Genetics Exam 2

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any disease caused by a mutation in a gene

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single gene disorder

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any disease caused by excess or deficiency of whole chromosomes or chromosome segments

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Chromosome disorders

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any disease caused by interaction of genetic and environmental factors

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multifactorial diseases

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how many single gene disorders manifest after puberty?

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less than 10%

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the hereditary unit

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gene

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location of a gene on a particular segment of DNA is its:

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locus

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these are alternate forms of a gene

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alleles

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the first person you see manifesting the disease

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proband

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huntington disease has this inheritance

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single gene, autosomal dominant

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cystic fibrosis and sickle cell have this inheritance

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single gene, autosomal recessive

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hemophilia A, duchene muscular dystrophy have this inheritance

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single gene, x linked recessive

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most autosomal recessive diseases are due to:

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loss of function mutations

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most common, severe, autosomal recessive disease among caucasians

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cystic fibrosis
clinical symptoms: respiratory, GI, elevated sweat electrolytes. thick mucous in lungs that blocks alveoli and pancreatic duct

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cystic fibrosis frequency of affected per live birth

frequency of carriers

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1/2500

1/25 carrier in caucasians

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cystic fibrosis gene

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CFTR, 250kb, 27 exons, 1480 amino acids

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describe sodium and chloride channels in CF

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some Cl channel nonfunctional so more Na taken in to balance charge

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number of mutations listed for CFTR

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1400, most being missense

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most common mutation for CF

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deltaF508

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how many % of mendelian disorders are autosomal dominant?

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50%

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do patients with autosomal dominant mutations have new spontaneous mutations?

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yes, many patients with autosomal dominant diseases have new spontaneous mutations

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most common form of huntington disease

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adult onset in thirties to forties

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hereditary neurological disorder of CNS, causes progressive degeneration, caused by single gene defect on chromosome 4

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huntington disease

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disease with physical manifestation of jerky, involuntary movements, balance/coordination problems, hesitant speech, dementia, swallowing problems

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huntingon

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nancy wexler worked with this disease in venezuela

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huntington disease

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trinucleotide repeat at end of huntingtin gene

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CAG CAG CAG CAG (for glutamine) (polyglutamine expansion)

more repeats means earlier symptoms

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huntingtin gene. ok go:

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chromosome 4p16.3, cag normally 6-35 times; mutation is 36-121 times

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risk of HD and number of CAG repeats

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less than 26: normal range, no HD
27-35 next generation at risk
36-39 some may get HD, next generation at risk
more than 40: will develop HD

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how many cases of achondroplasia are due to new mutations

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80%

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achondroplasia. go!

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autosomal dominant

incompletely dominant disorder of short limbed dwarfism and large heads

normal intelligence, lead normal lives

caused by mutations in FGFR3 gene
1138G->A 98%
1138G->C 2%

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occurs when all or part of a parents' germline is affected by a disease mutation but somatic cells are not

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germline mosaicism

in embryonic development of one parent, a mutation occurred that affected all or part of the germline but few or none of the somatic cells. parents do not express the disease but have the mutation

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probability that a gene will have any phenotypic expression at all. all or nothing

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penetrance

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seen in individuals who have the genotype for a disease but may not show the disease phenotype at all

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reduced penetrance

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is polydactyly a case of reduced penetrance or variable expressivity?

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reduced penetrance

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severity of expression of the phenotype of individials with the same genotype

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expressivity

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reasons for variable expressivity:

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environment
modified genes
allelic heterogeneity (different mutations in same gene cause different phenotypes)

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neurofibromatosis shows:

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variable expressivity, with 100% penetrance

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disease characterized by:
growth of fleshy tumors in skin
cafe au lait spots - brown spots
tumors on iris - lisch nodules
mental retardation

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neurofibromatosis

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marfan has this inheritance

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autosomal dominant, variable expressivity

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symptoms of this disease include skeletal, optical and cardiovascular abnormalities

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marfan syndrome

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genes that exert effects on multiple aspects of physiology or anatomy are:

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pleiotropy

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when different genotypes cause the same phenotype

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locus heterogeneity

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retinitis pigmentosa can be caused by at least 43 different loci, meaning it has

