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Chromosomal Basis of Inheritance

Chromosomal Basis of Inheritance. Chapter 15. Chromosome Theory. “ Mendelian genes have loci on chromosomes. It is chromosomes that undergo segregation and independent assortment” Support from improved microscopy that allowed visualization of mitosis and meiosis (1875ish)+

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Chromosomal Basis of Inheritance

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  1. Chromosomal Basis of Inheritance Chapter 15

  2. Chromosome Theory • “Mendelian genes have loci on chromosomes. It is chromosomes that undergo segregation and independent assortment” • Support from improved microscopy that allowed visualization of mitosis and meiosis (1875ish)+ • Parallels noted to Mendel’s ideas

  3. Thomas Hunt Morgan • Fruit flies • Drosphilia melanogaster • Prolific breeders • 2 wk generation time • N=4

  4. Sex linkage • When cross white eyed male w/ red eyed female • F1 all red eyes • F2 3:1, but only males had white eyes!! • Gene traced to X chromosome • 1st gene traced to any chromosome • Males express sex-linked traits more often • Examples? • IQ15.1, 15.5

  5. Gene Linkage • Genes located on the same chromosome tend to be inherited together • Can only be separated by crossover • Gene Mapping (w/Sturtevant) • Recombination frequencies reflect distances between genes • Create a linkage map • Unit = map unit = 1% recombination frequency • Gives a sequence of genes, but may not translate directly into lengths

  6. Do IQ 15.4

  7. Sex chromosomes • Humans XY system • Birds, fish etc. ZW system • Females determine sex • ZW = female, ZZ = male • Bees, ants: haplo-diploid system • No sex chromosomes • Fertilized eggs = females, unfertilized = males

  8. X inactivation • One X in each cell of female becomes a Barr body in embryonic development • Methylation • Random, therefore females are mosaic • Ex. Tortoise shell cat

  9. Alteration in Chromosome Number • Nondisjunction • Failure of chromosomes to separate in meiosis • Trisomy or monosomy • Usually lethal (except trisomy 21 or sex chromosomes) • Nondisjuction in mitosis can lead to problems if early in development

  10. Alterations in chromosome structure • Deletion – can occur during cross over • Duplication – “ • Inversion • Translocation

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