9Why do we care?Many diseases and birth defects are a direct result of missing, broken, or extra chromosomes.Down SyndromeCri du chat SyndromePatau Syndrome
10Mutations 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 missingTRISOMY - three homologsNULLISOMY- one entire homologous pair is missing.
14Figure: 07-04Caption:Incidence of Down syndrome births contrasted with maternal age.
15How does it happen? Nondisjunction Figure: 07-01Caption:Nondisjunction during the first and second meiotic divisions. In both cases, some of the gametes formed either contain two members of a specific chromosome or lack that chromosome. Following fertilization by a gamete with a normal haploid content, monosomic, disomic (normal), or trisomic zygotes are produced.Each chrom.has twochromatids
16Trisomy: Patau Syndrome 1/20,000 birthssevere mental retardationheart and organ defectspolydactylydeath by the age of one year
20How 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.
21Breaks 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.)
22Cri-du-chat Syndrome _________________ Mental retardation Slow motor skill developmentLow birth weight and slow growthSmall head (microcephaly)Partial webbing of fingers or toesWide-set eyes (hypertelorism)High-pitched cry