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M2M2 > Numerical Chromosome Abnormalities > Flashcards

Numerical Chromosome Abnormalities Flashcards

1
Q

2 key differences between mitosis and meiosis::

A
  1. Paternally and maternally derived homologous chromosomes pair at the onset of meiosis (prophase I) whereas the two homologs segregate independently in mitosis
  2. Recipricol recombination events between maternal and paternal sister chromatids generate chiasmata (physical linkages) between homologs. In contrast, recombination between homologs is rare during mitosis
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2
Q

Bivalents

A

Structures formed when maternal and paternal homologs of each chromosome become paired (or synapse) along their entire lengths during prophase I of meiosis

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3
Q

Maternal and Paternal pairing during prophase I to form bivalents requires formation of wht proteinaceous structure?

A

synaptonemal complex

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4
Q

The synaptonemal complex forms during which phase of meiosis and what does it promote?

A

prophase I

inter-homolog interactions

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5
Q

Reciprocal recombination events during the formation of bivalent in prophase I results in physical links between homologs, these attachments or crossovers are also known as_______

A

chiasmata

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6
Q

How many crossovers, on average, occur on each chromosome during chiasma formation?

A

2-3

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7
Q

What is the consquence of corssingover during prophase I of meiosis

A

Genetic reassortment between chromosomes

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8
Q

When does the synaptonemal complex disassemble?

A

Then end of prophase I

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9
Q

What is the important consequence of synaptonemal complex disassembly at the end of prophase I?

A

The bivalents are now only held together by chiasmata

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10
Q

In which phase of meiosis are homologs separated to opposite poles?

A

first meiotic division (anaphase I)

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11
Q

What is the most common error in meiosis?

A

Chromosome nondisjunction in meiosis I

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12
Q

3 Genetic consequences of meiosis?

A

reduction of chromosome number from diploid to haploid

random segregation give 2^23 combos

random shuffling due to crossover, increases genetic variability even more`

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13
Q

How many rounds of chromosome segregation occur in meiosis vs. mitosis

A

one in mitosis - yields daughter cell identical is chromosomal content to parental cell

two in meiosis - yield 4 cells with half the chromosomal content to parental cell

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14
Q

DNA replication proceeds each round of chromosome segregation in mitosis, what about meiosis?

A

Nope! two rounds of chromosome segregation occur without an intervening round of DNA replication

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15
Q

Is there pairing of homologous chromosomes in mitosis?

A

No

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16
Q

Is there pairing of homologous chromosomes in meiosis?

A

Yes! requires pairing of homologous chromosomes and recombination for its successful completion

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17
Q

Does recombination occur in mitosis

A

infrequently

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18
Q

When do the centromeres on paired sister chromatids segregate in mitosis

A

EACH anaphase

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19
Q

When do centromeres on paired sister chromatids divide in meiosis

A

ONLY anaphase II

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20
Q

What cells undergo mitosis?

A

somatic cells and germ line precursor cells prior to entry into meiosis

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21
Q

What cells undergo meiosis?

A

only germ line

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22
Q

Metacentric chromosomes

A

the centromere is located in the middle

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23
Q

submetacentric chromosomes

A

centromere is slightly removed from the center

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24
Q

acrocentric choromosomes

A

centromere is near one end of the chromosome

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25
Q

Treating chromosomes with Giemsa and DAPI results in what?

A

Unique banding patterns due to regions high in G+C and A+T –> banding pattern is unique to each human chromosome and allows for unequivocal identification

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26
Q

ISCN

A

International System for Human Cytogenetic Nomenclature - determines chromosome naming

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27
Q

Short arms =

Long arms =

A

p

q

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28
Q

When do we visualize chromosomes?

A

metaphase

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29
Q

1-22 human chromosomes are arranged by?

A

size

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30
Q

p and q are the short and long arm respectively, how do we subdivide

A

regions 1–> outward
then Bands 1—> outward
so p11 would be petite arm region 1 band 1 – right next to centromere

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31
Q

On the acrocentric chromosome there are little secondary constriction balls that are highly polymorphic, what are these called and what are they comprised of?

A

Satellites :)

Repetitive satellite and ribosomal DNA

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32
Q

Note that not all specimens are created equally, what does this mean?

