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P1EXAM > Original Genetics > Flashcards

Original 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 - homologous chromosome pairs cross over then separate (still diploid)
Meiosis II - sister chromatids separate (now haploid)

Produces sperm/ova

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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
Cardiac - VSD/ASD
Left lip/palate
Holoprosenecephaly
Microphthalmia/anophthalmia
Polydactyly
Scalp defects /Omphalocele/Renal anomalies

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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 - ?familial/de-novo
- If it is familial and the parent carrying the balanced translocation is healthy, the baby is unlikely to have problems.
- If it is de novo, it may be associated with problems and 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 -1 base sub
2) Frameshift mutation -1+ base insert/deleted
3) Nonsense mutation -1 base sub-> short protein
4) Splice site mutation - intron/extron

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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 bases (not a multiple of 3) are inserted/deleted –> different stop codon placement –> truncated proteins

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26
Q

What is a nonsense mutation?

A

A single base substitution –> 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.
Introns removed from primary transcript and exons joined at splice site

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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
uE3
inhibin A

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

NT scanning
b-hCG
PAPP-A

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

T
C
U

pyrimidine ‘(nice) To Cee 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

Haemophilia A - eight/’Aight’

111
Q

Wha clotting factor is deficient in Haemophilia B

A

IX

Goes in order:
Haemophilia A - eight
Haemophilia B - nine

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

119
Q

What does non-disjunction of chromosomes during meiosis cause?

A

Aneuploidies (trisomy/monosomy)

P1EXAM (27 decks)

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