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MRCOG Part one > Genetics: Single gene and chromosome abnormality > Flashcards

Genetics: Single gene and chromosome abnormality Flashcards

1
Q

What is the meaning of haploid cells?

A

Haploid cells only contain 23 chromosomes, the germ cells.

All somatic cells have 46 chromosomes, the diploid number

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2
Q

Describe what happens in meiosis 1

A
  1. DNA duplication of chromosomes
  2. Homologous chromosome recombinant with non identical chromatids (mixes maternal & paternal DNA)
  3. Duplicated chromosomes then split to form 2 new daughter cells (with 2 x chromatids in each)
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3
Q

Describe what happens in meiosis 2

A

Single chromosome split into sister chromatatids & pulled to side of cell before cytokinesis.

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4
Q

How many daughter cells are made from one dividing germ cell during meiosis

A

4 - each containing 23 chromosomes

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5
Q

What is the difference between the development of the germ cell between ovum and sperm after meiosis

A

Female: 3 daughter cells form polar bodies and will not survive, 1 becomes ovum
Men: All develop into spermatozoa

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6
Q

What is the meaning of aneuploidy?

A

Abnormality of number of chromosomes

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7
Q

What are the main mechanisms resulting in aneuploidy

A

Non-disjunction

Translocation

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8
Q

How does nondisjunction occur?

A

Pairs of homologous chromosomes at meiosis 1 or sister chromatids during mitosis do not separate so both chromosomes are passed onto on daughter cell (trisomy), whilst one is left missing one chromosome (monosomy)

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9
Q

Name some trisomy conditions

A 
Downs syndrome 
Edwards Syndrome
Patau syndrome 
Klienfelter (XXY)
Triple X syndrome (XXX)
XXY syndrome (XXY)
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10
Q

What % of does syndrome is caused by nondisjunction?

Other cause?

A

95%

Robertsonian Translocation

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11
Q

What medical conditions are often present in those with Downs Syndrome?

A 
Learning difficulty 100%
Cardiac problems atrioventricular defects (40-50%)
Hypothyroidism 20-40%
Dementia 10-15%
Acute Lymphoblastic anaemia
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12
Q

What chromosome is trisomy in edwards syndrome?

A

18

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13
Q

What are the clinical features of Edwards Syndrome?

A 
Babies likely to die within 1 year
Profound learning difficulty 
Congential heart disease (90%, VSD)
Facial clefts
Spina bifida 
Clenched hands
Rockerbottom feet
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14
Q

What chromosome does Patau syndrome effect?

A

13

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15
Q

What are the clinical features of patau syndrome?

A 
High rate miscarriage 
Profound learning difficulty
Congential heart disease 
Postaxial polydactyly 
Cleft palate
Microphthalmia
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16
Q

Clinical features of klinefelter syndrome

A

Slightly descreased IQ
Tall stature
Infertility
Transient gynaecomastia

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17
Q

Whats the risk of nondisjunction at 20 vs 40 yrs

A

20: 1 in1500
40: 1:100

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18
Q

Name a monosomy resulting from nondisjunction

A

Turners syndrome 45X

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19
Q

What are the clinical features of Turner syndrome?

A

Mildly Lower IQ
Many do not survive due to fetal hydrous
High risk of miscarriage
Consider NT >4mm

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20
Q

What is the difference between balanced and unbalanced reciprocal translocation?

A

Chromosome rearrangement between 2 nonhomologus chromosomes.

Balance: No phenotypic consequence
Non balances: 1 chromosome with missing genetic material (monosomy) and 1 with duplicate genetic material (trisomy)

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21
Q

If a parent with balanced translocation has children with partner with normal chromsomes what are the possible outcomes

A
  • 25% normal chromsome
  • 25% balanced translocation
  • 50% unbalanced translocation → risk sport miscarriage or go to term with child with high risk learning difficulty etc
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22
Q

Explain robertsionian translocations

A

Chromsome rearrangement that involves fusion of the long arms of 2 afrocentric chromosome and their short arms are lost.

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23
Q

Which chromosomes can have robertsonian translocations?

A

13, 14, 15, 21 & 22

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24
Q

13q, 14q and 14q,21q are the most common robertsoninan translocations, which conditions are these?

A

13q14q patau

14q 21q downs

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25
Q

What is imprinting and which chromosomes are vulnerable?

A

One parent allele is preferentially silenced

14&15

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26
Q

What conditions is paternal uniparental disomy (UPU15)

Some features?

A

Angelman syndrome characteristic face and ataxic fain severe learning difficulty

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27
Q

What conditions is maternal uniparental disomy (UPU15)

Some features?

A

Prader-willi poor feeders in neonatal period, insatiable appetite from childhood, hypotonic and short, learning difficulty

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28
Q

What technique is used to identify micro deletions or micro duplications?

A

aCGH

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29
Q

In micro deletions what mechanisms abrogate gene function:

A 
missense mutation
truncating mutation (frameshift, nonsense)
Splice site 
partial gene deletion
whole gene deletion 
triplet repeat expansion
30
Q

What common conditions are from triplet repeat expansion?

A

Huntingdons, myotonic dystrophy, fragile x

31
Q

Which micro deletion is williams syndrome

32
Q

what are the features of williams syndrome

A

mild/severe learning
Cardiac aortic/pulmonary stenosis
renal artery stenosis
cocktail party personality

33
Q

which micro deletion if diGeorge

34
Q

what are the features of diGeorge

A 
Leanrind difficulty 
cardiac fallot, vid
short 
cleft palate 
low calcium
35
Q

4 ways genes can be inherited?

