Chapter 15 Human Genetics and Disorders AP Biology

Chapter 15: Human Genetics and Disorders AP Biology 2006 -2007

Human Chromosome Disorders § Some have high frequency in humans most embryos are spontaneously aborted u developmental problems result from biochemical imbalance u § Certain conditions are tolerated upsets the balance less = survivable u characteristic set of symptoms = syndrome u AP Biology

Chromosomal Abnormalities § Nondisjunction § chromosomes don’t separate properly during meiosis § Chromosomal Mutations § § AP Biology deletion duplication inversion translocation

replication error of Changes in chromosome structure § deletion u § duplication crossing over u error of loss of a chromosomal segment repeat a segment § inversion u reverses a segment § translocation u AP Biology move segment from one chromosome to another

Autosomal Disorders § Dominant u Achondroplasia § Recessive u § Dwarfism § Lethal in homozygous cond. u Acromegaly § Melanin pathway mutated § photosensitivity u § Gigantism § Overactive pituitary u AP Biology Phenylketoneuria § Cannot break down phenylalanine § Lead to mental retardation Huntington’s § Degeneration of nervous system § Manifests around 40 years of age Albinism u Cystic fibrosis § Cl- channel problem § Mucus buildup § Death around 40

Nondisjunction § Problems with meiotic spindle cause errors in daughter cells u u u 2 n 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

Nondisjunction § Cause of aneuploidy (wrong chromosome #) u trisomy § cells have 3 copies of a chromosome u monosomy § cells have only 1 copy of a chromosome n+1 n n n-1 AP Biology trisomy monosomy 2 n+1 2 n-1

Down syndrome § Trisomy 21 3 copies of chromosome 21 u 1 in 700 children born in U. S. u § Chromosome 21 is the smallest human chromosome u but still severe effects § Frequency of Down syndrome 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: § 1970 s 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 u Cause is nondisjunction § But produces a variety of distinct syndromes in humans u u AP Biology XXY = Klinefelter’s syndrome male XXX = Trisomy X female XYY = Jacob’s syndrome male XO = Turner syndrome female

Genetic testing § Amniocentesis in 2 nd trimester sample of embryo cells from fluid u stain & photograph chromosomes u § Analysis of karyotype AP Biology

Polyploidy §Occurs when eukaryotes have more than 2 n chromosomes Major method of speciation in plants § As the number of chromosomes increase so does the possible cell size ex: huge strawberries! u. Often lethal in animals u AP Biology

Gene Linkage §Genes found on same chromosome are considered LINKED! §How close or far away are they? §Fewer gamete possibilities the closer they are! §Why? Less possibility for crossing over to occur. §Independent assortment does not apply §No linkage if genes are on separate chromosomes (# of recombinants increases) AP Biology

AP Biology

This shows linked genes with no crossing over and crossing over. How can we test to see if genes are linked?

Mendelian Testcross no linkage Phenotypic ratio: 1 : 1 Frequency: 25% AP Biology

Morgan’s Testcross showing linkage: AP Biology

Chromosome Mapping § Calculate the frequency of recombinant offspring. § Recombination frequency = # map units u Ex: 13% recombinant frequency = 13 map units § Greater % = greater distance § Lower % = closer distance AP Biology

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