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EMS - Genetics in Medicine > Chromosomal Basis of Inheritance (6) > Flashcards

Chromosomal Basis of Inheritance (6) Flashcards

1
Q

Cytogenetics

A

Study of chromosomes, anything more than a single gene

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2
Q

Autosomes

A

Chromsomes 1-22

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3
Q

Why cytogenetics?

A

0.7% livebirths

5% stillbirths

50% miscarriages

Up to 100% cancers

Up to 40% of all conceptions!

> 140 known syndromes

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4
Q

Conventional cytogenetic analysis

A

G-banding, metaphase chromosome analysis

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5
Q

Molecular cytogenetic analysis

A

Molecular resolution at all stages of cell cycle (DNA/in situ), FISH, microarray CGH, next generation sequencing, MLPA, QF-PCR, qPCR

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6
Q

Mitosis

A 
Interphase
Prophase
Metaphase
Anaphalse
Telophase
Cytokinesis
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7
Q

How long does it take to do one cell division?

A

24 hours

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8
Q

G-banding

A

Each chromosome band has 50 genes, alternating light and dark bands

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9
Q

Types of cytogenetic abnormality

A

Numerical/structural

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10
Q

How do you produce abnormal phenotype?

A
  1. Dosage effect
  2. Disruption of gene
  3. Effect due to parental origin
  4. Position effect
  5. Unmasking of recessive disorder
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11
Q

Dosage effect

A

A gain/loss, most important and frequent > lead to change in function of genes, cope with a gain of genes more than a loss

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12
Q

Disruption of gene

A

Breakpoint/inappropriate activation/inactivation

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13
Q

Effect due to parental origin

A

Genomic imprinting (maternal age increases aneuploidy, paternal doesn’t)

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14
Q

Position effect

A

A gene in a new chromosomal environment functions inappropriately

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15
Q

Phenotypic severity

A
  • Many lethal in utero (spontaneous abortion)
  • Survival imbalances (organ malformation, reduced intelligence, facial dysmorphism)
  • Sex chromosome imbalance
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16
Q

Is sex chromosome or autosomal imbalance more severe?

A

Sex chromosome imbalance

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17
Q

Diploidy

A

2 copies of each chromosome

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18
Q

Aneuploidy

A

Gain (trisomy) or loss (monosomy)

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19
Q

Polyploidy

A

Gain whole set (triploidy/tetraploidy)

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20
Q

Mosaicism

A

Diploidy and aneuploidy

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21
Q

Causes of chromosomal imbalance

A

Gametogenesis/meiosis, Fertilisation and Early cleavage

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22
Q

Errors at gametogenesis

A

Increase in maternal age > increase in aneuploidy, increase in paternal age > no significant risk

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23
Q

Meiotic errors (non-disjunction)

A

Failure of chromosome/chromatid separation > disomic/nullisomic/normal gametes

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24
Q

Which chromosomes does autosomal aneuploidy usually effect?

A

Trisomy 13, 18, 21 - small chromosomes so they have few genes on them, can just about tolerate gains

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25
Q

Trisomy 21

A

Down’s syndrome

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26
Q

How common is trisomy 21?

A

1/700

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27
Q

How many spontaneously abort with trisomy 21?

A

75%

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28
Q

What are symptoms of trisomy 21?

A
  • Head (eyes upward slanting, brush field spots, small nose, ears abnormally shaped/low set, tongue protruding/small mouth, flat face, brachycephalic, short neck)
  • Neurological (learning disabilities, mild > moderate 30-60 IQ)
  • Hands and feet (short and broad, single palmar crease, 5th finger clinodactly, wide sandal gap)
  • Fertility - males infertile
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29
Q

What is average life expectancy of somebody with trisomy 21?

A

55-68 years

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30
Q

What medical problems are associated with trisomy 21?

A

Increased risk of cancer (leukaemia), Alzheimer’s, hypothyroid, obesity, coeliac, arthritis, diabetes, hearing loss, seizures

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31
Q

Trisomy 18

A

Edwards syndrome

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32
Q

How common is trisomy 18

A

1/6000 live births

33
Q

How many spontaneously abort with trisomy 18?

A

95%, and only 10% survive > 1 year

34
Q

What are symptoms of trisomy 18?

A
  • Head (microcephaly, low set ears, micrognathia - small jaw, cleft lip and palate)
  • Hands and feet (clenched and overlapping fingers, rocker bottom feet)
  • Low birth weight, short sternum, severe mental retardation
  • Organ malformations (umbilical/inguinal hernia, congenital HD 90%, congenital kidney abnormalities, eye abnormalities - cataracts, micropthalmia)
35
Q

Female meiosis begins when?

A

At 5 months, eggs are suspended until puberty

36
Q

Male meiosis begins when?

A

At puberty

37
Q

Autosomal aneuploidy and maternal age

A

Unfavourable chiasmata distribution (foetus) and age - dependent deterioration of meiotic structures (10-40years later - alcohol and hormonal changes)

38
Q

Sex chromosome aneuploidy

A

No age-related risk, phenotype less severe than autosomal, sexual orientation not affected

39
Q

45 X

A

Turner’s syndrome

40
Q

How common is Turner’s syndrome?

