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Download Notes GENES ON CHROMOSOMES
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GENES ON CHROMOSOMES Chapter 11 & Chapter 13 I. II. Introduction A. Chromosome Theory of Heredity 1. Def. – genes are located on chromosomes a. Locus – point on a chromosome where a given gene is located. 2. Reason: early geneticists saw a relationship between Mendel’s discoveries and the way chromosomes move during meiosis. Tests of the Chromosome Theory of Heredity A. Sex Determination 1. Discovered through Morgan’s work with Drosophila a. Good subject for genetic studies: 1. small 2. easily handled 3. produce a large number of offspring in a short period of time. b. Chromosomes of fruit fly - diagram c. Gametes of fruit fly: 1. Egg - 3 autosomes and X 2. Sperm – 3 autosomes and X or Y Result of fertilization: 50:50 chance of producing male or female 2. Relation to Chromosome Theory of Heredity: a. Presence of “Y” determines traits in fruit flies: ! Examples: sex in fruit fly Pigmentation of abdomen 3. Sex determination in animals is not always”Y” for males: p.341 ! Examples: some birds have : female = xy and male =xx X-Linkage: 1. Def. Traits carried on the sex chromosomes 2. Discovery: Morgan’s work with fruit flies 3. Relationship to C.T. of H. – again links trait with chromsomes: 4. Other examples in humans: p. 350 ! Red-green colorblindness ! Hemophilia ! Carriers: females who are heterozygous for the trait because there is a 50:50 chance that they will possess the trait to their male offspring. 5. X-inactivation and Barr Bodies: p. 351 d. B. a. General Description: female mammals show the presence of a “Barr Body” when stained during interphase. 1.) Barr Body a.) Def. – a dense stainable structure which is one of the X chromosomes b.) Why is it inactive? Is coiled 2.) Explanation: During early embryonic development in the female, some of the X’s are inactivated. Therefore there are two populations of traits in one organism and two different gene expressions. 3.) Example: Calico cat P. 363 # 1,5,6,7,15 " P. 365 #2,3 Chromosome Abnormalities: A. Usually two types: 1. involve chromosome # 2. involve chromosome structure " III. B. Types: 1. Nondisjunction(C.N.) p. 352 Definition: failure of homologous chromosomes to separate during meiosis. ! Examples: Down’s Dyndrome Klinefelter and Turner Polyploidy(C.N.) a. Def.: complete nondisjuntion of entire gamete ! Not a problem in plants; can actually introduce new combinations but is lethal in animals. Structural Alterations page 308 ! When- during crossing over a. Types: 1.) Deletion a.) Def.: segment of chromosome is lost b.) Examples: some forms of leukemia 2.) Inversion a.) Def.: segment is turned around b.) Examples 3.) Translocation a.) Def.: segment of a chromosome breaks off and attaches to a nonhomologue b.) Examples ! 2. 3. c.) IV. V. VI. Cause: chemicals, xrays, radiation Linkage page 279 A. Def.- more than one gene on a chromosome B. Behavior- do not follow the Law of Independent Assortment, segregate together. C. Linkage and variety: reduces variety ! Producing variety: crossing over Chromosomal Maps A. How derived: determined by frequency of crossing-over ! Meaning: genes that are linked crossover more frequently B. What is allows: drawing genetic maps ! Def.: shows the location of genes on chromosomes ! P. 283 # 10 ! P. 285 # 5,7,8 Other Genetic Characteristics: ! Multiple Alleles: more than one allele determines a trait. Page 273 ! Example: Blood types page 344 Three alleles: IA, IB, IO " IA & IB are codominant (definition: both alleles are expressed) " IO is recessive " P. 363 # 4 " P. 365 # 4,6