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Clinical Cytogenetics and Molecular Genetics > Chromosome Numerical Abnormalities > Flashcards

Chromosome Numerical Abnormalities Flashcards

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1
Q

Term for number of homologous chromosome sets present in a cell or organism

A

Ploidy (n)

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2
Q

Define human haploidy, diploidy and polyploidy

A

Haploid = n = 23
Diploid = 2n = 46
Triploidy = 3n = 69
etc.

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3
Q

Define aneuploidy and euploidy

A

Euploidy - exact multiple of haploid set of chromosomes
Aneuploidy - incomplete haploid set - abnormal chromosome numbers (45, 47 etc)

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4
Q

Most common parent and phase of meiosis for aneuploidy?

A

Maternal meiosis I

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5
Q

Incidence of ALL chromosome abnormalities in newborns?

A

1 in 150

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6
Q

Incidence of ALL aneuploidy in newborns?

A

1 in 250

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7
Q

Incidence of X and Y aneuploidy in newborns?

A

1 in 475

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8
Q

Incidence of autosome aneuploidy in newborns?

A

1 in 700

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9
Q

Incidence of balanced structural abnormalities in newborns?

A

1 in 500

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10
Q

Incidence of ALL structural abnormalities in newborns?

A

1 in 375

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11
Q

Incidence of unbalanced structural abnormalities in newborns?

A

1 in 1600

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12
Q

Incidence of trisomy 21 in newborns?

A

1 in 700 (to 1,000) livebirths

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13
Q

Incidence of trisomy 18 in newborns?

A

1 in 6000 (to 10,000)

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14
Q

Incidence of trisomy 13 in newborns?

A

1 in 10,000 (to 16,000)

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15
Q

Most common trisomy in miscarriage?

A

Trisomy 16 - 80%

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16
Q

___% of sperm and ___% of oocytes show chromosome abnormalities

A

10% of sperm
20-25% of oocytes - aneuploidy > polyploidy > structural abnormalities

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17
Q

Four tolerated aneuploidies in conceptus?

A

45,X and trisomies 16, 21 and 22

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18
Q

Three tolerated autosomal trisomies?

A

13, 18, 21 (also most common cause of stillbirths)

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19
Q

Cause of 95-99% of SABs?

A

Turner syndrome 45, X

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20
Q

Crossing over occurs in what phase of Meiosis?

A

Prophase I
4c, 2n

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21
Q

At the end of male meiosis I what is the ploidy?

A

2c, 1n
haploid with 2 copies

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22
Q

At the end of male meiosis II what is the ploidy?

A

1c, 1n
haploid with 1 copy

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23
Q

5 phases of prophase I?

A

leptotene - beginning of chromosome condensation , telomeres touch nuclear membrane
zygotene - homologs pair and synapsis
pachytene - crossing over
diplotene - homologs separate, tethered by chiasma
diakenesis - separation of homologs

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24
Q

Eggs are suspended in which meiosis phase until fertilization?

A

Prophase I

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25
Q

Extra phase of prophase I in eggs?

A

dictyotene - stage after pachytene (crossing over)
= male diplotene

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26
Q

Completion of female meiosis occurs when?

A

After fertilization

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27
Q

How many sister chromatids are involved in each crossover event?

A

One

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28
Q

1 centimorgan (cM) = ___% chance for recombination

A

1%

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29
Q

Number of chiasma/chromosome arm required for normal segregation?

A

1
*typically 46-53 chiasmata total and correlates with length of chromosome arm

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30
Q

Recombination occurs more in males or females?

A

Females

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31
Q

Recombination occurs more in centromeres or telomeres?

A

Telomeres

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32
Q

Chance of same exact random segregation of chromosomes?

A

1 in 2^23 (1 in 8 million)
*recombination makes it even smaller

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33
Q

Define nondisjunction

A

Failure of homologous chromosomes (MI) or sister chromatids (MII, mitosis) to separate;

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34
Q

Parental origin of trisomy is most often paternal or maternal and what stage of meiosis?

A

maternal meiosis I

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35
Q

Define heterodisomy

A

Two copies of the chromosome from the same parent (homologs)

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36
Q

Define the two results of meiosis I nondisjunction

A

Heterodisomy (same parent homologs)
Nullisomy

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37
Q

Define the result of meiosis II nondisjunction

A

Isodisomy - two sister chromatids from the same parent

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38
Q

Timing on nondisjunctions in oocytes

A

14 days before ovulation (meiosis I - first polar body formed)
Hours after fertilization (meiosis II

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39
Q

Mechanism of nondisjunction?

