Chapter 12 Chromosome Variations and Human Genetics

Chapter 12 Chromosome Variations and Human Genetics -...

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Chapter 12 Chromosome Variations and Human Genetics I. Too Young to Be Old A. Because of a rare defect in an autosomal dominant gene, persons suffering from progeria syndrome will age a lifetime in the first decade of life. B. The analysis of human inheritance is a fascinating but often baffling field of study. II. Chromosomal Theory of Inheritance A. Return of the Pea Plant 1. By 1882, Walther Flemming had observed threadlike chromosomes in the nuclei of dividing cells. 2. By 1887, August Weismann had suggested that meiosis halves the number of chromosomes when gametes are made. 3. By 1900, Mendel’s work was finally appreciated, namely his view that diploid cells have two units for each trait and the units segregate during gamete formation. B. Autosomes and Sex Chromosomes 1. Most of the chromosomes are of the same quantity and type in both sexes and are called autosomes (44 in humans). 2. Sex chromosomes determine gender. a. Human females have two X chromosomes; males have one X and one Y. b. Each human egg will contain twenty-two autosomes plus one X; but sperm will carry twenty-two autosomes plus either an X or a Y. c. The Y chromosome carries the gene for TDF (testis determining factor). 3. Chromosomes are visualized in a lab preparation called a karyotype. 4. Nonsexual traits are also coded for on the sex chromosomes. C. Linkage and Crossing Over 1. Linkage is the tendency of genes located on the same chromosome to be transmitted together in inheritance. 2. Linkage can be disrupted by crossing over. a. Crossing over is an exchange of parts of homologous chromosomes. b. The probability that crossing over will lead to the separation of two genes on a
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This note was uploaded on 01/04/2012 for the course BSC BSC1005 taught by Professor Orlando,rebecca during the Fall '10 term at Broward College.

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Chapter 12 Chromosome Variations and Human Genetics -...

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