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Medical genetics > Polygenic diseases and population genetics > Flashcards

Polygenic diseases and population genetics Flashcards

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USMLE® Step 1 flashcards Biology 101 flashcards
1
Q

How is bias of ascertainment a flaw in twin studies?

A

People focus on twins with similar behavioural traits

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2
Q

What is meant by penetrance?

A

Proportion of individuals carrying disease causing allele who express disease phenotype

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3
Q

What is meant by expressivity?

A

Severity of phenotype in affected individuals (degree to which mutant phenotype develops in someone with mutant genotype)

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4
Q

Why do monogenic disorders show expressivity and variable penetrance (3)?

A

Environmental factors

Other genes (e.g. modifier genes affect transcription of primary causative gene)

Preferential activation of normal or abnormal X chromosome

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5
Q

What are polygenic disorders?

A

Disorders not controlled by single gene (but several genes as well as environment)

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6
Q

Compare spectrum of diseases in terms of entirely environmental and entirely genetic. Give an example

A

Entirely genetic: Duchenne’s

Entirely environmental: Scurvy

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7
Q

Most diseases lie where on the spectrum between environment and genetic cause

A

In the middle, they are polygenic and multifactorial

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8
Q

Are polygenic disorders inherited in autosomal dominant/recessive manner?

A

No

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9
Q

Are polygenic diseases more common in the population than monogenic? Why?

A

Yes

Polygenic diseases often have a more moderate effect than single gene disorders.

Offspring carrying many genes contributing to a diseased phenotype are likely to survive to reproduction and pass the genes down to the next generation.

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10
Q

What causes polygenic disorders?

A

Interactions between many genes and environmental factors

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11
Q

Give examples of congenital symptoms associated with polygenic disorders

A 
Spina bifida
Cleft lip/palate
Congenital hip dislocation
Clubfoot
Congenital heart defect
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12
Q

What are some examples of non-congenital polygenic diseases

A

Asthma, diabetes, hypertension, Parkinson’s, autism , Psoriasis

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13
Q

What do associated alleles indicate for a specific polygenic disease?

A

Risk of devleoping disease (not a cause of polygenic disease)

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14
Q

4 characteristics of polygenic diseases

A

Are common

Multi gene involvement (polygenic)

Major non genetic influences

Unclear transmission patterns

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15
Q

Why are single gene diseases rarer?

A

Mutations causing single gene disease have more impact on gene product function so sufferers are less likely to reproduce successfully

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16
Q

Describe effects of different associated genes in polygenic disease

A

Each gene has varying effects on trait occurence and development, each gene may have small additive effect

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17
Q

Describe the incidence of polygenic and monogenic diseases in close relatives

A

Lower incidence of polygenic diseases in close relatives than monogenic diseases

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18
Q

What is meant by an additive effect describing a associated gene in polygenic disorder?

A

No gene dominant/recessive to each other, their effect is cumulative

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19
Q

What are two ways phenotypes can be inherited in the population?

A

Continuous (normal distribution)

Discontinuous (have it or not)

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20
Q

How can discontinuous diseases resemble a continuous phenotype?

A

When they are polygenic and many genes act together

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21
Q

What does liability mean with a discontinuous trait?

A

Certain cumulation of disease causing genes

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22
Q

What affects whether you cross the threshold in a distribution of liability?

A

Combination of genes you have inherited and exposure to environmental factors

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23
Q

What is the threshold in a discontinuous phenotype?

A

The certain cumulation of disease causing genes that means the diseased phenotype is expressed.

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24
Q

What is recurrence risk?

A

Risk that a disease will occur elsewhere in a pedigree, given that at least one member of the pedigree exhibits the disease.

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25
Q

What increases as number of affected family members increases?

A

Recurrence risk

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26
Q

What happens to the liability of a sibling of an affected person?

A

Higher liability (compared to general pop) so bell shaped curve moves right

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27
Q

What is relative risk?

A

Probability of individual developing condition compared to all others in general population

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28
Q

What is heritability?

A

Proportion of total phenotypic variance of a condition which is caused by additive genetic variance

Basically how much is the disorder ‘genetic’ vs environmental

29
Q

What does it mean to analyse the heritability of a disorder?

A

Estimate what proportion of phenotype can be ascribed to genetic factors / environmental factors

The more heritable the more genetic

30
Q

Why is it important to know if heritability is high?

A

Family members may consider retesting

31
Q

List the ways heritability can be assessed

A

Studies of migrant groups moving to population with different incidence of disease

Degree of family clustering

Twin studies

32
Q

How can degree of family clustering help assess variability, what is the flaw?

A

Ratio of risk to siblings of affected indiviudals with general pop
(recurrence risk)

Flaw: families tend to share environmental factors

33
Q

How do we use twin studies to study heritability?

A

Compare concordance rates in monozygotic and same sex dizygotic twins.

Greater concordance rates in monozygotic compared to DZ twins indicates more important genetic component

34
Q

What is concordance?

