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Cytogenetics: Chromosome Mutations, Aberrations & Evolution Chromosomes Prokaryote Eukaryote Eukaryotic chromosomes Human karyotype Why do we care? Many diseases and birth defects are a direct result of missing, broken, or extra chromosomes. • Down Syndrome • Cri du chat Syndrome • Patau Syndrome Mutations at the level of the homologous pair • EUPLOIDY: "true" ploidy, meaning two members of each homologous pair. • ANEUPLOIDY: "not true" ploidy, meaning more or fewer members than two of each homologous pair. – MONOSOMY - one homolog; partner is missing – TRISOMY - three homologs – NULLISOMY- one entire homologous pair is missing. Monosomy and Trisomy Down Syndrome How does it happen? Nondisjunction Each chrom. has two chromatids Trisomy: Patau Syndrome • 1/20,000 births • severe mental retardation • heart and organ defects • polydactyly • death by the age of one year Chromosomal Abnormalities Occurring in Human Fetuses % spontaneously aborted % fetuses with the fetuses with the abnormality that Type of Abnormality abnormality survive to term All abnormalities 50 5 Autosomal Trisomies 16 7.5 0 13, 18, & 21 4.5 15 All others 13.8 0 Trisomies of Sex c hromosomes XXX, XXY, XYY 0.3 75 Monosomy for X (XO) 8.7 1 Structural A bnormalities 20 45 Structural Changes • • • • Deletions (deficiencies) Duplications Inversions Translocations Deletions (deficiencies) w How can chromosomes break? Ionizing radiation (production of free radicals, which act like little atomic "cannon balls", blasting through strands of DNA or c'somes. Chemical insult. Why do they rejoin? Break points of chromosomes are highly reactive ("sticky"), whereas normal ends of c'somes are capped by telomeres, which do not readily bond to other molecules. • Breaks that occur ______ __________________ __________________ will affect both newly formed chromatids, & all daughter cells arising from them. • Breaks that occur ______ ____________________ ____________________ may affect only one chromatid. (Thereafter, only the progeny carrying the broken chromatid will be affected.) Cri-du-chat Syndrome _________________ _________________ Mental retardation Slow motor skill development Low birth weight and slow growth Small head (microcephaly) Partial webbing of fingers or toes Wide-set eyes (hypertelorism) High-pitched cry Structural Changes • • • • Deletions (deficiencies) Duplications Inversions Translocations Duplication Duplications Bar eye: caused by duplication Duplications: source of evolutionary novelty? Duplication is a source of new genes over evolutionary time: e.g., gene families like globins and MHC genes • • • • ______________ ______________ ______________ ______________ Structural Changes • • • • Deletions (deficiencies) Duplications Inversions Translocations Inversions