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Genetics of Complex Disease > Complex Disease > Flashcards

Complex Disease Flashcards

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0
Q

What percent of human disease is due to Mendelian disease?

A

1-4%

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1
Q

What percent of human disease is due to chromosomal abnormalities?

A

<1%

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2
Q

What percent of human disease is due to complex disease?

A

95%

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3
Q

What are chromosomal abnormalities with relation to human disease?

A

Abnormalities resulting in visible alteration of chromosomes or resulting from specifically chromosomal mechanisms

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4
Q

Chromosomal abnormalities can be constitutional or… ?

A

Somatic - i.e. not present in all cells

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5
Q

What are numerical chromosomal abnormalities?

A

Alterations in the number of chromosomes, e.g. triploidy, tetraploidy and aneuploidy

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6
Q

What do the terms triploidy and tetraploidy mean?

A

When an individual’s cells contain three or four copies of the whole genome (rather than two) - this is LETHAL

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7
Q

How does triploidy/tetraploidy occur?

A

As a result of dispermy, diploid gamete or endomitosis

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8
Q

What is aneuploidy?

A

When an individual has more or less copies of one or more chromosomes

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9
Q

What is trisomy?

A

When an individual has an extra copy of one chromosome

e.g. Down’s Syndrome - extra copy of chromosome 21

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10
Q

What is monosomy?

A

When an individual has one less copy of a chromosome

e.g. Turner’s Syndrome - one less X chromosome

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11
Q

How is Mendelian disease defined?

A

A trait caused by a mutation in a single gene (i.e. monogenic) with a known pattern of inheritance

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12
Q

In what case is a genotype at a locus both necessary and sufficient for a trait to be expressed?

A

Mendelian disease

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13
Q

What factors can influence inheritance of Mendelian disease?

A 
incomplete penetrance
locus heterogeneity 
allelic heterogeneity
clinical heterogeneity 
genetic anticipation 
genetic imprinting
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14
Q

what is penetrance?

A

the proportion of individuals with a particular genetic mutation who exhibit signs of the mutation (i.e. the trait)

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15
Q

what is locus heterogeneity?

A

mutations at many different loci (in many different genes) giving the same phenotype

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16
Q

what is allelic heterogeneity?

A

different mutations in one gene giving the same phenotype

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17
Q

what is clinical heterogeneity?

A

the same mutation giving different phenotypes

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18
Q

what is genetic anticipation?

A

disease increasing in severity with each passing generation

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19
Q

what is genetic imprinting?

A

when a different phenotype is seen depending on whether mutation was inherited from mother or father

20
Q

how can a genetically complex disease be defined?

A

a disease where one or more alleles acting alone or in concert increase or decrease the risk of developing a trait

21
Q

is genetically complex disease monogenic or polygenic?

A

genetically complex disease is usually polygenic, monogenic complex disease is very rare

22
Q

which shows higher penetrance, Mendelian or complex disease?

A

Mendelian disease - complex disease usually shows low penetrance (having the allele only increases the risk of disease)

23
Q

In what genetic inheritance is the presence of the susceptibility allele neither necessary nor sufficient to trait development?

A

Genetically complex inheritance

24
Q

what is disease prevalence?

A

the total number if cases at any given time

25
Q

what is disease incidence?

A

the number of new cases/unit of time

26
Q

how is sibling risk ratio calculated?

A

the incidence/prevalence in siblings divided by the incidence/prevalence in population

27
Q

How can the heritability of a genetic disease be assessed?

A 
calculating sibling risk ratio 
family and twin studies
segregation analysis 
linkage analysis
association analysis (GWAS/GWLS)
28
Q

What are twin studies?

A

Disease concordance rates (degree of “trait sharing”) compared in monozygotic and dizygotic twins

29
Q

What can twin studies show?

A

If concordance rate is higher in monozygotic twins compared to dizygotic twins, there is a significant genetic component

30
Q

What is a disadvantage of twin studies?

A

twins have a shared environment from birth therefore results may be influenced by environment

31
Q

how can a shared environment in twin studies be controlled for?

A

adoption studies looking at clustering of disease in biological vs. adoptive family

32
Q

what is concordance in genetics?

A

the presence of the same trait in two individuals

33
Q

What are two approaches to identifying susceptible alleles in complex disease?

A

Linkage and association analyses

34
Q

What is required to carry out a linkage analysis study?

A

the collection of DNA from informative families - i.e. family pedigrees or sibling pairs

35
Q

can DNA from monozygotic twins be used in linkage analyses?

A

no, as monozygotic twins have identical genes and alleles

36
Q

what is a LOD score?

A

a statistical estimate of whether two genes are located near each other on a chromosome and therefore are likely to be inherited together (i.e. the degree of LINKAGE of two genes)

37
Q

what is linkage of two genes defined as?

A

linkage is a measure of association between genes on the same chromosome - the closer the two genes are on the chromosome (i.e. the higher the degree of linkage) the more likely they are to be inherited together during meiosis

38
Q

How is a LOD score calculated?

A

via parametric linkage analysis

39
Q

what does a LOD score > 3 indicate?

A

Evidence for linkage between two genes i.e. the odds of the linkage observed NOT being due to chance is 1000 to 1

40
Q

what is linkage disequilibrium?

A

if two genes are in linkage disequilibrium, certain alleles of each gene are inherited together more often than would be expected by chance

41
Q

what is linkage equilibrium?

A

linkage equilibrium refers to two genes inherited completely independently in each generation - e.g. two loci on two different chromosomes that encode unrelated, non-interacting proteins

42
Q

what can genetic association studies show?

A

the statistical likelihood of developing a certain trait based on the presence of a certain allele

43
Q

what is the general difference between linkage analyses and association analyses?

A

linkage analyses are carried out in families, association analyses are carried out in a population

44
Q

what are advantages of genome-wide association analyses? (GWAS)

A
  • can look at susceptibility alleles in sporadic and infectious disease
  • hypothesis-free therefore does not require previous knowledge of disease
  • can identify novel susceptibility loci/associated alleles
45
Q

how were GWAS carried out in the pre-genome era?

A

by searching for non-random segregation of variable number tandem repeats (VNTRs) or “microsatellites”

46
Q

what is the outcome of a GWAS?

A

an odds ratio (OR) score

47
Q

what does an odds ratio (OR) of p < 5x10-7 show in GWAS?

A

that the association between a particular SNP (allele) and phenotype is significant

48
Q

which two different alleles did two different studies identify as the primary susceptibility alleles for PBC?

A

HLA-DQA1*0401 and DRB1-08:01-DQ4 haplotype

Genetics of Complex Disease (6 decks)

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