A

1/2500

41
Q

Symptoms of Turner’s syndrome

A
  • Reproductive (loss of ovarian function, no puberty, infertility)
  • Lymphatic (obstruction) (webbed neck, swelling of hand/feet)
  • Short stature
  • Coarctation of aorta
  • IQ normal/reduced compared to siblings
42
Q

47 XXY

A

Klinefelter syndrome

43
Q

How common is Klinefelter’s syndrome?

A

1/1000

44
Q

What percentage of people with Klinefelter’s syndrome are undiagnosed?

A

64%, identified through infertility and/or hypogonadism

45
Q

What percentage of Klinefelter’s syndrome are 47 XXY?

A

80%, 20% mosaic/variant

46
Q

Symptoms of Klinefelter’s syndrome

A
  • Infertility (lack of secondary sexual characteristics, testicular dysgenesis)
  • Growth (normal infants, then accelerates, long arms and legs)
  • IQ (norma, may decrease with increase of Xs)
  • Female body characteristics - breasts, body shape
47
Q

By how much does Klinefelter’s syndrome increase your risk of breast cancer?

A

20 times

48
Q

What percentage of people with Klinefelter’s syndrome develop gynaecomastia?

A

30-50%

49
Q

Errors at fertilisation

A

Polyploidy and molar pregnancy

50
Q

What is molar pregnancy?

A

Double paternal gene and no maternal genetic content, empty egg with massive cystic placenta

51
Q

How common is triploidy?

A

2% of all pregnancies, 1/57,000 live births

52
Q

What percentage of triploidy pregnancies spontaneous abort?

A

99.9%

53
Q

Triploidy

A

69 XXY, 69 XYY, 69 XXX

54
Q

Digyny

A

2N from egg, 1N from sperm

55
Q

Diplospermy

A

2N from sperm, 1N from egg

56
Q

Dispermy

A

1N from 2 sperm, 1N from egg

57
Q

What happens to foetus if you have double maternal genes?

A

Small placenta, macrocephaly as placenta concentrates on the head, significant growth delay

58
Q

What happens to foetus if you have double paternal genes?

A

Huge over developed cystic placenta, slight growth retardation, risk of developing polycarcinoma in future

59
Q

How common is molar pregnancy?

A

1/1000

60
Q

How is molar pregnancy detected?

A

Ultrasound or by heavy bleeding early on in pregnancy

61
Q

Mosaicism

A

Mitotic non-disjunction/failure to separate

62
Q

Trisomic zygote rescue

A

In early division - kick out extra 21

63
Q

Consequences of mosaicism

A
  • Variable phenotype
  • Variable lethality vs extra embryonic
  • Non-identical twin (occur post-zygotically in one twin)
  • Tissue-specificity (lateral asymmetry - effects certain parts of body)
  • Uniparental disomy (UPD)
  • Recurrence risk (if gonadal)
  • All females are mosaic - X-inactivated in some cells
64
Q

Chromosomal rearrangement - balanced

A

Translocations and inversions (doesn’t really effect phenotype)

65
Q

Chromosomal rearrangement - unbalanced

A

Deletions and duplications (effects phenotype)

66
Q

Reciprocal translocation

A

Break and exchange, 1/500, 5-10% phenotype risk, reproductive risk, change shape/size of chromosome, may break important genes

67
Q

Robertsonian translocation

A

Only acrocentrics (13, 14, 15, 21, 22, long q arm, short p arm), 2 q arms join together after break, 1/1000, no phenotype risk, reproductive risk, decrease number of chromosomes

68
Q

Inversions

A

2 breaks, 180 degree rotation and then rejoin, 1/1000, 5-10% phenotype risk, reproductive risk

69
Q

Pericentric inversion

A

1 break either side of centromere

70
Q

Paracentric inversion

A

2 breaks in 1 arm, 1 side of centromere

71
Q

Deletions and duplications

A

Loose large chunks of DNA, usually sporadic, more common in males, 1/2000, copy number variation (CNV)

72
Q

Interstitial deletion

A

Segment within chromosome is lost

73
Q

Terminal deletion

A

End of chromosome is lost

74
Q

Duplications

A

Gain of segment, material copied, direct/inverted

75
Q

Ring chromosome

A

Breaks at each end, then circulation

76
Q

Trisomy 13

A

Patau syndrome

77
Q

What are the symptoms of trisomy 13?

A
  • Small
  • Head (severe mental retardation, microcephaly/sloping forehead, holoprosencephaly (absence of forebrain))
  • Eyes (micropthalmia, coloboma, retinal dysplasia, palpebral fissures slanted)
  • Cleft lip/palate, ears abnormal and low, polydactyly, fingers flexed, heart defect, abnormal genitalia
78
Q

How common is Trisomy 13?

A

1/12,000

79
Q

Survival trisomy 13

A

95% spontaneously abort

EMS - Genetics in Medicine (14 decks)

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