A

peripheral blood chromosomes are different looking than bone marrow than amniotic fluid

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33
Q

Ploidy

A

Number of homologous chromosome sets present in a cell or organism -
so humans are diploid (2n)

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34
Q

Diploid

A

having two sets of chromosomes

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35
Q

haploid

A

having one set of chromosomes (gamete cells)

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36
Q

euploidy

A

“good ploidy” - full set(s) of chromosomes (46)

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37
Q

polyploidy

A

“many ploidy” having a chromosome number that is more than double the basic or haploid number - 69, etc.

triploid - 3 set (69)
tetrapolid - 4 set (92)

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38
Q

Aneuploidy

A

“not good ploidy” - incomplete set

trisomy - 47
monosomy - 45

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39
Q

When can aneuploidy occur?

A

meiosis I (maternal/paternal)

meiosis II (maternal/paternal)

post-zygotic

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40
Q

aneuploidy occuring in meiosis I

Discovery?

A

sponatneous abortions

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41
Q

Meiosis II aneuploidy discovery?

A

Stillbirths

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42
Q

Post-zygotic aneuploidy discovery?

A

Newborns

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43
Q

When does the synaptonemal complex form?

A

When we enter meiosis I

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44
Q

Nomenclature for numerical abnormalities

triploidy?

A

69,XXX
69,XXY
69,XYY

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45
Q

Nomenclature for numerical abnormalities?

trisomy

A

47,XX,+21

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46
Q

Nomenclature for numerical abnormalities?

Monosomy

A

45,X

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47
Q

Nomenclature for chromosomal abnormalities?

Mosaicism

A

45,X/46,XX

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48
Q

Aneuploidy (cells contain abnormal chromosome #) frequently results from

A

nondisjunction - the missegregation of chromosomes at metaphase in either mitosis or meiosis, such that daughter cells receive extra or fewer than the normal number of chromosomes

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49
Q

Monosomy

A

cell lacks one copy of a chromosome

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50
Q

trisomy

A

extra copy of chromosome

51
Q

autosomal monosomies? viable?

A

no - result in early embryonic lethality

52
Q

x monosomy?

A

Yes - 45,X = Turner syndrome

53
Q

Autosomal trisomies? viable?

A

More so than monosomies, most trisomies are compatible with at least some fetal development, but often result in spontaneous abortion

54
Q

What is the predominate cause of most trisomies?

A

maternal errors in the first meiotic division - increasing proximity to menopause is thought to increase risk

55
Q

What is the most common human chromosomal disorder ascertained in liveborn infants?

A

Trisomy 21 (1/800)

56
Q

More than 95% of trisomy 21 cases are due to ?

A

additional chromosome 21 of maternal origin

57
Q

of the additional chromosome 21 from maternal origin, which nondisjunction event is responsible for the vast majority?

A

maternal meiosis I

58
Q

What is essential for tethering homologous chromosomes during the first meiotic division?

A

meiotic recombination events (Crossovers)

59
Q

Nondisjunction events can be related to (2)

A
  1. positioning of the chiasmata

2. frequency of crossover events

60
Q

In what way is nondisjunction related to the positioning of chiasmata?

A

crossover events that occur too near or far from the centromere increase the odds of nondisjunction -

distal exchanges are less effective in ensuring spindle attachment

proximal exchanges can entangle paired homologs - that then undergo reduction division leading to what appears to be MII errors

61
Q

In what way is nondisjunction related to the frequency of crossover?

A

reduction of crossover (Recombination) increases the likelihood of nondisjunction

62
Q

How many cross over events normally take place / pair of homologous chromosomes?

A

2-3

63
Q

cross-over event generates what?

A

chiasma - physical link between homologues which is critical for normal disjunction

64
Q

in addition to being important for segregation, recombination is also important for

A

genetic diversity

65
Q

2 key causes of genetic variability arising from meiosis

A

independent assortment of maternal and paternal chromosomes

recombination during prophase I

66
Q

Consequence of nondisjunction in meiosis I

4 gametes what do they have?

A 
N+1 (2)
N+1 (2)
N-1 (0)
N-1 (0)
Thus, 100% abnormal...The N+! have one maternal and one paternal from normal meiosis II following nondisjunction in meiosis I
67
Q

Consequence of nondisjunction in meiosis II

A

N
N
N+1
N-1

If we have normal meiosis I then maternal and paternal will separate appropriately
Then if one of the second rounds (meiosis II) undergoes nondisjunction we will have one with double maternal (or paternal) and one with none
The other cell should have undergone normal to give rise to one each of normal maternal (or paternal)

*see slide 18

68
Q

Incidence of aneuploidy (varies depending on age)

spontaneous abortions?
stillbirth?
newborns?

A

Spontaneous abortions -50-60%
stillbirth - 4-6%
newborns 0.6%

69
Q

When does aneuploidy primarily take place?

A

first trimester?