A 
AD
AR
X linked r
X linked d 
mitochondrial
36
Q

Main features of tuberous sclerosis

A

AD
multisystem, cutaneous (shagreen patches)
neurological (brain hamartoma - seizure)
prenatal: rhabdosacroma

37
Q

Main features of neurofibromatosis type 1

A

AD
Cafe-au-let spots
neurofibromatoma
inguinal freckling and optic glioma

38
Q

Main features of HNPCC

A

AD

cancer: colon, endometrium, gastric and ovarian

39
Q

Main polycystic kidney disease

A

AD
Cysts in the kidney, liver, pancreas & spleen
CV abnormality intracranial anrueysm mitral valve prolapse

40
Q

Main features of haemochromatosis

A

AR

excess iron absorption in liver, pancreas and skin (cirrhosis, DM, bronze skin)

41
Q

Main features of alpha 1 antitrypsin

A

AR

no elastase, emphysema at young age and cirrhosis

42
Q

congenital adnreal hyperplasia (21 hydroxyls deficiency)

A

AR
Unable to synthesis cortisol
female virilisation
precocious puberty +/- salt wasting

43
Q

Main features of duchenne muscular dystrophy

A

X linked R

Progressive myopathy. death in 3rd decade resp failure

44
Q

Main features of Haemophilia A

A

X linked R

Deficiency factor VIII, abnormal clotting

45
Q

Main features of Fragile X syndrome

A

X linked R
Expansion of CGG
Learning difficulty and appearance

46
Q

What is the likely outcome for a male fetus that inherits an X linked dominant condition

A

spent loss of pregnancy

early neonatal death

47
Q

Main features of Rett Syndrome

A

Cognitive regression and learning difficulty

hand wringing, sterotypical movement

48
Q

At what weeks can’t the nuchal translucency be tested?

A

11-14 weeks

49
Q

What other findings other than nuchal translucency in 1st trimester are associated with trisomy 21

A

Absent or hypoplastic nasall bone

Absant or reversed A wave in blood flow of ductus arterisosis

50
Q

What is included in the triple test?

How accurate is it for detecting trisomy 21

A

NT, bHCG and PAPPA

90%

51
Q

Positive finding of triple test?

A

High NT
high bHCG
low PAPPA

52
Q

What tests are perfromed in the triple screen

A

aFP
bHCG
Estriol
70%

53
Q

Findings of triple test in Downs

A

afp & estriol low

bHCG high

54
Q

finding triple test in edwards

55
Q

high alpha fetoprotein is associated with what?

A

Neural tube defects

56
Q

What tests are in the quadruple test? Accuracy

A 
aFP
bHCG
Estriol 
Inhabin 
81%
57
Q

when is triple test/quadruple test performed

A

16-18 weeks

58
Q

What is cell-free fetal DNA testing? What is it tested for?

A

Non invasive, small amount fetal DNA used to determine gender, rhesus status + trisomy 13,18,21

59
Q

When can CVS happen?

What is aspirated?

A

11 weeks-14weeks

Placental tissue

60
Q

What is the difference in sampling cytotrophoblast vs mesenchymal cells during CVS?

A

Cytotrophoblasts are rapidly dividing, results in 2-3 days

mesenchymal cells 1-3 weeks but more accurate

61
Q

Risk of false positive in CVS & why

A

1.5% Placental mosaicism, present in placenta but not fetal tissue

62
Q

When is amniocentesis performed?
When cells are tested?
How long for result & why?

A

15 weeks
Amniocytes (in amniotic fluid), fetal skin, urinary tract & GI tract
1-3 weeks - no rapidly dividing cells.

63
Q

Miscarriage rate of amniocentesis and CVS

64
Q

When can cordcentbesis be performed?

What can is test for?

A

From 18 weeks
anaemia, prenatal infection
1-2% misc rate

65
Q

Which molecular genetic techniques measure gene dosage at particular locations in the genome?

A

FISH (Fluorescence in situ hybridisation)
QF PCR (Quantative fluorescence polymerase chain reaction)
MLPA (Multiplex ligation probe aplification)

66
Q

Which molecular genetic techniques measure gene dosage over the entire genome?

What is the benefit of this over the other methods mentioned?

A

aCGH (microarray comparative genetic hybriddisation)

Can also detect micro deletions or micro duplications as we’ll as aneuploidy

67
Q

What is the lifetime risk of developing ovarian cancer in the general population?
What % is due to BRCA1&2 genes?

A

1 in 70

6-8%

68
Q

Lifetime risk BRCA1 in developing breast & ovarian cancer?

A

Breast 60-90%

Ovarian 40-60%

69
Q

Lifetime risk BRCA2 in developing breast & ovarian cancer?

A

Breast 45-85%

Ovarian 10-30%

70
Q

What % of overall breast cancers are due to BRCA1&2

71
Q

In HNPCC lifetime risk of endometrial cancer risk?

72
Q

In Peutz-Jegher syndrome, what is the lifetime risk of endometrial cancer?

A

40%
30% bowel cancer
- pigmented macules in mucus membranes and skin + GI

MRCOG Part one (28 decks)

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