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MTM > WEEK 7 > Flashcards

WEEK 7 Flashcards

1
Q

What does Mendelian Inheritance refer to?

A

Single gene disorders

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2
Q

What are the types of single gene disorders?

A

1) Autosomal dominant
2) Autosomal recessive
3) X-linked recessive

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3
Q

What is autosomal dominant inheritance?

A

Heterozygotes with one copy of an altered gene have the condition

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4
Q

What is autosomal recessive inheritance?

A

Homozygotes with two copies of an altered gene have the condition

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5
Q

What is X-linked recessive inheritance?

A

Males with one copy of altered gene on X-chromosome have the condition

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6
Q

What is a heterozygote?

A

2 different alleles at locus

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7
Q

What is a homozygote?

A

2 alleles which are the same at locus

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8
Q

What are autosomal dominant conditions?

A

Describes any trait expressed in a heterozygote due to everyone who inherits altered gene showing clinical signs

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9
Q

Give three characteristics of autosomal dominant conditions?

A

1) M+F affected equally
2) Transmitted from one generation to the next (“vertical transmission”)
3) All forms of transmission are observed (M-M, F-F, M-F, F-M)

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10
Q

What is the result of most mutations in autosomal dominant disorders?

A

Loss of allele function

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11
Q

What is the result of the majority of mutations in autosomal recessive disorders?

A

Abolish action of allele

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12
Q

What is the result of autosomal dominant/autosomal recessive in a heterozygote?

A

dominant=clinical signs

recessive=no clinical signs

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13
Q

LOOK AT PEDIGREE DIAGRAM FOR:

A

AUTOSOMAL DOMINANT CONDITION

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14
Q

What is the aim for diagnosing autosomal dominant mutations using DNA?

A

To determine a sequence/copy number variant in order to demonstrate mutation in other affected family member/show it isn’t present in unaffected relatives

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15
Q

What are the implications of exceptions to Mendel’s rules in autosomal dominant inheritance?

A

Causes a person to appear not to have an autosomal dominant condition when they have the genotype

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16
Q

Give five examples of exceptions to Mendel’s rules in autosomal dominant inheritance

A

1) Variation in expression
2) Reduced penetrance
3) New mutation
4) Anticipation
5) Mosaicism

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17
Q

What is variation in expression?

A

When other genes modify the phenotype to cause family members to have different signs for the same disease

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18
Q

Give an example of variation in expression

A

Neurofibromatosis Type 1 causing café au lait skin patches and multiple neurofibroma but some can appear to be skin tags

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19
Q

What is reduced penetrance?

A

When someone who is heterozygous shows no clinical signs of a condition (“skipped generation”)

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20
Q

Give an example of reduced penetrance

A

Huntington’s disease gene containing between 36-39 CAG repeats has reduced penetrance, HD is a progressive neurological disorder resulting in involuntary movements, dementia and psychiatric disturbance

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21
Q

What is age-related penetrance?

A

When there is a delayed onset of genetic disease (BRCA mutation)

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22
Q

What is new mutation?

A

When there is a new mutation in egg/sperm of the parent of affected offspring

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23
Q

Give an example of new mutation

A

Achondroplasia causing short stature due to new mutation in sperm, the chance of which increases with paternal age

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24
Q

What is anticipation?

A

Unstable, expanding trinucleotide repeat mutation resulting in reduced age of onset and/or increased phenotype severity in successive generations

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25
Q

Give an example of anticipation

A

Muscular dystrophy (CTG repeats) causing muscle weakness and impaired muscle contraction after relaxation (myotonia)/Huntington’s disease (CAG repeats)

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26
Q

What is mosaicism?

A

Formation of two populations of cells with different genetic constitutions usually due to mitotic error

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27
Q

Give an example of mosaicism

A

Severe osteogenesis imperfecta (brittle bone disease)

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28
Q

What are autosomal recessive conditions?

A

Conditions that manifest only in a homozygous state

29
Q

Give four characteristics of autosomal recessive conditions

A

1) Can’t follow the disease through pedigree diagrams
2) “Horizontal transmission”-see siblings affected
3) M+F affected equally
4) May be evidence of consanguinity due to increased chance of two carriers

30
Q

What is consanguinity?

A

When parents have a biological ancestor in common (eg. siblings)

31
Q

LOOK AT PEDIGREE DIAGRAM FOR:

A

AUTOSOMAL RECESSIVE CONDITION

32
Q

What are X-linked recessive conditions?

A

Disorders which are sex-linked and normally manifest only in males

33
Q

Give an example of an X-linked recessive condition

A

Duchenne Muscular Dystrophy (DMD)

34
Q

What is the permutation of males with an affected X chromosome?

A

They have the disorder due to hemizygosity

35
Q

What is hemizygosity?

