ANTB15H3 Lecture Notes - Effective Population Size, Brca2, Brca1

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9 Dec 2013
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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 4,5,6th 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
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- 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
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