change in gene structure and expression due to addition, deletion or change in nucleotides.

• A number of viewpoints were put forward prior to Mendel to explain the transmission of characters from parents to offspring. 
• They are often called theories of blending inheritance as they believed that parents blended or got mixed during their transmission to the offspring.

1. Moist Vapour Theory: This theory was advocated by Pythagoras. He believed that the male body produced some sort of a moist vapour during coitus, which helped in the development of the body parts of the embryo. 
2. Reproductive Blood Theory: This theory was propounded by Aristotle. He was of the belief that both the males and the females produced reproductive blood. But the male reproductive blood was purer than the female reproductive blood. When the two reproductive drops of blood coagulated to form the embryo, it was due to the male’s pure blood that the characteristics of the male contributed more than the impure blood of the female.
3. Preformation Theory: This theory was given by Swammerdam. He believed that the organism already existed or was pre-formed in the eggs or sperm in a very minute form. This miniature was called the homunculus, which required fertilisation to speed up its growth.
4. Theory of Epigenesis: The preformation theory was discarded by Wolff, a German scientist who came up with the theory of epigenesist. He believed that the organism did not develop as a homunculus in sperms or eggs. The formation of the body parts of the embryo took place step by step. It was only after the fertilization that this formation began.
5. Theory of Acquired Characters: According to French biologist Lamarck, a new character is passed on to the progeny of an individual once it has been acquired by the same individual. This theory was later rejected by a biologist who experimented on at least 20 generations of a rat.
6. Theory of Pangenesis: Charles Darwin, the father of evolution, theorized that miniature and invisible body parts exist in the blood called gemmules and are transmitted to sex organs and assembled in the gametes. After the fertilization process, these gemmules develop into natural body parts and organs.
7. The Germ Plasm Theory: This theory was propounded by a German biologist August Weismann. He theorized that there were generally types of body tissues- germplasm and somatoplasm. Germplasm tissues were the reproductive tissues that helped in the production of gametes. On the other hand, somatoplasm were tissues other than the reproductive ones.

• Gregor Mendel systematically studied the patterns of inheritance which involved the transfer of characters from one generation to the next. He is known as the Father of Genetics.
• Although Mendel described his results in 1866, his work was recognized only in 1900, when Mendel’s laws were rediscovered simultaneously by Hugo de varies - a Dutch biologist, Carl Correns - a German botanist and Erich von Tschermak - an Austrian botanist.
• Mendel conducted experiments on pea plants (Pisum sativum) and studied the inheritance of certain traits.
  
  
  
  

Some General Terms Used by Mendel

Monohybrid Inheritance and the Law of Segregation

• A cross which involves only a single pair of contrasting characters is called a monohybrid cross.
• Consider pea plants with a pair of contrasting characters—tallness and dwarfness—with respect to the height of the stem. Because pea plants are self-fertilising, Mendel bred them for several generations till he obtained pure varieties. The parental generation P₁ has the genotype TT for a pure tall parent plant and tt for a pure dwarf plant. The two parents are homozygous with one type of gamete each, i.e. T from tall parent and t from dwarf parent. These two plants are crossed with one another.
• Mendel found that all plants were tall. He called them first filial or F₁ generation seeds. The F₁ generation has genetic constitution Tt. It is genotypically a hybrid and a heterozygous plant with two different alleles. Phenotypically, the plant is tall because the allele or the gene T for tallness masks the effect of its corresponding recessive gene t.
  
  

• Gives an idea of new combination in the new generation.
• Enables to predict the frequency of characters in the new generation.
• Better quality plant and animal breeds can be produced.
• By the technique of hybridisation, new type of plants with new combination of useful characters can be produced.