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Human Genetics and Disorders AP Biology 2006-2007 Human Chromosome Disorders Some have high frequency in humans most embryos are spontaneously aborted developmental problems result from biochemical imbalance Certain conditions are tolerated upsets the balance less = survivable characteristic set of symptoms = syndrome AP Biology Chromosomal Abnormalities Chromosomal Mutations deletion duplication inversion Translocation Nondisjunction chromosomes don’t separate properly during meiosis AP Biology replication error of Changes in chromosome structure deletion duplication crossing over error of loss of a chromosomal segment repeat a segment inversion reverses a segment translocation AP Biology move segment from one chromosome to another Autosomal Disorders Dominant Acondroplasia Recessive Dwarfism Melanin pathway Lethal in mutated photosensitivity homozygous cond. Acromegaly Phenylketoneuria Gigantism Cannot break down Overactive pituitary phenylalanine Lead to mental retardation Huntington’s Degeneration of nervous system Manifests around 40 years of age AP Biology Albinism Cystic fibrosis Cl- channel problem Mucus buildup Death around 40 Nondisjunction Problems with meiotic spindle causes errors in daughter cells 2n homologous chromosomes do not separate properly during Meiosis 1 sister chromatids fail to separate during Meiosis 2 too many or too few chromosomes - aneuploidy n-1 n n+1 n AP Biology Alteration of chromosome number error in Meiosis 1 error in Meiosis 2 all with incorrect number AP Biology 1/2 with incorrect number Down syndrome Trisomy 21 3 copies of chromosome 21 1 in 700 children born in U.S. Frequency correlates with the age of the mother AP Biology Down syndrome & age of mother Mother’s age Incidence of Down Syndrome Under 30 <1 in 1000 30 1 in 900 35 1 in 400 36 1 in 300 37 1 in 230 38 1 in 180 39 1 in 135 40 1 in 105 42 1 in 60 44 1 in 35 46 1 in 20 48 1 in 16 49 1 in 12 AP Biology Rate of miscarriage due to amniocentesis: 1970s data 0.5%, or 1 in 200 pregnancies 2006 data <0.1%, or 1 in 1600 pregnancies Sex Chromosomes Abnormalities Human development more tolerant of wrong numbers in sex chromosome Cause is nondisjunction But produces a variety of distinct syndromes in humans AP Biology XXY = Klinefelter’s syndrome male XXX = Trisomy X female XYY = Jacob’s syndrome male XO = Turner syndrome female Genetic testing Amniocentesis in 2nd trimester sample of embryo cells from fluid Karyotype of baby Chorionic villus sampling (CVS) – placental cells AP Biology Polyploidy Having more than 2n chromosomes Major method of speciation in plants As the number of chromosomes increase so does the possible cell size ex: huge fruits! AP Biology Often lethal in complex animals Gene Linkage Genes found on same chromosome are considered LINKED! How close or far away are they? Fewer gamete possibilities the closer they are! (more will have same genotype as parent) Why? Less possibility for crossing over to occur which creates variability. Independent assortment does not apply The expected ratio for the cross would be significantly different in the observed. AP Biology Ex: not following 9:3:3:1 or 1:1:1:1 Parental – genes are the same as parent Recombinant – genes are different from parent How do we test for linkage? Do a test cross! Compare the expected phenotypic frequencies with the observed phenotypic frequencies Use Chi square to compare data APMader Biology2007-08 Testcross showing no linkage! Phenotypic ratio: 1 : 1 : 1 : 1 Frequency: 25% : 25%: 25% : 25% AP Biology EXPECTED FREQUENCIES Morgan’s Testcross showing linkage: Parental Genotype: B+b vg+vg Genes: Body color and wing shape are linked! Pay attention to recombinant numbers! OBSERVED FREQUENCIES AP Biology What are the phenotypic frequencies now? 42% : 41% : 9% : 8% Now… compare the offspring phenotypic frequencies of expected vs. observed Expected: Frequency: 25% : 25%: 25% : 25% Observed: Frequency: 42% : 41% : 9% : 8% They DO NOT MATCH! Gene Linkage!! AP Biology Chromosome Mapping Calculate the frequency of recombinant offspring. Recombination frequency = # map units Ex: 13% recombinant frequency = 13 map units Greater % = greater distance Lower % = closer distance AP Biology Mapping Genes Start with the largest map distance and work down from there. AP Biology