A

Diploid individual with only one gene at a specific locus (due to XY)

36
Q

What are the permutations of females with an affected X chromosome?

A

1) They have the disorder due to homozygosity

2) They are a carrier of the disorder due to heterozygosity

37
Q

Give three characteristics of X-linked recessive conditions

A

1) All daughters of males with the condition are obligate heterozygotes (but may be homozygotes)
2) When a mother is a carrier and the father is unaffected, each son has a 1/2 chance of having the condition; each daughter has a 1/2 chance of being a carrier (“Knight’s move” pattern of transmission-mother to son)
3) Affected males cannot transmit disorder to their sons

38
Q

What are the reasons that may cause a female to show signs of an X-linked recessive disease?

A

1) Homozygous for allele
2) Has single X chromosome (Turner’s syndrome)
3) Has structural rearrangement of X chromosome
4) Is heterozygous but has skewed or non-random X-inactivation (carrier with mild symptoms)

39
Q

How does an X-linked dominant disease’s pedigree diagram differ slightly from that of an autosomal dominant condition?

A

1) Excess affected females

2) No male-to-male transmission

40
Q

What is the result of lyonisation?

A

Females being less severely affected than males by X-linked dominant conditions

41
Q

What is lyonisation?

A

Inactivation of an X chromosome

42
Q

Give an example of an X-linked dominant condition

A

Vitamin D resistant rickets

43
Q

What is a characteristic in X-linked dominant disease?

A

M-F transmission always results in the disorder being passed on

44
Q

How does mitochondrial inheritance occur?

A

Exclusively via the mother as sperm mitochondria is actively expelled from the fertilised egg

45
Q

What is multifactorial inheritance?

A

The type of hereditary pattern seen when there is more than one genetic factor involved and, sometimes, also environmental factors

46
Q

What is the continuum of effect of DNA variants?

A

Responsible for normal variation—>Modulate disease susceptibility or resistance to environmental triggers—>Directly cause disease

47
Q

What are DNA sequence variants?

A

DNA sequences with varying effects on health depending on where they occur and whether they alter essential gene function and/or their controlling elements

48
Q

Give the characteristics for Non-Mendelian Inheritance

A

Polygenic
Multifactorial
Maternal inheritance (mitochondrial)

49
Q

What does multifactorial inheritance cause?

A

Common complex disorders

50
Q

What two factors go into multifactorial inheritance?

A

Inherited predisposition/genetic susceptibility (genetic factors) and environmental factors`

51
Q

What is the characteristic of inheritance patterns for common conditions due to multifactorial inheritance that is different to mendelian inheritance?

A

No obvious pattern

52
Q

Give examples of common complex conditions

A 
CHD
DM
Hypertension
Cerebrovascular disease
Schizophrenia
Breast and bowel cancers
53
Q

How do you measure how much of disease aetiology is due to genetic or environmental factors in multifactorial conditions?

A

Observational studies of disease incidence in different groups of people (eg. family clustering, twin/adoption studies, population/migration studies)

54
Q

What is family clustering?

A

Analysing risk to family members of someone with a common complex disease

55
Q

How do you determine whether environmental factors influence common complex conditions by twin studies?

A

Assess monozygotic (all genes shared) and dizygotic (50% genes shared) twins

56
Q

What is meant by polygenic?

A

Multiple genes which each make a small additive contribution to the final phenotype

57
Q

What is polygenic inheritance the basis for?

A

Continuous traits following normal distribution in the population

58
Q

Give examples of polygenic/continuous traits

A

Blood pressure, height, IQ

59
Q

How do single genes contribute to a continuous characteristic?

A

Additive effects (they blend)

60
Q

Give an example of single genes contributing to a continuous characteristic

A

Eye colour

61
Q

What is the result of genetic and environmental factors varying from person to person?

A

They produce differing liabilities to a multifactorial condition for each individual

62
Q

What is liability threshold?

A

The threshold for liability above which a person will develop a multifactorial condition

63
Q

What is the shift in liability curve for relatives of a family member with a multifactorial condition?

A

Leftward due to increased likelihood of similar genetic and environmental factors

64
Q

What is the trend in recurrence risk when moving away from the origin relative?

A

It decreases

65
Q

What is empiric risk?

A

The chance that a disease will occur in a family based on experience with the diagnosis, past history and medical records rather than theory

66
Q

What may SNPs identify in common conditions?

A

They may increase or decrease risk or have indirect association/act as a marker for nearby major genetic influence

67
Q

What is the 100,000 Genome Project?

A

Government plan to sequence full genomes of up to 100,000 NHS patients with cancer, rare inherited diseases or infectious diseases

68
Q

What is hypothesised with genetic testing?

A

Showing individualised risk could provide motivation to change behaviour to cause decrease in risk

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