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MRCOG - Part 1 > Genetics > Flashcards

Genetics Flashcards

1
Q

What is the haploid number of chromosomes?

A

23 - as is found in the germ cells

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2
Q

What is the diploid number of chromosomes?

A

46 - as is found in the somatic cells

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3
Q

What are the different types of chromosomes?

A

1) Metacentric chromosome - when the short and long arms are approx. equal length
2) Submetacentric chromosome - when one arm is obviously longer than the other
3) Acrocentric chromosome - when the centromere is situated at the top of chromosome

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4
Q

What are the chromosome arms called?

A

p and q arms

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5
Q

How do somatic cells divide?

A

Mitosis

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6
Q

How do germ cells divide?

A

Meiosis

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7
Q

How does meiosis occur?

A

Meiosis I - chromosomes duplicate, paternally and maternally inherited chromosomes are recombined and diploid is reduced to haploid number.
Meiosis II - there is then splitting of the sister chromatids

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8
Q

What are the most common trisomy conditions?

A

Down syndrome - trisomy 21
Edward’s syndrome - trisomy 18
Patau syndrome - trisomy 13
Klinefelter syndrome - 47XXY

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9
Q

What is the most common monosomy?

A

Turner syndrome - 45X

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10
Q

What proportion of Down syndrome is accounted for by non-disjunction?

A

95% - when there is 3 separate copies of chromosome 21

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11
Q

What are the features of Down syndrome?

A 
Learning difficulties
Atrioventricular septal defect
Upslanting palpebral fissures
Epicanthic folds
Flat midface
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12
Q

What are the features of Edwards syndrome?

A 
Profound learning difficulties
Congenital heart disease, commonly VSD
Facial clefts
Clenched hands
Rocker-bottom feet
Spina bifida
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13
Q

What are the features of Patau syndrome?

A 
Profound learning difficulties
Congenital heart disease
Scalp defects
Omphalocele
Renal anomalies
Microphthalmia/anophthalmia
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14
Q

What is more common, Edwards or Patau?

A

Edwards

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15
Q

What proportion of Turner’s syndrome foetuses make it to term?

A

1%, they don’t tend to survive due to fetal hydrops

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16
Q

What is the consequence of balanced reciprocal translocation?

A

Usually there is no phenotypic consequences

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17
Q

What are the possible outcomes for a woman with a balanced reciprocal translocation?

A

1) A normal pregnancy with normal karyotype
2) A normal pregnancy with the familial balanced reciprocal translocation
3) A spontaneous miscarriage owing to an unbalanced product of the familial translocation
4) A pregnancy that goes to term but the child has a high likelihood of learning difficulties and possibly other congenital anomalies associated with an unbalanced product of the familial reciprocal translocation

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18
Q

What do you do if you detect an unbalanced reciprocal translocation prenatally?

A

Counsel parents that it is likely to be associated with fetal anomaly

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19
Q

What do you do if you detect balanced reciprocal translocation prenatally?

A

Karyotype the parents - if it is familial and the parent carrying the balanced translocation is healthy, the baby is unlikely to have problems. If it is found to be de novo, it may be associated with problems and therefore a microdeletion should be investigated for using an array comparative genomic hybridisation (aCGH)

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20
Q

What is imprinting?

A

The process by which one parental allele is preferentially silenced according to its parental origin

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21
Q

What imprinting disorder does paternal UPD15 (uniparental disomy) cause?

A

Angelman syndrome

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22
Q

What imprinting disorder does maternal UPD15 cause?

A

Prader-Willi syndrome

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23
Q

What are the different types of mutation?

A

1) Missense mutation
2) Frameshift mutation
3) Nonsense mutation
4) Splice site mutation

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24
Q

What is a missense mutation?

A

Single base substitutions occurring in a coding region of the disease

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25
Q

What is a frameshift mutation?

A

One or more base, that isn’t a multiple of 3, are inserted or deleted to/from a sequence, disrupting the normal reading frame, ultimately resulting in a different stop codon placement and therefore resulting in an abnormal/truncated proteins

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26
Q

What is a nonsense mutation?

A

A single base substitution resulting in a premature stop codon = truncated protein

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27
Q

What is a splice site mutation?

A

When there is a mutation to the nucleotides at the splice site, i.e. the junction between the introns/exons

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28
Q

What are the common triplet repeat conditions?

A

1) Fragile X syndrome
2) Huntington’s
3) Myotonic dystrophy

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29
Q

What features are common to triplet repeat conditions?

A

1) Variability of phenotype
2) Anticipation - progression in triplet repeat size and phenotype severity as the repeat is passed on
3) Parent-of-origin effect - the triplet may expand if passed on by the mother rather than the father or vice versa

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30
Q

What are some common microdeletion syndromes?

A

1) Cri du chat
2) Williams
3) DiGeorge

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31
Q

What’s the lifetime risk of breast/ovarian cancer in BRCA1?

