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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
Trisomy 21
Down's syndrome
26
How common is trisomy 21?
1/700
27
How many spontaneously abort with trisomy 21?
75%
28
What are symptoms of trisomy 21?
- 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
29
What is average life expectancy of somebody with trisomy 21?
55-68 years
30
What medical problems are associated with trisomy 21?
Increased risk of cancer (leukaemia), Alzheimer's, hypothyroid, obesity, coeliac, arthritis, diabetes, hearing loss, seizures
31
Trisomy 18
Edwards syndrome
32
How common is trisomy 18
1/6000 live births
33
How many spontaneously abort with trisomy 18?
95%, and only 10% survive > 1 year
34
What are symptoms of trisomy 18?
- 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
Female meiosis begins when?
At 5 months, eggs are suspended until puberty
36
Male meiosis begins when?
At puberty
37
Autosomal aneuploidy and maternal age
Unfavourable chiasmata distribution (foetus) and age - dependent deterioration of meiotic structures (10-40years later - alcohol and hormonal changes)
38
Sex chromosome aneuploidy
No age-related risk, phenotype less severe than autosomal, sexual orientation not affected
39
45 X
Turner's syndrome
40
How common is Turner's syndrome?
1/2500
41
Symptoms of Turner's syndrome
- 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
47 XXY
Klinefelter syndrome
43
How common is Klinefelter's syndrome?
1/1000
44
What percentage of people with Klinefelter's syndrome are undiagnosed?
64%, identified through infertility and/or hypogonadism
45
What percentage of Klinefelter's syndrome are 47 XXY?
80%, 20% mosaic/variant
46
Symptoms of Klinefelter's syndrome
- 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
By how much does Klinefelter's syndrome increase your risk of breast cancer?
20 times
48
What percentage of people with Klinefelter's syndrome develop gynaecomastia?
30-50%
49
Errors at fertilisation
Polyploidy and molar pregnancy
50
What is molar pregnancy?
Double paternal gene and no maternal genetic content, empty egg with massive cystic placenta
51
How common is triploidy?
2% of all pregnancies, 1/57,000 live births
52
What percentage of triploidy pregnancies spontaneous abort?
99.9%
53
Triploidy
69 XXY, 69 XYY, 69 XXX
54
Digyny
2N from egg, 1N from sperm
55
Diplospermy
2N from sperm, 1N from egg
56
Dispermy
1N from 2 sperm, 1N from egg
57
What happens to foetus if you have double maternal genes?
Small placenta, macrocephaly as placenta concentrates on the head, significant growth delay
58
What happens to foetus if you have double paternal genes?
Huge over developed cystic placenta, slight growth retardation, risk of developing polycarcinoma in future
59
How common is molar pregnancy?
1/1000
60
How is molar pregnancy detected?
Ultrasound or by heavy bleeding early on in pregnancy
61
Mosaicism
Mitotic non-disjunction/failure to separate
62
Trisomic zygote rescue
In early division - kick out extra 21
63
Consequences of mosaicism
- 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
Chromosomal rearrangement - balanced
Translocations and inversions (doesn't really effect phenotype)
65
Chromosomal rearrangement - unbalanced
Deletions and duplications (effects phenotype)
66
Reciprocal translocation
Break and exchange, 1/500, 5-10% phenotype risk, reproductive risk, change shape/size of chromosome, may break important genes
67
Robertsonian translocation
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
Inversions
2 breaks, 180 degree rotation and then rejoin, 1/1000, 5-10% phenotype risk, reproductive risk
69
Pericentric inversion
1 break either side of centromere
70
Paracentric inversion
2 breaks in 1 arm, 1 side of centromere
71
Deletions and duplications
Loose large chunks of DNA, usually sporadic, more common in males, 1/2000, copy number variation (CNV)
72
Interstitial deletion
Segment within chromosome is lost
73
Terminal deletion
End of chromosome is lost
74
Duplications
Gain of segment, material copied, direct/inverted
75
Ring chromosome
Breaks at each end, then circulation
76
Trisomy 13
Patau syndrome
77
What are the symptoms of trisomy 13?
- 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
How common is Trisomy 13?
1/12,000
79
Survival trisomy 13
95% spontaneously abort

EMS - Genetics in Medicine (14 decks)

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