A

Aberrant recombination
- achiasmate (absent) or reduced recombination
- suboptimal locations of crossovers (pericentromeric)

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40
Q

Premature separation of sister chromatids leads to _____ and _____ gametes

A

disomic and nullisomic

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41
Q

Parental gamete and stage of meiosis for nondisjunction?

A
  • Oocyte
  • Meiosis I
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42
Q

Nondisjunction in postzygotic mitosis leads to ________

A

Mosaicism

43
Q

Most common trisomy in early pregnancy loss (<10 weeks)?

A

Trisomy 16

43
Q

_____ is a ring-shaped protein complex that organizes the genome, enabling its condensation, expression, repair and transmission. The loss of it results in loss of chiasmata and premature separation.

A

cohesin

43
Q

Advanced maternal age

A

35

44
Q

Aneuploidy present in _____% of sperm and ______% of eggs

A

Sperm = 10%
Oocytes = 20-25%

45
Q

_________ and __________ cause more severe phenotypic consequences than trisomy and duplications

A

Monosomy, deletions

46
Q

Sex chromosome abnormalities of live birth?

A

45, X and 47 XXX and 47 XYY and 47 XXY

47
Q

____% of 45, X conceptuses are spontaneously aborted

A

99%

48
Q

Imbalance of _______ bands (gene‐rich) more severe than _______ bands (gene‐poor)

A

G-negative (rich)
G-positive (poor)

49
Q

The tolerated trisomies are gene ______ autosomes

A

Gene poor

50
Q

Gautier discovered the first medical condition shown to result from a chromosome abnormality (1959)

A

Trisomy 21 (DS)

51
Q

Prevalence of Trisomy 21 in live births?

A

1 in 800

52
Q

_____% of Trisomy 21 conceptuses survive until birth

A

20%

53
Q

The most common genetic aberration for intellectual disabilities

A

Trisomy 21

54
Q

Trisomy 21 features (10)

A

Hypotonia
Almond‐shaped eyes, epicanthal folds, brushfield spots
Flat nasal bridge
Excess nuchal skin
Single palmar flexon crease; clinodactyly
Cardiovascular defects ‐ 40%
ID
Transient myeloproliferative disorders (MDS) in newborns (associated with
GATA1), and 500-fold higher risk of
leukemia (AML and ALL ~1:1)
Premature dementia (Alzheimer’s disease)
onset 50 yo, due to APP in 21q21.3

55
Q

Down syndrome critical region

A

“DS critical region” (DSCR): 21q22.1q22.3

56
Q

Down syndrome Mechanism (hint 5)

A
  1. 85-90% Maternal meiosis (75% Meiosis I, 25% Meiosis II)
  2. 3-5% Paternal meiosis (II = 75%, I = 25%)
  3. 4% Robertsonian
  4. 3-5% Mitosis
  5. Other
57
Q

Mechanism of 85-90% of trisomy 21

A

Maternal meiosis (I (75%) > II (25%)

58
Q

Recurrence risk for down syndrome with maternal age <30

A

1%

59
Q

Three possible explanations for recurrent trisomy 21

A
  1. Random
  2. Parental gonadal mosaicism
  3. Predisposition to nondisjunction
60
Q

Nomenclature for robertsonian translocation with chromosome 14 (q10;q10)

A

46,XX,der(14;21)(q10;q10),+21

61
Q

Empiric risk for robertsonian translocation (female vs male carriers)?

A

1/3 empiric risk
10-15%
vs
0-2%

62
Q

___% of trisomy 21 embryos become mosaic with a nondisjunction event and trisomy rescue during mitosis. Varied phenotype

A

2%

63
Q

Risk = ____% for i(21)(q10) except for gonadal mosaicism

A

100%

64
Q

Trisomy 18 incidence in live births?

A

1 in 6,000 (to 8,000)

65
Q

Trisomy 18 is associated with paternal/maternal meiosis ____

A

maternal meiosis II

66
Q

Trisomy 18 phenotype

A

IUGR
Microcephaly
Omphalocele
Cardiac defects
Severe ID
Clenched hand position w/ 2&5 over 3&4
Clubfoot or rocker-bottom feet
Small, low set ears

67
Q

Trisomy 13 incidence in live births?

A

1 in 10,000 (to 16,000)

68
Q

Trisomy 13 phenotype?

A

Midline defects:
Holoprosencephaly (failure of brain development) / microcephaly
Cleft lip/palate
Cutaneous scalp defects (cutis aplasia)
Congenital heart defects
Omphalocele
Polydactyly
Renal abnormalities
Severe ID
IUGR

69
Q

Trisomy 8 mosaic syndrome incidence?

A

1in 25,000

69
Q

Trisomy 8 mosaic syndrome name and karyotype?