A 
Breast = 80%
Ovarian = 40%
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32
Q

What’s the lifetime risk of breast/ovarian cancer in BRCA2?

A 
Breast = 45%
Ovarian = 15%
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33
Q

What are the characteristics of autosomal dominant pedigrees?

A

1) Individuals in each generation are affected
2) Males and females affected in roughly equal numbers
3) Both men and women can have affected children

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34
Q

What are the characteristics of autosomal recessive pedigrees?

A

1) Only one generation is affected

2) Parents of affected children are consanguineous

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35
Q

What are the characteristics of x-linked recessive pedigrees?

A

1) Only male individuals are affected, if females are affected they may have a milder phenotype
2) No male-to-male transmission
3) The condition may seen to skip generations

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36
Q

What are the characteristics of x-linked dominant conditions?

A

Results in spontaneous loss or pregnancy or early neonatal death in males. Therefore, they seem to be restricted to live female individuals

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37
Q

What are some examples of mitochondrial disease?

A

1) MELAS - mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes
2) Leigh’s disease - subacute necrotising encephalomyopathy
3) MERRF - myoclonic epilepsy with ragged red fibres

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38
Q

What is nuchal translucency?

A

The songraphic appearance of subcutaneous fluid found at the back of the fetal neck between weeks 11 and 14.

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39
Q

What does the combined test comprise of?

A

The NT scan + b-hCG + PAPP-A

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40
Q

What is the detection rate of trisomy 21 when using NT in isolation?

A

70-75%, with a 5% false-positive rate

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41
Q

What is the detection rate of trisomy 21 with the combined test

A

90%

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42
Q

What may be seen in a Trisomy 18 pregnancy?

A

Polyhydramnios or oligohydramnios

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43
Q

What are the AFP/b-hCG levels in Down’s syndrome?

A 
AFP = reduced
b-hCG = increased
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44
Q

What may cause reduced PAPP-A?

A

Aneuploidy, e.g. Down’s, or IUGR

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45
Q

From what gestation can CVS be performed?

A

Week 11

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46
Q

From what gestation can amniocentesis be performed?

A

Week 15

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47
Q

On what chromosome is BRCA1 found?

A

Chromosome 17

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48
Q

On what chromosome is BRCA2 found?

A

Chromosome 13

49
Q

What is the inheritance type of the BRCA gene?

A

Autosomal dominant

50
Q

What type of inheritance does ‘familial’ Down’s syndrome refer to?

A

Robertsonian translocation

51
Q

Which chromosomes can Robertsonian translocations occur on?

A

Chromosomes 13, 14, 15, 21 and 22 only

52
Q

What is seen in a fetus with a complete absence of a-globulin chains, as with those with alpha-thalassaemia?

A

Hydrops fetalis

53
Q

What is the incidence of PKU in the UK?

A

1/14000

54
Q

What are the neurotoxic byproducts of phenylalanine?

A

Phenylpyruvic acid and phenylethylamine

55
Q

When should the heel prick test be performed?

A

> 12 hours after birth

56
Q

What are the 3 types of PKU?

A

1) Type 1 (autosomal recessive); 2) Type 2; 3) Malignant PKU

57
Q

What is the detection rate of Down’s using NT alone?

A

70%

58
Q

What is the detection rate of Down’s in the 1st trimester using both NT and serum screening?

A

85% detection rate

5% false +ve rate

59
Q

What is the risk of Down’s in a woman >/= 45?

A

> 1/50

60
Q

What is the risk of Down’s in a woman @ 20 years old?

A

1/1500

61
Q

What is the most common enzyme deficiency causing CAH?

A

21-hydroxylase

62
Q

What is 21-hydroxylase deficiency caused by?

A

Abnormal CYP21A genes

63
Q

What is the pattern of serum markers in Down’s?

A

PAPP A, AFP and uE3 = down

inhibin A and b-hCG = up

64
Q

What is the pattern of serum markers in Edward’s?

A

PAPP A, AFP, UE3, inhibin A and b-HCG = down

65
Q

What should be offered to all women with beta-thalasaemia in pregnancy?

A

1) Folic acid 5mg from 3/12 prior to conception
2) Specialist cardiac assessment at 28/40
3) Serial fetal biometry scans every 4/52 from 24/40
4) Regular blood transfusion aiming for a pre-transfusion Hb of 100
5) If have undergone splenectomy and have a plt count >600, they should have LMWH + 75mg aspirin

66
Q

How is beta-thalasaemia inherited?

A

Autosomal recessive

67
Q

What does beta-thalasaemia cause?

A

Major (homozygous) = severe transfusion dependant anaemia

Minor (heterozygous) = mild microcytic anaemia

68
Q

What does the triple test consist of?

A

AFP
b-hCG
uE3

69
Q

What does the quadruple test consist of?

A

AFP
b-hCG
inhibin A
uE3

70
Q

At what gestations can the quadruple test be used?

