Extra credit problem forLecture #4An agouti mouse is crossed to a white mouse and all the F1offspring are agouti.An F1 female is crossed to an F1 male, and the offspringare:11 agouti: 5 white: 4 blackQ: Test the hypothesis that the original parentalgenotypes were BBCC and bbcc. Give the χ2 value,the df, the P value, and state whether or not you rejectthe hypothesis.Cytogenetics: ChromosomeMutations, Aberrations &EvolutionChromosomes Prokaryote EukaryoteEukaryotic chromosomesHuman karyotypeWhy do we care?Many diseases and birth defects are adirect result of missing, broken, or extrachromosomes.• Down Syndrome• Cri du chat Syndrome• Patau SyndromeMutations at the level of thehomologous pair• EUPLOIDY: "true" ploidy, meaning twomembers of each homologous pair.• ANEUPLOIDY: "not true" ploidy, meaningmore or fewer members than two of eachhomologous pair.– MONOSOMY - one homolog; partner is missing– TRISOMY - three homologs– NULLISOMY- one entire homologous pair ismissing.Monosomy and TrisomyDown SyndromeHow does it happen? NondisjunctionEach chrom.has twochromatidsTrisomy: Patau Syndrome• 1/20,000 births• severe mentalretardation• heart and organdefects• polydactyly• death by theage of one yearChromosomal Abnormalities Occurring in Human FetusesType of Abnormality% spontaneously abortedfetuses with theabnormality% fetuses with theabnormality thatsurvive to termAll abnormalities505Autosomal Trisomies 167.50 13, 18, & 214.515 All others13.80Trisomies of Sex chromosomesXXX, XXY, XYY0.375Monosomy for X (XO)8.71Structural Abnormalities2045Structural Changes• Deletions (deficiencies)• Duplications• Inversions• TranslocationswDeletions (deficiencies)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.Break points of chromosomes are highly reactive("sticky"), whereas normal ends of c'somes arecapped by telomeres, which do not readily bondto other molecules.Why do they rejoin?• Breaks that occur __________________________________________will affect both newlyformed chromatids, & alldaughter cells arisingfrom them.• Breaks that occur ______________________________________________may affect only onechromatid. (Thereafter,only the progeny carryingthe broken chromatid willbe affected.)Cri-du-chat Syndrome__________________________________Mental retardationSlow motor skilldevelopmentLow birth weight and slowgrowthSmall head (microcephaly)Partial webbing of fingers ortoesWide-set eyes(hypertelorism)High-pitched cryStructural Changes• Deletions (deficiencies)• Duplications• Inversions• TranslocationsDuplicationDuplicationsBar eye: caused by duplicationDuplications: source ofevolutionary novelty?• ______________• ______________• ______________• ______________Duplication is a source of new genes overevolutionary time: e.g., gene families like globinsand MHC genesStructural Changes• Deletions (deficiencies)• Duplications• Inversions•
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