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Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 Chapter Contents Mendel’s Laws of Inheritance Inheritance of One Gene Law of Dominance Law of Segregation Exceptions to Mendelian Principles Inheritance of Two Genes Law of Independent Assortment Two Genes Interaction w.r.t. Post-Mendelism Chromosomal Theory of Inheritance Linkage and Recombination Sex Determination Mutation Genetic Disorders Pedigree Analysis Mendelian Disorders Chromosomal Disorders Cytoplasmic Inheritance The transfer of characters from parents to offspring is known as inheritance. Variation is the degree by which progeny differ from their parents. The branch of science which deals with the inheritance as well as the variation of characters from parents to offspring is Genetics. Early agriculturists (8,000 –10,000 B.C.) knew that one of the causes of variation was hidden in sexual reproduction. Because of it, they successfully bred domesticated varieties from wild plants and animals through selective crossing and artificial selection. Indian cow (e.g., Sahiwal of Punjab) is domesticated form of an ancestral wild cow. MENDEL’S LAWS OF INHERITANCE Gregor Johann Mendel was the first to demonstrate the scientific basis of inheritance and variation by conducting hybridisation experiment. His experiments were in fact the extension and development of hybridisation experiments on pea conducted by earlier workers like Knight and Goss. He was the first to consider one to three characters at one time which was perhaps the reason of his success. Mendel was born on July 22, in 1822. He worked on Pisum sativum (Garden pea or Edible pea) for 7 years (1856–1863) and proposed the law of inheritance in living organisms. Character Trait Feature of the individual. , Stem height e.g. Distinguishable feature of a character and its detectable variant. e.g., Tallness or dwarfness. Selection of pea plant : The main reasons for adopting garden pea for experiments were as follows : (1) Pea has many distinct alternative traits (clear contrasting traits). Chapter 14 Principles of Inheritance and Variation
62 Principles of Inheritance and Variation NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 (2) It produces a large number of seeds and completes its life cycle in one season. (3) Flowers show self (bud) pollination, so are true breeding. (4) It is easy to artificially cross-pollinate the pea flowers. The hybrids thus produced were fertile. Mendel conducted artificial pollination or cross-pollination experiments using several true-breeding pea lines. A true breeding line is one that having undergone continuous self-pollination, shows the stable trait inheritance and expression for several generations. Mendel selected 14 true-breeding pea plant varieties, as pairs which were similar except for one character with contrasting traits. It means, Mendel selected 7 characters in pea plant which are listed in following table. S.No. 1. 2. 3. 4. 5. Seed shape Seed colour Flower colour Pod shape Pod colour Character Dominant trait Recessive trait Contrasting traits Round Yellow Violet Full/Inflated Green Wrinkled Green White Constricted Yellow 6. 7. Flower position Stem height Axial Tall Terminal Dwarf Location of traits (On chromosome No.) 7 1 1 4 5 4 4
NEET Principles of Inheritance and Variation 63 Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 Note : 1. Initially Mendel took 34 varieties of pea plants, then 22 but ultimately worked with only 7 pairs of varieties. 2. Term 'Pure line' was coined by Johannsen in 1900. Reasons for Mendel’s Success (1) Mendel applied statistical methods and mathematical logics for analysing his results. (2) He kept accurate records of his experiments, giving all the details of number and type of individuals, which are a necessity in the genetic studies. (3) Mendel experimented on a number of plants for the same trait and obtained hundreds of offspring. A large sampling size gave credibility to his results. Chances of error are little in large samples. (4) He tried to formulate theoretical explanations for the observed results. These explanations were further tested by conducting experiments for successive generations of the test plants, that proved his results pointed to general rules of inheritance rather than being unsubstantiated ideas. INHERITANCE OF ONE GENE Study of inheritance of single pair of contrasting traits of a character at a time is called one gene inheritance. Mendel crossed true breeding tall variety (6–7 ft.) and true breeding dwarf variety (0.75–1 ft.) pea plants to study the inheritance of one gene. The plants used in initial cross are referred to as P1 and P2 or parents. Since pea is self-fertilising, the anthers should be removed from the female parent before maturity for the purpose of cross pollination. The method of removal of anthers from bisexual flowers of female parent plant is called emasculation. The pollens, then at the dehiscence stage, is brought from the male parent and is dusted on the stigma of emasculated flower. He collected the seeds produced as a result of this cross and grew them to generate plants of the first hybrid generation. This generation is also called the filial1 (offspring) progeny or the F1. Petal Stigma Anther Stamen Carpel Removal of anthers (Emasculation) Parent (P )2 Transfer of pollen (Pollination) Parent (P )1 Fig. : Steps in making a cross in pea Seeds
64 Principles of Inheritance and Variation NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 Mendel found that all F1 progeny plants were tall (6-7 ft), like one of its parents; none were dwarf. He made similar crosses with other pairs of contrasting traits and in every case the result was the same. In each, F1 plants were identical to one of the parents. When Mendel self-pollinated the tall F1 plants, both tall and dwarf plants were obtained in F2 generation. Offspring derived from selfing of the F1 are termed as second filial or F2 generation. The proportion of plants that were tall were th 3 4       of the F2 population while th 1 4       of the F2 population were dwarf. We must note here that dwarfness which disappeared in F1 generation, reappeared in F2. The tall and dwarf traits in F2 generation were identical to their parental type and did not show any blending, i.e., all the offsprings were either tall or dwarf, none were of in-between height. In the example discussed here, cross is performed involving single pair of contrasting traits of a character. Such cross is known as monohybrid cross. Tall (P )1 Dwarf (P )2 Parental F generation 1 F generation 2 Tall Selfing Tall Tall Tall Tall Dwarf Fig. : Diagrammatic representation of monohybrid cross He made similar crosses with other pairs of contrasting traits and observed the F2 generation in which both the traits were expressed in the proportion of 3 : 1. Note : 1. Mendel also worked on two other plants namely, Hawkweed (Hieracium) and Lablab and he failed to obtain same results as found in garden pea due to parthenogenesis non-availability of pureline seeds and parthenogenesis. 2. SBE - 1 gene is responsible for the synthesis of an enzyme SBE (starch branching enzyme) essential for producing round seeds. Mutation in gene leads to failure in the production of this enzyme and hence in complex metabolic disturbances producing wrinkled seeds. Concept of ‘Factors’ Based on these observations, Mendel proposed that something was being stably passed down, unchanged, from parent to offspring through the gametes, over successive generations. He called these things as 'factors'.