Genetics Flashcards

1
Q

Huntington’s disease (4)

A
  • genetic autosomal dominant disorder, but rare
  • trinucleotide repeat disorder
  • late onset (late 30s), but more repeats -> earlier onset
  • loss of physical control, emotional changes, mental deterioration, and death
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2
Q

sickle cell anemia (3)

A
  • autosomal recessive genetic disorder, but common in some areas
  • single point mutation that causes RBCs to sickle in homozygotes, heterozygotes not affected
  • 80% die before reproducing
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3
Q

single-gene disorders

A
  • cystic fibrosis, Tay Sachs, sickle-cell anemia, Hungtingtons
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4
Q

dominant

A
  • condition expressed in heteroxygote
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5
Q

recessive

A
  • condition not expressed in heterozygotes
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6
Q

co-dominant

A
  • heterozygotes intermediate between two homozygotes
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7
Q

assumptions of Hardy-Weinberg equilibrium (5)

A
  • no selection
  • no mutation
  • no migration
  • random mating
  • large population size
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8
Q

is the assumptions of H-W equilibrium are met: (3)

A
  • frequencies of alleles and genotypes will remain constant through time
  • genotype frequencies can be inferred from allele frequencies and vice versa
  • H-W proportions of the genotypes are recovered in a single generation of random mating
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9
Q

H-W equilibrium equation

A

p^2 + 2pq +q^2 = 1

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10
Q

cystic fibrosis (3)

A
  • autosomal recessive
  • mucus build up in homozygotes leading to serious infections, digestive problems and early death (before reproductive age)
  • early onset (~2)
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11
Q

expected equilibrium frequency of deleterious recessive allele under mutation-selection balance equation

A

q hat = sqrt(mu/s)

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12
Q

what does mu mean in the expected equilibrium frequency of deleterious recessive allele under mutation-selection balance

A

mu = mutation rate

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13
Q

what does s mean in the expected equilibrium frequency of deleterious recessive allele under mutation-selection balance (3)

A

s = purifying selection coefficient
- number between 0 and 1 reflecting strength of selection against homozygotes for allele
s = 1 - w, where w = relative fitness of the homozygote

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14
Q

heterozygote advantage (3)

A
  • when heterozygotes have an advantage for fitness even though the recessive homozygous is deleterious
  • cystic fibrosis and sickle cell anemia
  • also called overdominance
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15
Q

what is the fate of a favoured allele (+) under directional selection

A

+ will eventually become fixed

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16
Q

fate of WT allele (+) under heterozygote advantage/overdominance

A

+ will reach an intermediate value/stable equilibrium which can be predicted by p hat +

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17
Q

predicted freq. of WT allele at equilibrium

A

p hat+ = (W+s - Wss)/(2W+s - W++ - Wss)

18
Q

long term effects of dominant favoured selection

A
  • dominant allele is fixed
19
Q

long term effects of recessive favoured selection

A
  • dominant allele is lost
20
Q

long term effects of heterozygote disadvantage selection/underdominance (3)

A
  • p hat+ will predict the unstable equilibrium
  • starting freq. above this values will fixate +
  • starting freq. below this value will lost +
21
Q

why hasn’t natural selection eliminated genetic diseases? (5)

A
  • heterozygote advantage
  • genetic drift & founder effects
  • recurrent mutation, with, perhaps mutational bias
  • late onset
  • fitness trade-offs
22
Q

genetic drift & founder effects (3)

A
  • long and mild expansion
  • ancient bottleneck + expansion
  • strong recent bottleneck + explosive growth
23
Q

hereditary tyrosinemia (3)

A
  • due to genetic drift & founder effects
  • failure to produce enzyme to break down tyrosine aa; autosomal recessive)
  • it is much more common in Quebec than worldwide due to defective gene present in founders of Quebec area
24
Q

genetics of Huntington’s (3)

A
  • dominant mutation on 4th chromosome involving a trinucleotide repeat (CAG CAG …)
  • number of repeats determines severity of disease
  • mutations exhibit length-dependent bias
25
length of repeats and severity of Huntingtons (3)
- <35 = no disease - >35 = disease - more repeats -> earlier onset
26
mutational bias (Huntingtons) (4)
- number of repeats more likely to increase than decrease - mutated alleles transmitted to offspring if occurring in germ line cells (genetic anticipation) - fathers more likely to transmit mutated alleles than mother - number of mutations (repeats) increases with father's age
27
number of cell divisions leading to gametes in females
- egg precursors divide only 24 times, all but once before birth
28
number of cell divisions leading to gametes in males
- sperm precursors divide 23 times per year after puberty
29
possible areas affected by fitness trade-offs (5)
- sexual selection (adult -> mating pairs) - fecundity (mating pairs -> gametes) - gametic selection (gametes -> zygote) - longevity (zygote -> adult) - viability (zygote -> adult)
30
fitness trade-offs
- traits that are favoured at some stages of the life cycle may be disfavoured at others
31
possible fitness trade-off in Huntingtons
- heterozygotes may be more fertile prior to onset of disease than WT individuals
32
linkage disequilibrium
- a measure of nonrandom associations among alleles at multiple loci
33
linkage disequilibrium causes (2)
- physical linkage on chromosome (LD decays over time) | - selection favouring particular allelic combinations ( can be enhanced by chromosomal inversions)
34
physical applications of linkage disequilibrium (2)
- estimating alelle age (young -> high LD; old -> low LD) | - detecting positive selection
35
linkage disequilibrium: detecting positive selection - low freq. of allele - high LD; young allele
- recent mutation
36
linkage disequilibrium: detecting positive selection - high freq. of allele - high LD; young allele (3)
- postive selection - founder event + population expansion - involves specific regions within the genome
37
linkage disequilibrium: detecting positive selection - low freq. of allele - low LD; old allele
- neutral or low selective advantage
38
linkage disequilibrium: detecting positive selection - high freq. of allele - low LD; old allele (2)
- drift | - applies throughout the genome
39
what is the purpose of purifying selection?
- purge deleterious variants
40
what is the purpose of positive selection and balancing selection?
- favour advantageous variants