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Human Genetics and Disorders

Learn about human genetics - disorders, chromosome abnormalities, autosomal disorders, and Down syndrome. Explore gene linkage, sex chromosome abnormalities, genetic testing, and chromosome mapping. Understand the impact of alterations in chromosome structure, like deletions and duplications.

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Human Genetics and Disorders

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  1. Human Genetics and Disorders

  2. 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

  3. Chromosomal Abnormalities • Nondisjunction • chromosomes don’t separate properly during meiosis • Chromosomal Mutations • deletion • duplication • inversion • translocation

  4. error ofreplication error ofcrossing over Changes in chromosome structure • deletion • loss of a chromosomal segment • duplication • repeat a segment • inversion • reverses a segment • translocation • move segment from one chromosome to another

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

  6. n n Nondisjunction • Problems with meiotic spindle cause errors in daughter cells • homologous chromosomes do not separate properly during Meiosis 1 • sister chromatids fail to separate during Meiosis 2 • too many or too few chromosomes - aneuploidy 2n n-1 n+1

  7. Alteration of chromosome number error in Meiosis 1 error in Meiosis 2 all with incorrect number 1/2 with incorrect number

  8. Nondisjunction • Cause of aneuploidy (wrong chromosome #) • trisomy • cells have 3 copies of a chromosome • monosomy • cells have only 1 copy of a chromosome n+1 n n n-1 monosomy 2n-1 trisomy 2n+1

  9. Down syndrome • Trisomy 21 • 3 copies of chromosome 21 • 1 in 700 children born in U.S. • Chromosome 21 is the smallest human chromosome • but still severe effects • Frequency of Down syndrome correlates with the age of the mother

  10. Down syndrome & age of mother Rate of miscarriage due to amniocentesis: • 1970s data0.5%, or 1 in 200 pregnancies • 2006 data<0.1%, or 1 in 1600 pregnancies

  11. 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 • XXY = Klinefelter’s syndrome male • XXX = Trisomy X female • XYY = Jacob’s syndrome male • XO = Turner syndrome female

  12. Genetic testing • Amniocentesis in 2nd trimester • sample of embryo cells from fluid • stain & photograph chromosomes • Analysis of karyotype

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

  14. 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)

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

  16. Mendelian Testcross no linkage Phenotypic ratio: 1 : 1 : 1 : 1 Frequency: 25% : 25%: 25% : 25%

  17. Morgan’s Testcross showing linkage:

  18. 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

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