Chapter 15 - Chapter 15 The Chromosomal Basis of...

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Chapter 15 The Chromosomal Basis of Inheritance Updated November 2008
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Overview: Locating Genes on Chromosomes Genes Are located on chromosomes Can be visualized using certain techniques Figure 15.1
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Concept 15.1: Mendelian inheritance has its physical basis in the behavior of chromosomes Several researchers proposed in the early 1900s that genes are located on chromosomes The behavior of chromosomes during meiosis was said to account for Mendel’s laws of segregation and independent assortment
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The chromosome theory of inheritance states that Mendelian genes have specific loci on chromosomes During meiosis chromosomes undergo: Segregation Independent assortment
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Meiosis Homologous chromosomes during meiosis account for the segregation of alleles at each locus to different gametes Nonhomologous chromosomes account for independent assortment of alleles
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0.5 mm Meiosis Metaphase I Anaphase I Metaphase II Gametes LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. 1 4 yr 1 4 Yr 1 4 YR 3 3 F 1 Generation 1 4 yR R R R R R R R R R R R R Y Y Y Y Y Y Y Y Y Y Y Y y r r r r r r r r r r r r y y y y y y y y y y y All F 1 plants produce yellow-round seeds (YyRr) 1 2 2 1
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Morgan’s Experimental Evidence: Scientific Inquiry Thomas Hunt Morgan In early 20 th century, first geneticist to associate a specific gene with a specific chromosome Provided convincing evidence that chromosomes are the location of Mendel’s heritable factors
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Morgan’s Choice of Experimental Organism Morgan worked with fruit flies Because they breed at a high rate A new generation can be bred every two weeks They have only four pairs of chromosomes (3 pairs of autosomes & 1 pair of sex chromosomes) And they are cheap and easy to get!
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Morgan’s Flies Morgan first observed and noted Wild type (normal) phenotypes that were common in the fly populations Traits alternative to the wild type Are called mutant phenotypes Figure 15.3
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Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type) The F 1 generation all had red eyes The F 2 generation showed the 3:1 red:white eye ratio, but only males had white eyes Morgan determined that the white-eyed mutant allele must be located on the X chromosome
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Fig. 15-4c Eggs F 1 CONCLUSION Generation Generation P X X w Sperm X Y + + + + + Eggs Sperm + + + + + Generation F 2 × w w w w w w w w w w w w w w w
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Morgan’s discovery that transmission of the X chromosome in fruit flies correlates with inheritance of the eye-color trait Was the first solid evidence indicating that a specific gene is associated with a specific chromosome (Morgan and his lab did a lot of fascinating fly research. He won a Nobel Prize in 1933.)
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Concept 15.2: Sex-linked genes
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Chapter 15 - Chapter 15 The Chromosomal Basis of...

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