Lecture 3_Mendelian Genetics

Lecture 3_Mendelian Genetics - Chapter 3: Mendelian...

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Chapter 3: Mendelian Inheritance Organisms possess a lot of variations. For example, many different breeds of dogs exist. No two humans (except identical twins) look the same. All this variability is due to different genetic combinations. The inheritance of genetic traits was first studied systematically by Gregor Mendel. Many others had done breeding experiments before, but they did not carry it out systematically and analyzed the data in a mathematical way. The study of traits from one generation to the next in individual matings is called Transmission Genetics or Mendelian Genetics. Genotype and Phenotype. Gene combination that an individual inherits is called its "Genotype ". The visible final outcome is called its "Phenotype". During the formation of the individual, the final outcome is due to the genotype as influenced by the environment. For example, an individual may have the genotype for tallness but due to nutritional deficiency during growth, it may end up as a short creature. The effects of the environment are most evident when one examines identical twins (with the same genotype) that have been reared apart. The trait that is present in most of the individuals in a population is called the “wild type ”. Hereditary variants are called “ mutants ”. Mutants arise due to changes in the DNA sequence of the relevant gene. They can be due to base-substitutions, small addition/deletions (frameshifts) or due to large deletions or insertions (such as those created by pieces of DNA that move around in the genome- transposable elements). Mutations due to the insertion of a large insertion in a gene, can be followed by Southern Blotting and labeled DNA probe hybridization as described in Ch. 2. In the blot, one sees a larger size DNA band relative to what is seen with the wild type gene ( Fig. 3.2 ). Mendel's Experimental Approach . He did a number of breeding experiments using garden peas. It is an organism that is easy to grow, cross-hybridize and it produces many offsprings (seeds) per generation. Mendel planned and carried out his experiments very meticulously. Most importantly, he only looked at 1 or a few characters at a time. He counted the CLASSES of PROGENY from each cross and how many plants in each CLASS. This quantitative analysis allowed him to formulate certain theories about the inheritance of traits. Self- versus Cross-fertilization. Self-fertilization is possible in many plants and some lower animals because they are hermaphrodites. In nature, pea plant is self-fertilized (however, cross-fertilization is experimentally possible, Fig. 3.3 ). Mendel began with a large number (34) of strains of peas, which he let self-fertilize for a number of generations, so that the starting plants were "true-breeding" or "pure-breeding ". Ultimately, only 7 traits, were used by Mendel for his genetic experiments ( Fig. 3.1 )- 1. Seed Shape- Smooth/Wrinkled 2. Seed Color- Yellow/Green 3. Flower color- Purple/White. 4. Pod Shape-
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This note was uploaded on 09/29/2011 for the course GENETICS 380 taught by Professor Glodowski during the Spring '08 term at Rutgers.

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Lecture 3_Mendelian Genetics - Chapter 3: Mendelian...

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