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Lecture #6 Date ________

Lecture #6 Date ________. Chapter 15~ The Chromosomal Basis of Inheritance. The Chromosomal Theory of Inheritance. Genes have specific loci on chromosomes and chromosomes undergo segregation and independent assortment. Chromosomal Linkage. Morgan

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Lecture #6 Date ________

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  1. Lecture #6 Date ________ • Chapter 15~ The Chromosomal Basis of Inheritance

  2. The Chromosomal Theory of Inheritance • Genes have specific loci on chromosomes and chromosomes undergo segregation and independent assortment

  3. Chromosomal Linkage • Morgan • Drosophilia melanogaster • XX (female) vs. XY (male) • Sex-linkage: genes located on a sex chromosome • Linked genes: genes located on the same chromosome that tend to be inherited together

  4. Genetic recombination • Crossing over Genes that DO NOT assort independently of each other • Genetic maps The further apart 2 genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency • Linkage maps Genetic map based on recombination frequencies

  5. Human sex-linkage • SRY gene: gene on Y chromosome that triggers the development of testes • Fathers= pass X-linked alleles to all daughters only (but not to sons) • Mothers= pass X-linked alleles to both sons & daughters • Sex-Linked Disorders: Color-blindness; Duchenne muscular dystropy (MD); hemophilia • X-inactivation: 2nd X chromosome in females condenses into a Barr body (e.g., tortoiseshell gene gene in cats)

  6. Chromosomal errors, I • Nondisjunction: members of a pair of homologous chromosomes do not separate properly during meiosis I or sister chromatids fail to separate during meiosis II • Aneuploidy: chromosome number is abnormal • Monosomy~ missing chromosome • Trisomy ~ extra chromosome (Down syndrome) • Polyploidy~ extra sets of chromosomes

  7. Chromosomal errors, II • Alterations of chromosomal structure: • Deletion: removal of a chromosomal segment • Duplication: repeats a chromosomal segment • Inversion: segment reversal in a chromosome • Translocation: movement of a chromosomal segment to another

  8. Genomic imprinting • Def: a parental effect on gene expression • Identical alleles may have different effects on offspring, depending on whether they arrive in the zygote via the ovum or via the sperm. • Fragile X syndrome: higher prevalence of disorder and retardation in males

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