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locus heterogeneity

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an autosomal recessive lysosomal storage disease caused by deficiency of hexosaminidase A

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tay-sachs
has adult onset and severe infantile form

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infantile and adult tay sachs are:
locus heterogeneity
allelic heterogeneity
genetic heterogeneity

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allelic heterogeneity

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mutations in different genes causing same/similar phenotypes

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locus heterogeneity

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different mutations at the same locus causing different phenotypes

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allelic heterogeneity

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dosage compensation aka

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x inactivation

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hypothesis concerning x inactivation

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lyon hypothesis

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evidence for lyon (x inactivation)

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enzyme studies (no more x enzymes in females than males)
barr bodies
lyon hypothesis

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when does barr body replicate

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later in cycle than other chromosomes

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how many barr bodies exist in a cell

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one less than the number of x chromosomes present in the cell;
from cells in normal females

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in carrier females, symptoms of Fabry depend on:

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what % of cells containing normal x chromosome are inactivated

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a manifesting heterozygote is a female who shows the phenotype of the disease and is an extreme case of

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unfavorable lyonization

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in lyonization, is every gene inactivated?

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no, some genes on the x chromosome are not inactivated
about 15%

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gene for x inactivation

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XIST, gene is transcribed only on the inactive chromosome, encodes a functional RNA, required for initiation but not maintenance of inactivation, methylation may be involved in inactivation
Trix - increase trix and both Xs stay on
Oct4 - turns cells back into embryonic stem cells

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gene dealing with x inactivation that turns cells back to embryonic stem cells

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OCT4

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what accounts for 40% of all x linked mental retardation

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fragile X
clinical symptoms besides mental retardation include abnormal facial appearance with large ears and long face, hypermobile joints and increased testicular volume
x chromosome shows elongated decondensed region near tip of long arm

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transmission of fragile x

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x linked dominant with reduced penetrance in females
males in a family can carry and not show symptoms, are called transmitting males
in affected family, each generation's chances increase

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explain sherman's paradox

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deals with fragile x
daughters are never affected but males are
lead to finding cause of fragile x as an expansion of a CGG repeat in 5' untranslated region of the gene, occurs during meiosis of female, fully mutated males have no mRNA from fragile x gene

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what gene contains CGG repeat in fragile X

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FMR 1

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describe mtDNA

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2-10 copies of circular per cell
very little repetitive
no introns
93% coding

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three differences between mito and nuclear genetic codes

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AGA/AGG = stop in mito and Arg in nuclear
UGA = Trp in mito and stop in nuclear
no recombination

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mitochondrial disorders show distinctive patterns of inheritance, due to

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replicative segregation
homoplasmy and heteroplasmy
maternal inheritance

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mitochondrial diseases:

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myopathy
cardiomyopathy
dementia
deafness
blindness
anemia

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cause of leber hereditary optic neuropathy

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missense mutation in mtDNA

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leber hereditary optic neuropathy gene

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1178A > G of ND4
strong sexual bias, males more commonly affected

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the differential expression of genetic material, depending on whether the genetic material has been inherited from the male or female

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parent of origin effects

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what level does imprinting occur

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transcriptional level, involves methylation

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cause of prader willi

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deletion of 15Q12 on parental

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cause of angelman

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deletion of 15q12 on maternal

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inheritance of two chromosomes of a given pair from only one parent

same copy?

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uniparental disomy

uniparental isodisomy

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what is responsible for HIV resistance

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homozygous deltaCCR5, a mutation in the CCR5 cytokine receptor

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what does the delta stand for before a gene

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mutant

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mutation frequency =

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D/2N where D is affected people in studied population and N is number of people in the population

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5 hardy weinberg conditions

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random mating
large population
no mutations
no migration
no selection

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how does stratification affect HW

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stratification means there are subgroups in the population, like religion and race

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random fluctuations of gene frequencies in small populations

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genetic drift

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founder effect is an example of

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genetic drift

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disease in the amish with disproportionate dwarfism, polydactyly

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ellis-van creveld syndrome

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slow diffusion of genes across a racial barrier, a process that involves a large population and a gradual change in gene frequencies

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gene flow

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the mechanism of gene flow is

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migration