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302 scientific basis of medicine > Genetics 1 & 2) Genetics, inheritance and populations > Flashcards

Genetics 1 & 2) Genetics, inheritance and populations Flashcards

1
Q

What are types of genetic abnormalities for disorders?

A 
Single gene (monogenic)
Mitochondrial disorder
Chromosomal imbalance
Polygenic
Multifactorial
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2
Q

What is OMIM?

A

Comprehensive authoritative compendium of human genes and genetic phenotypes

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3
Q

Describe the architecture of a gene

A

Promoter region
Start codon
Stop codon
Spliceosome enzymes

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4
Q

What are the bases of a promoter region?

A

TATA

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5
Q

What are the bases of a start codon?

A

ATG

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6
Q

What are the bases of a stop codon?

A

TAG
TAA
TGA

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7
Q

What bases usually make up exon-intron boundaries?

A

GT

AG

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8
Q

What are the classes of mutations?

A

Deletion - gene, exon, nucleotides
Insertions
Single base substitutions - missense, nonsense, splice site
Frameshift
Dynamic mutations e.g. trinucleotide repeat expansion

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9
Q

What is an autosomal recessive disorder?

A

Caused by mutations that result in loss of functional gene product

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10
Q

Describe the frequency of cystic fibrosis

A

Very common in Northern Europe
Carrier frequency is 1 in 23 individuals
Heterozygous carriers are resistant to dehydration so this is advantageous when population experience bouts of water-borne diseases that cause severe diarrhoea e.g. typhoid and cholera

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11
Q

Describe the frequency of Tay-Sachs disease

A

Common in Ashkenazi jews

History of migration in small population numbers means ancestral Ashkenazi populations experience genetic bottlenecks

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12
Q

What is a genetic bottleneck?

A

Occur when populations are small enough that genetic diversity is rapidly and randomly lost each generation leading to random fluctuation of allele frequencies regardless of whether they are beneficial or detrimental leading to genetic drift

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13
Q

Describe the frequency of sickle cell anaemia

A

In malaria endemics, carriers of the mutation have some resistance to the disease

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14
Q

Describe the cystic fibrosis gene

A

Resides on chromosome 7 and gives rise to cystic fibrosis transmembrane conductance regulator

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15
Q

What is the most common CF defect?

A

Deletion of 3 nucleotides (CTT) - 508 mutation

Phenylalanine loss at position 508 in CFTR protein so ATT is now followed by GGT

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16
Q

What type of mutation is the CF 508?

A

Class II

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17
Q

Describe a class I CF mutation

A

Nonsense / frameshift / splicing mutation

No protein / decreased protein

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18
Q

Describe a class II CF mutation

A

Missense or in-frame deletion

Block in protein processing

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19
Q

Describe a class III CF mutation

A

Missense mutation

Defective CFTR regulation

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20
Q

Describe a class IV CF mutation

A

Missense mutation

Altered conduction of CFTR

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21
Q

Describe a class V CF mutation

A

Promoter region mutation

Reduced level of normal protein

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22
Q

Describe a class VI CF mutation

A

Nonsense mutation

Instability at cell surface

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23
Q

Describe the mutation causing phenylketonuria

A

Mutation affecting enzyme phenylalanine hydroxyls (PAH) which normally metabolises L-phenylalanine to L-tyrosine
Autosomal recessive chromosome 12q22-24.1

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24
Q

What is the consequence of phenylketonuria?

A 
Accumulation of phenylalanine
Severe learning difficulties
Fair skin
Eczema
Epilepsy
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25
Q

Describe the size of the PAH gene

A

79kp
450 different mutations
Mutations mainly in exon 7 compromising catalytic domain

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26
Q

What is consanguinity?

A

Describes a situation when 2 relatives have offspring

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27
Q

What is the consequence of consanguinity?

A

Relatives are genetically more similar than non relatives so are more likely to share the same mutations
Means there is a greater chance their offspring will inherit 2 copies of the mutation shared by their parents

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28
Q

What is the inbreeding coefficient for siblings?

A

1/4

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29
Q

What is the inbreeding coefficient for half siblings

A

1/8

30
Q

What is the inbreeding coefficient for uncle / niece?

A

1/8

31
Q

What is the inbreeding coefficient for 1st cousins?

A

1/16

32
Q

What is the inbreeding coefficient for second cousins?

A

1/32

33
Q

What is genomic imprinting?

A

Mammalian epigenetic process that distinguishes maternal and paternal alleles to ensure parent-specific expression of imprinted genes
Some genes in the allele from one of the parents are inactivated (methylation)

34
Q

Give an example of an allele only expressed when inherited from the father

A

The gene encoding insulin-like growth factor 2

35
Q

What controls methylation of genes?

A

Imprinting centres located near the imprinted areas on the same chromosome

36
Q

Give examples of conditions caused by loss of function mutations in the long arm of chromosome 15

A

Prader Willi syndrome

Angelman syndrome

37
Q

What is Prader Willi syndrome caused by?

A

Loss of function in paternal chromosome 15 (deletion of genes in 15q11-13)

38
Q

What are the clinical features of Prader Willi?

A

Mental retardation
Hypotonia
Gross obesity (diabetes)
Male hypogenitalism

39
Q

What is Angelman syndrome caused by?

