Chapter 15 THE CHROMOSOMAL BASIS OF INHERITANCE

Chapter 15 THE CHROMOSOMAL BASIS OF INHERITANCE - CHAPTER15...

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CHAPTER15 THE CHROMOSOMAL BASIS OF INHERITANCE I. Relating Mendelism to Chromosomes A. Mendelian inheritance has its physical basis in the behavior of chromosomes during sexual life cycles 1860s: Mendel proposed that discrete inherited factors segregate and assort independently during gamete formation 1875: Cytologists worked out process of mitosis 1890: Cytologists worked out process of meiosis 1900: Three botanists (Correns, de Vries, and von Seysenegg) independently rediscovered Mendel's principles of segregation and independent assortment 1902: Cytology and genetics converged as Walter Sutton, Theodor Boveri, and others noticed parallels between the behavior of Mendel's factors and the behavior of chromosomes. For example: - Chromosomes and genes are both paired in diploid cells. - Homologous chromosomes separate and allele pairs segregate during meiosis. - Fertilization restores the paired condition for both chromosomes and genes. Based upon these observations, biologists developed the chromosome theory of inheritance. According to this theory: - Mendelian factors or genes are located on chromosomes. - It is the chromosomes that segregate and independently assort. B. Morgan traced a gene to a specific chromosome: science as a process Thomas Hunt Morgan from Columbia University performed experiments in the early 1900s which provided convincing evidence that Mendel's inheritable factors are located on chromosomes. 1. Morgan's choice of an experimental organism Morgan selected the fruit fly, Drosophila melanogaster , as the experimental organism
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- Are easily cultured in the laboratory - Are prolific breeders - Have a short generation time - Have only four pairs of chromosomes which are easily seen with a microscope There are three pairs of autosomes (Il, III, and IV) and one pair of sex chromosomes. Females have two X chromosomes, and males have one X and one Y chromosome. Morgan and his colleagues used genetic symbols that are now convention. For a particular character: - A gene's symbol is based on the first mutant, non-wild type discovered. - If the mutant is recessive, the first letter is lowercase. (e.g., w = white eye allele in Drosophila.) - If the mutant is dominant, the first letter is capitalized. (e.g., Cy = "curly" allele in Drosophila that causes abnormal, curled wings.) - Wild-type trait is designated by a superscript +. (e.g., Cy + = allele for normal, straight wings.) Wild type = Normal or most frequently observed phenotype Mutant phenotypes = Phenotypes which are alternatives to the wild type due to mutations in the wild-type gene 2. Discovery of a sex linkage After a year of breeding Drosophila to find variant phenotypes, Morgan discovered a single male fly with white eyes instead of the wild-type red. Morgan mated this mutant white-eyed male with a red-eyed female. The cross is outlined below. w = white-eye allele
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This note was uploaded on 03/18/2008 for the course BS 110 taught by Professor S.lawrence during the Spring '07 term at Michigan State University.

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Chapter 15 THE CHROMOSOMAL BASIS OF INHERITANCE - CHAPTER15...

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