A

14+2/40 —> 20+0/40

71
Q

What does the combined test consist of?

A

PAPPA + b-hCG + NT scanning

72
Q

At what gestations can the combined test be used?

A

11+2/40 —> 14+1/40

73
Q

At what gestation could the mid-trimester anomaly scan occur?

A

18+0/40 —> 20+6/40

74
Q

What proportion of breast cancers are caused by BRCA?

A

5%

75
Q

What proportion of inherited cancers are caused by BRCA?

A

25%

76
Q

How is Noonans inherited?

A

Autosomal dominant

77
Q

How is Cri-du-Chat inherited?

A

Microdeletion of chromosome 5

78
Q

What is the inheritance pattern of adult PKD?

A

Autosomal dominant

79
Q

At what stage of meiosis is the oocyte after ovulation?

A

Metaphase II

80
Q

How many genes in the human genome?

A

20,000

81
Q

What is the risk of non-disjunction at age 40?

A

1/100

82
Q

What is the risk of non-disjunction at age 45?

A

1/30

83
Q

What chromosomes are most commonly involved in reciprocal translocations?

A

Chromosome 11 and 22

84
Q

What chromosomes are involved in Robertsonian translocations?

A

13; 14; 15; 21 and 22

85
Q

What are the most common Robertsonian translocations?

A

Fusion of chromosome 13 to 14

Fusion of chromosome 14 to 21

86
Q

What is the method by which microdeletions may be detected?

A

Microarray aCGH

87
Q

What are some common autosomal dominant conditions?

A
  1. Tuberous sclerosis
  2. Marfan’s
  3. HNPCC
  4. Neurofibromatosis 1
  5. Huntingtons
  6. Achondroplasia
  7. Autosomal dominant polycystic kidney disease
88
Q

What are some common X-linked dominant conditions?

A

1) Incontinentia pigmenti

2) Rett syndrome

89
Q

What may cordocentesis be used for? And when?

A

From week 18

Can be used to assess anaemia and diagnose prenatal infection

90
Q

What is the most common trisomy in miscarried fetuses?

A

Trisomy 16

91
Q

What does the ‘double bubble’ sign suggest?

A

Duodenal atresia, which in itself suggests Down’s syndrome

92
Q

What type of chromosome is the X chromosome?

A

Metacentric chromosome

93
Q

How is vitamin D resistant rickets inherited?

A

X-linked dominant

94
Q

What is a genetic cause of hypergonadotrophic hypogonadism?

A

Klinefelters

95
Q

What are nucleotides made up of?

A

A sugar
A nitrogenous base
A phosphate group

96
Q

How many carbons on each sugar? And what are the different types of sugar?

A

5 carbons to each sugar

Different types = deoxyribose (for DNA), ribose (for RNA)

97
Q

Where on the sugar does the nitrogenous base attach to form a nucleotide?

A

Base attaches to carbon 1

98
Q

Where on the sugar does the phosphate group attach to form a nucleotide?

A

Phosphate group attaches to carbon 5

99
Q

How many bonds form between C and G?

A

3 H bonds

100
Q

How may bonds form between A and T/U?

A

2 H bonds

101
Q

What are the purine nitrogenous bases?

A

A

G

102
Q

What are the pyrimidine nitrogenous bases?

A

C
T
U

103
Q

What is the largest chromosome?

A

Chromosome 1

104
Q

What is the smallest chromosome?

A

Chromosome 22

105
Q

What enzymes are involved in DNA replication?

A

Unwinding of strands - DNA helicase
Copying of strands - DNA polymerase
Winding back of strands - DNA ligase

106
Q

When does the majority of non-disjunction occur?

A

70% at meiosis 1

107
Q

Which cardiac defect is associated with Turner’s syndrome?

A

Coarctation of the aorta

108
Q

What is a Barr body?

A

An inactivated X chromosome present if there are >2 X chromosomes in a cell

109
Q

What clotting factor is deficient in Christmas disease?

A

XI

110
Q

What clotting factor is deficient in Haemophilia A?

A

VIII

111
Q

Wha clotting factor is deficient in Haemophilia B

A

IX

112
Q

Where is the CF mutation mutation located?

A

Mutated CFTR gene on chromosome 7

Most common mutation = F508

113
Q

What causes aplastic crisis in sickle cell disease?

A

Parvovirus B19

114
Q

What are the features of hyposplenism seen on blood film in sickle cell disease?

A

Codocytes

Howell-Jolly bodies

115
Q

Where is the highest incidence of thalassaemia in the world?

A

Maldives

116
Q

What is Bart’s hydrops?

A

A type of alpha thalassaemia whereby 4 genes are affected (severity is determined by the number of genes affected) - it causes in utero death

117
Q

What is the incidence of beta thalassaemia in the UK?

A

1/10,000

118
Q

Which hormones are elevated in Turner’s syndrome?

A

LH and FSH are elevated in Turner’s syndrome

MRCOG - Part 1 (13 decks)

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