A

Deletion in maternal chromosome 15 (15q11-13)

40
Q

What are the clinical features of Angelman syndrome?

A 
Mental retardation
Lack of speech
Growth retardation
Hyperactivity
Inappropriate laughter
41
Q

What are autosomal dominant disorders?

A

Those that result from mutations that often manifest in gene products with novel functions or with genes expressed in an unregulated fashion

42
Q

Give examples of autosomal dominant disorders

A

Achrondoplasia

Retinoblastoma

43
Q

What is achondroplasia?

A

Mutation in fibroblast growth factor receptor 3 gene
Abnormality of cartilage formation
7/8 cases are caused by new mutations

44
Q

Why is retinoblastoma not expressed in the mother of affected children?

A

Mother is an obligate carrier

Mutation has a 90% penetrance

45
Q

What is Huntington’s disease?

A

Progressive neurodegenerative disease striking later in life

Causes dementia, severe depression and chorea

46
Q

What causes Huntington’s disease?

A

Trinucleotide repeat expansion in Huntington gene

47
Q

What is anticipation?

A

Age of onset and/or severity is worse in successive generations

48
Q

What does less than 35 CAG repeats mean for Huntington’s test?

A

Negative test result

49
Q

What does 36-39 CAG repeats mean for Huntington’s test?

A

Uninformative test result

50
Q

How many repeats are needed for a positive Huntington’s test?

A

Over 40

51
Q

What is variable expression?

A

Individuals inheriting the same mutation can manifest a disorder with varying degrees

52
Q

What is neurofibromatosis?

A

Disorder of nervous system, eye and skin

53
Q

Compare X and Y chromosomes

A

X chromosome is relatively large spanning over 153 million base pairs and 800 protein coding genes
Y chromosome only has 70 genes

54
Q

Describe the genetics of Fragile X disorder

A

X-linked disorder with triplet repeat expansion

Excessive CGG expansion within FMR1 gene

55
Q

What is Fragile X disorder?

A

Genetic syndrome that is the most widespread single-gene cause of autism and inherited cause of mental retardation among boys

56
Q

Describe the range of repeats for Fragile X disorder

A

Normal range is less than 44
55-200 is known as pre-mutation where there is reduced penetrance
Above 200 CGG repeats is when disorder manifests

57
Q

What is X-chromosome inactivation / lyonisation?

A

Females have 2X the magnitude of gene expression as males need to function with just a single X chromosome
Early during embryogenesis one of the female X chromosomes condenses down and becomes inactive - Barr body

58
Q

What happens to females who inherit a fault gene on the X chromosome that is condensed?

A

Display reduced penetrance

59
Q

What happens if mutation occurs in one of the germline precursors as an embryo?

A

They grow up free of condition as none of the somatic cells have the condition.
However the offspring will have the disorder

60
Q

Describe the mitochondrial genome

A

16.5kb

37 genes encoding 13 polypeptides, 22 transfer RNAs and 2 ribosomal RNAs

61
Q

Give an example of a mitochondrial disorder

A

Leber hereditary optic neuropathy

62
Q

What are the types of genetic testing?

A

Prenatal screening
Newborn screening
Carrier screening
Diagnostic screening, pre symptomatic diagnosis

63
Q

What are the reasons for genetic testing?

A

Help diagnose suspected genetic condition
Identify specific underlying genetic change causing a genetic condition
Determine chance of developing a genetic condition in the future
Determine whether someone is a carrier of a genetic condition
Clarify chance of passing a genetic condition onto children
Find out whether a developing baby or embryo has particular genetic condition

64
Q

What are the methods of genetic testing?

A 
Blood sample from the arm
Saliva samples
Skin biopsies
Amniotic fluid
Chorionic villus sampling
65
Q

What is the function of diagnostic genetic tests?

A

Identify or exclude specific genetic conditions in an individual who has symptoms or signs of a genetic condition
Often use to confirm a diagnosis
Results can help with healthcare management and provide individuals and families with an explanation for the symptoms experienced

66
Q

What is the function of carrier genetic tests?

A

See if an individual is a carrier of the genetic condition as there may be implications for children

67
Q

When are predictive / pre symptomatic genetic tests done?

A

When individual has a family history of a serious later-onset genetic condition e.g. Huntington’s disease
Only offered to adults due to need to respect individual autonomy

68
Q

What are whole exome tests?

A

New type of genetic testing when you don’t know where in a person’s genetic makeup to look for causes of the condition
Looks at all genes in 1 test

69
Q

What is chorionic villus sampling?

A

Offered from 11 weeks of pregnancy

Involves taking a sample from placenta wall with needle through abdomen

70
Q

What is amniocentesis?

A

From 15 weeks of pregnancy

Needle through belly to sample amniotic fluid

71
Q

What conditions can newborn blood spot screening test?

A 
Sickle cell disease
Phenylketonuria
Cystic fibrosis
Congenital hypothyroidism
Medium-chain acyl CoA dehydrogenase deficiency
72
Q

What is the parental conflict hypothesis?

A

Inequality between parental genomes due to imprinting is a result of the differing interests of each parent in terms of the evolutionary fitness of their genes. Paternally expressed genes tend to be more growth promoting whereas maternally expressed genes tend to be more growth limiting.

302 scientific basis of medicine (28 decks)

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