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ANTB15- Oct 28, Lec 5- Tay Sachs.docx

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Larry Sawchuk

Oct 28, 2013- Lecture 5 Tay-Sachs Disease • Genetic condition • Passed on from family members to their children • Why carry a gene that’s Why Tay Sachs Has Received So Much Attention Because of its severity (death in childhood) It is simple biochemical structure It is easy to undertake genetic screening Child dies usually at 4,5,6 year of life You can do biological screening -> ethical dilemma > TS-V (symbol for disease)- on exam! If both husband and wife are carriers of the disease, there’s ¼ of a chance of producing a child with TS-V Tay Sachs gene in higher freq Has been found in Ashkenazi Jews To a much extent in other Groups - French Canadians in Quebec - Cajun families in sw Louisiana - Pennsylvania Dutch FACTORS RESPONSIBLE FOR ETHNIC VARIATION IN DISEASE RATES  Artifactual -Bias of Ascertainment problems in determination of rates  Real Environmental factors Cultural factors: culture MAY play a role, even though it’s a genetic disease Genetic factors Why the higher rate of TS among the Jews 1. Differential Mutation mutation would have to be 80X higher 2. Ascertainment Bias Jews seeks physicians care more often than non-Jews (MacMahon & Koller 1957) Two Theories that have been forth 1. Selection: natural selection 2. Random Genetic Drift - Founder Effect - Bottle Neck Effect  Tay sachs Disease  Gaucher’s Disease  Niemann Pick Disease  all involve defects in degradation of sphingo-lipids (class of lipids in Central Nervous System)  accumulate in the brain & neural tissue Gaucher’s Disease  Sphenomegaly  bone & joint pain  variable age of onset  non-lethal  prevalence j=1/3000  nj=very rare  In 1882, French medical student Phillipe Charles Ernest Gaucher saw a 32- year old woman with an enlarged liver and spleen. After the woman's death, Gaucher examined the cells of the spleen.  80 genetic changes that cause Gaucher's disease have been described.  Of these, four mutations account for over 50% of the symptomatic patients.  Despite the many genetic variants, there are only three type of Gaucher's Niemann-Pick (type A)  Abdominal enlargement  physical & mental retardation by 6 months  death by 3 years of age  prevalence j=>1/100,000 nj=1/500,000 TSD  First described by Warren Tay 1881  symptoms appeared at 6 months and inevitably fatal  1887 Bernard Sachs provided the first clinical & pathologic description TSD  Simple autosomal recessive  complete penetrance: if you have the gene, you have the disease  carriers appear normal and only distinguished by a reduced amount of beta- hexosaminidase A T.S.D  Mental & physical retardation @ 4-6 months  dementia, blindness by 2-3 yrs  death by 4 yrs  prevalence j=1/3000 nj=very rare  TSD  Overall heterozygote frequency 3-4%  biological fitness of homozygous is 0  some clinical heterogeneity arising from the wide variety of mutations that cripple the catalytic activity of Hex A Hex A Located at 15q23-q24 Located on long arm 78 mutations - 65 single base substitutions - 1 large & 10 small deletions - 2 small insertions - On Exam: Point mutation: simple little changes in the bases that’s causing the horrible diseases  One insertion, consisting of 4 bases within exon 11 is found in 80% of the carriers of TSD  [exon the coding regions to form mRNA] Frequency of TSD - 1/6000 Ashkenazie births - 1/500,000 non-Jewish births - <1/500,000 Oriental or Sephardic Jews They’re all Jews, but they’re different kinds of Jews Main point: This problem is among Ashkenazie Jews mainly > there’s something about this grup that makes them more prone to the disease Ashkenazim  80% of the 13-14 million Jews  Hebrew term for German  migrated to Rhineland in the 9th century  moved to present day Poland, Lithuania, Belarus, the Ukraine & Russia The disease appeared after 1400, not before it This mutation occurred after 1400, which is after the Middle Ages Variation among Ashkenazim  A = highest proportion from descendents of individuals from s. Lithuania & N.E. Poland  B = lower in Ukrainian region  C = very few from the western Balkans (or Germany)  5 fold differential from A to C The disease appeared after 1400, not before it This mutation occurred after 1400, which is after the Middle Ages Heterozygote Advantage Myrianthopoulous & Aronson have demonstrated that
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