A

When both/neither person in a twin is affected

35
Q

Describe genetic differneces between monozygotic and dizygotic twins

A

Monozygotic twins generally identical

Dizygotic twins share 50% of genes on average

36
Q

Why is it best to separate twins at birth?

A

Monozygotic twins more likely to share environmental factors (than DZ twins)

37
Q

What are 5 flaws in twin studies?

A

Intrauterine differences

Bias of ascertainment

MZ twins are same sex but DZ not always same sex

MZ may be treated differently than DZ twins

38
Q

How are intrauterine differences a flaw for twin studies?

A

MZ twins share more intrauterine tissue than DZ twins during gestation

Makes it more difficult to separate intrauterine environmental causes from genetic causes

39
Q

How are MZ twins treated differently than DZ twins, what’s the solution?

A

Treated as same person so may influence behavioural traits

Solution: study twins separated at birth

40
Q

What does the Hardy Weinberg principle state?

A

(For diploid sexually reproducing organisms)

Allele and genotype frequencies remain constant from generation to generation

41
Q

What are the assumptions for Hardy Weinberg (5)?

A

Large population (so no genetic drift)

Random mating

No migration (no gene flow)

No new mutations

No selection pressures for/against given genotype

42
Q

What is a human example where criteria for HW are met?

A

Human blood type

Enzyme variants

43
Q

What does HW enable?

A

Predict incidence of diseased individuals and unaffected carriers if p and q are known

44
Q

What is gene pool?

A

Alleles in a population

45
Q

What is p^2, q^2 and 2pq

chance of being (genotypes)?

A

AA
aa
Aa (carrier)

46
Q

What is the frequency of carriers?

A

2pq

47
Q

What are the Hardy Weinberg equations?

A

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

p + q = 1

48
Q

What does HW assume about alleles in a population?

A

2 alleles for a gene

49
Q

How does real pop differ from assumptions of HW (6)?

A

Assortive/ non random mating

Selection

Positive selection

Mutations (source of variation)

Genetic drift (in small pops)

Migration (thus gene flow)

50
Q

How can non random mating prevent HW conditions being met?

A

Heterozygoes more likely to mate than expected by chance random mating.

More homozygotes (more disease) than predicted by Hardy Weinberg

Also less variation expected (people mate with similar people)

51
Q

How can selection prevent HW conditions being met

A

Some mutations affect reproductive fitness or ability to survive

Some of these mutations only have onset after reproductivity so passed on

52
Q

How do mutations prevent conditions for HW being met?

A

Mutations likely to be eliminated unless compensating advantage

53
Q

How can genetic drift prevent HW conditions being met?

A

Allele frequences change over time (esp in small populations)
Over time one allele is fixed and another eliminated

54
Q

What is founder effect when and where does it occur?

A

Type of genetic drift, occurs in small recently isolated populations with reduced genetic variation

55
Q

When does migration and subsequent gene flow have its largest effect?

A

Populations are small and allele frequency differences are large

56
Q

What are polymorphism?

A

Coexistence in a population of two or more alleles at a genetic locus

Natural variations in a gene that have no adverse effects and occur at high frequency in general population

57
Q

What are balanced polymorphisms?

A

System of genes in which two alleles are maintained in stable equilibrium because the heterozygote is more fit than either of the homozygotes.

58
Q

Who has the advantage in a balanced polymorphism, why?

A

Heterozygote advantage - more likely to survive than those with just one version of a gene/one allele

59
Q

What might ethnic differences in disease frequencies depend on?

A

Open/closed population
Survival advantages
Consanguinity

60
Q

What are examples of ethnic differences in disease frequencies?

A

Mediterranean and Africa - high prob of sickle cell and thalassaemia

Ashkenazi Jews: Tay Sachs disease, familal breast cancer

Amish: Maple syrup urine disease

61
Q

Why is consanguinity important?

A

Likelihood of some genetic conditions increases if partners are related.

62
Q

How does autosomal recessive inheritance contribute to increased risk if partners are related?

A

Both partners may be carriers of a mutation but aren’t affected by condition as they have normal gene copy as back up

If partners are related, they are more likely to have mutations in the same genes.

63
Q

What is absolute risk?

A

The chance of an individual developing a condition in their lifetime is the

64
Q

What is prevalence?

A

Prevalence is the proportion of the population that has a certain condition.

65
Q

How to work out the frequency of patients with the disease from the carrier rate?

A

Carrier rate x carrier rate x 1/4

(Aa x Aa) gives 1/4 aa

66
Q

True or false? Polygenic disease is caused by the interaction of specific disease-associated alleles

A

False, not their interaction but cumulation

67
Q

What is expressivity?

A

The degree to which a mutant phenotype develops in an individual who inherits a mutant genotype

68
Q

What is epistasis

A

The interaction of genes that are not alleles, in particular the suppression of the effect of one such gene by another.

Medical genetics (6 decks)

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