Chapter 15 Notes

Chapter 15 Notes - [CAMPBELL BIOLOGY: CHAPTER 15 NOTES]...

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P a g e | P a g e | [ CAMPBELL BIOLOGY: CHAPTER 15 NOTES ] Chapter 15: The Chromosomal Basis of Inheritance Overview: Locating Genes on Chromosomes Today we know that genes—Gregor Mendel’s “hereditary factors”—are located on chromosomes. A century ago, the relationship of genes and chromosomes was not so obvious. Many biologists were skeptical about Mendel’s laws of segregation and independent assortment until evidence mounted that they had a physical basis in the behavior of chromosomes. Concept 15.1 Mendelian inheritance has its physical basis in the behavior of chromosomes Around 1900, cytologists and geneticists began to see parallels between the behavior of chromosomes and the behavior of Mendel’s factors. ° Using improved microscopy techniques, cytologists worked out the process of mitosis in 1875 and meiosis in the 1890s. ° Chromosomes and genes are both present in pairs in diploid cells. ° Homologous chromosomes separate and alleles segregate during meiosis. ° Fertilization restores the paired condition for both chromosomes and genes. Around 1902, Walter Sutton, Theodor Boveri, and others noted these parallels and a chromosome theory of inheritance began to take form: ° Genes occupy specific loci on chromosomes. ° Chromosomes undergo segregation during meiosis. ° Chromosomes undergo independent assortment during meiosis. The behavior of homologous chromosomes during meiosis can account for the segregation of the alleles at each genetic locus to different gametes. The behavior of nonhomologous chromosomes can account for the independent assortment of alleles for two or more genes located on different chromosomes. Morgan traced a gene to a specific chromosome. In the early 20th century, Thomas Hunt Morgan was the first geneticist to associate a specific gene with a specific chromosome. Like Mendel, Morgan made an insightful choice in his experimental animal. Morgan worked with Drosophila melanogaster , a fruit fly that eats fungi on fruit. ° Fruit flies are prolific breeders and have a generation time of two weeks. ° Fruit flies have three pairs of autosomes and a pair of sex chromosomes (XX in females, XY in males). Morgan spent a year looking for variant individuals among the flies he was breeding. ° He discovered a single male fly with white eyes instead of the usual red. The normal character phenotype is the wild type. Alternative traits are called mutant phenotypes because they are due to alleles that originate as mutations in the wild-type allele. ° When Morgan crossed his white-eyed male with a red-eyed female, all the F 1 offspring had red eyes, suggesting that the red allele was dominant to the white allele. Crosses between the F
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This note was uploaded on 07/24/2008 for the course BIO 120 taught by Professor Staff during the Fall '07 term at USC.

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Chapter 15 Notes - [CAMPBELL BIOLOGY: CHAPTER 15 NOTES]...

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