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Content and pages
* 5.1 Mendel’s Experiment
* 5.2 Inheritance of traits in humans
* 5.3 Genes and Chromosome
* The passing on genetic instructions from generation to generation is called inheritance. * The scientific study of inheritance is called genetics. * It was in the 1850’s that the study of inheritance began. * Discoveries made by Gregor Mendel, an Austrian monk, contributed to the basis of modern genetics. * Mendel chose pure-breeding pea plants to study the inheritance of several characteristics. * A pure-breeding plant is obtained after many generations of self-pollination. * They produce identical offspring and the offspring show the same traits as their parents. * Table 5.1 shows the difference in meaning between characters and traits. Character | Trait |
Plant height | Tall or short (dwarf) |
Seed color | Green or yellow |
Seed shape | Round or wrinkled |
* In one of his experiment, Mendel chose two parent plants, one a pure-breeding tall plant and other a pure breeding short plant. * He called this generation the parental generation or P generation. * He carried out cross-pollination on the two plants by transferring the pollen grains from the tall plant onto the stigma of the short plant. * He collected the seed, planted them and found that all grew to become tall plants. * The results of the parental cross appeared in the first generation called the first filial generation or F1 generation. Mendel’s experiment with long and short pea plants
Schematic diagram of Mendel’s experiment
* Mendel then allowed the plants of the F1 generation to self pollinate to produce the second filial generation or F2 generation. * He discovered that when he planted the seeds from the F2 generation, about three quarters of the offspring were tall and one quarter was short. The ratio tall to short is 3:1. * Mendel found similar results when he crossed pure-breeding pea plants for each of the other characteristics. Character and traits of pea plants studied by Mendel
A pair of homologous chromosomes
Result of his experiment
a) Inheritance depends on the transfer of heredity factors from parents to offspring. There is a heredity factor that determines particular characteristics. b) Each characteristic is controlled by a pair of factors. c) The factors are passed from generation to generation unaltered. d) The factors may be dominant or recessive. Some factors are not expressed in every generation. e) The factors segregate or separate during gamete formation so that each gamete contains only one pair of factors for a given characteristic.
Genes and alleles
* Genes are the basic units of inheritance which occupy specific positions on chromosomes. * The position of the gene is called its locus.
* Gene determines specific characters in an organism.
* For example, in Mendel’s experiments there is a gene which...