A

Warkany syndrome
mos 47,XX,+8/46,XX.

70
Q

Trisomy 8 mosaic syndrome phenotype?

A

thick lips
camptodactyly
clinodactyly
deep plantar and palmar skin furrows
ID
vertebral or hip anomalies
joint
kidney
cardiac,
skeletal abnormalities
skin hyper/ hypopigmentation
Males are more frequently affected than females

71
Q

_______ ______________ two aneuploidy events in the same individual (i.e. Down Syndrome with XXY (Klinefelter))

A

Double aneuploidy

72
Q

________ results in 12% of spontaneous abortions and are the result of an additional haploid genome set

A

Polyploidy

73
Q

The two exceptions to the mechanism of aneuploidy being nondisjunction in maternal meiosis I?

A
  1. Trisomy 16 = meiosis II
  2. 45, X = paternal nondisjunction
74
Q

Risk for aneuploidy?
1. Maternal age 35
2. Maternal age 42

A
  1. 1 in 350
  2. 1 in 42
75
Q

_____ aneuploidy is when only part of the body is involved

A

mosaic

76
Q

_____ aneuploidy is when only part of the chromosome is involved

A

partial
-deletions, duplications and unbalanced translocations
-Pallister Killian Syndrome

77
Q

Pallister Killian syndrome chromosome

A

iso 12p

78
Q

Isochromosomes form by fusion of homologous ______ arms into one chromosome and the fusion of homologous ____ arms

A

p, q

79
Q

Isodicentric chromosomes form from the ______ ___________ of one complete arm and partial other arm

A

Inverted duplication

80
Q

Cateye syndrome chromosome

A

Idic (22)

Coloboma of the iris
Anal atresia with fistula
Heart and renal malformation

81
Q

_______ chromosomes are the result from breaks at both ends of a chromosome or fragment and are sometimes the result of complex inversions, duplications, or deletions with crossing over

A

Ring

82
Q

Pallister Killian is a tetrasomy and is always _______

A

mosaic

83
Q

Pallister Killian syndrome occurs ____ ________ and has a _______ (high/low) recurrence risk

A

de novo
low

83
Q

Pallister Killian syndrome results from Iso 12p and results in the ___________ of 12p

A

Tetrasomy

84
Q

Tetrasomy 12p phenotype

A

Course face
High forehead
Sparse scale hair
Hypertelorism
Epicanthal folds
Severe ID
Hypotonia
Hypopigmented streaks/spots
Accessory nipples
Cardiac anomalies
Diaphragmatic hernia

85
Q

A small (size < chromosome 20) supernumerary chromosome, whose identity is not determined using classical methodology, is called a ___________ chromosome

A

marker (mar)

86
Q

Marker chromosomes usually arrive from __________ chromosomes (____, ______, ______, ______ and _____)

A

acrocentric (13, 14, 15, 21 and 22)

87
Q

40 - 70% of marker chromosomes are comprised of chromosome _____??

A

15 (break points 1 and 2 without PWS or AS)

88
Q

Small supernumerary marker chromosome incidence in livebirths?

A

1 in 500

89
Q

Large inversion duplication of 15 (break points 1-3+) can cause two syndromes, what are they

A

Prader Willi Syndrome
Angelman syndrome

90
Q

Large inversion duplication of 15 (break points 1-3+) are usually _______ (paternal/maternal) in origin?

A

Maternal

91
Q

Isodicentric (15)/Inv dup 15 occurs in 1 in __________ live births and occurs in a ______ ratio of males to females?

A

30,000
1:1

92
Q

Probe for chromosome 15 in the PWS/AS region and further down on the q arm

A

SNRPN
PML

93
Q

Isodicentric chromosomes have ______ centromeres

A

Two

94
Q

Idic(15) phenotype

A

Autistic/ID
Hypotonia/lax ligaments
Epilepsy
Facial dysmorphisms (long philtrum, full cheeks, bulbous nose)

**brain abnormalities, GU defects, growth retardation
** congenital heart defects, microcephaly

95
Q

3 chromosome 22 syndromes?

A
  1. DiGeorge
  2. Cateye syndrome (idic 22)
  3. Der (22)
96
Q

Nomenclature for supernumerary and non-supernumerary ring chromosomes that usually arise de novo

A

47, r
46, r

97
Q

Pathogenicity prediction = ____% for ring chromosomes

A

60%

98
Q

Pathogenicity prediction = ____% for Non-satellited/non-acrocentric marker chromosomes

A

15%-29%

99
Q

Pathogenicity prediction = ____% for Satellited/Acrocentric marker chromosomes

A

0-11%

Clinical Cytogenetics and Molecular Genetics (20 decks)

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