70
Q

Tolerated aneuploidies

Conceptions

A 
many
45,x,
trisomy16
trisomy21
trisomy22
71
Q

Tolerated anueploidies

Live Births

A

trisomy 13, 18, or 21
Sex chromosome anueploidy
- gains of x and y
- loss of sex chromosome, e.g, monosomy x

72
Q

Maternal age effect - on aneuploidy?

Two hit theory

A
  1. Diminished recombination

2. Diminished ability to complete segregation if unfavorable recombination

73
Q

Maternal age effect - cohesin theory?

A

Postulates the degradation of cohesin complex over span of extended meiosis I arrest results in precocious homolog separation

74
Q

What kind of cells must cytogenetic studies use?

A

Viable - because we need them to be in the mitotic phase of the cell cycle

75
Q

What would the ISCN be for trisomy 21?

A

47,XY,+21

47,XX,+21

76
Q

Down Syndrome - Trisomy 21 Clinical features

A 
Characteristic facies
Short Stature
Hypotonia 
Hearing loss
Moderate intellectual impairment
Early alzheimers
Leukemia in perinatal period
77
Q

What would the ISCN be for a Down’s child who acquired luekemia and peripheral blood cell contained extra acquired X

A

48,XX+X,+21c

c is for constitutional, to distinguish it from the acquired leukemic extra X

78
Q

Extra acquire X is a common finding in what

A

Leukemia

79
Q

What is the most common chromosomal abrnomality in live born? How common?

A

Trisomy 21

1/800 - 1/900

80
Q

What is the most common genetic cause of moderate intellectual disability

A

Trisomy 21

81
Q

-4% of Trisomy 21 is associated with what?

A

balanced translocation in parent

82
Q

What causes the majority of trisomy 21

A

maternal meiosis I nondisjunction errors

83
Q

What percent of Trisomy 21 Conceptus survive to birth?

A

20-25%

84
Q 
Risk of Trisomy 21
Age
30
35
40
A

30 -1%
35 -2.8%
40 - 10.4%

85
Q

Cause of maternal risk for trisomy with age?

A

Due to meiosis I suspension from pre-birth until sexual maturity

86
Q

Trisomy 18 - Edwards syndrome

Clinical features

A 
Small for gestational age
Microcephaly
Clenched fingers
Rocker bottom feet
Heart/brain abnormalities
87
Q

Trisomy 18 - frequency

A

1/8000 live births

88
Q

Trisomy 18 - perinatal lethality?

A

90%

89
Q

Turner syndrome ISCN?

A

45,X

90
Q

Most common chromosome abnormality in spontaneous abortions?

A

Turner Syndrome 45,X

91
Q

What percent of Turner Syndrome fetuses survive to term?

A

1%

Thus 99% do not

92
Q

Turner syndrome incidence

A

1/2,500 female live births

93
Q

Turner syndrome clinical presentation

A 
prenatal lymphedema
Cystic hygroma (large cyst-like cavity containing lymph)
Congenital heart disease
Coarctation of the aorta
Gonadal dysgenesis 
Short stature*
Webbed neck*
94
Q

Most classic phenotype of Turner

A

Webbed neck and short stature

95
Q

Possible Turner syndrome karyotypes…

A 
Don't need to know them all 
45,X (50%)
46,X,i(Xq)
mos45X,46XX
mos45,X/46X,i(Xq)
Other

The take home point is that only 50% are seen in peripheral blood as pure 45X
25% have known mosaic in the peripheral blood

96
Q

What is hypothesized about the mosaicism seen in 25% of Turner peripheral blood?

A

Contributes to survival

97
Q

Turner Syndrome variants should remind you to consider what?

A

To always consider mosaicism including tissue specific mosaicism

98
Q

Which Turner variants have a more severe phenotype?

A

The 46X tiny ring (X)

99
Q

Trisomy 13 =

A

Platau syndrome

100
Q 
Trisomy 13 (Platau)
clinical features
A 
characteristic facies
severe intellectual deficits
congenital malformations
holoprosencephaly (failure to cerebrum to divide)
facial clefts
polydactyly
renal anomalies
101
Q

47, XXY =

A

Klinefelter Syndrome

102
Q

Is Klinefelter Syndrome common?

A

Yes, 1/1000 liveborn males

103
Q

Is Klinefelter lethal in utero?

A

no

104
Q

Klinefelter phenotype

A

Tall
Hypogonadism
Underdevelopment 2nd sex
Learning disability and poor psychosocial development

105
Q

Klinefelter parent of origin?

A

50% paternal meiosis I error

50% maternal meiotic errors (75% in Meiosis I)

106
Q

Klinefelter mosaicism

Most common?

A

47,XXY/46,XY
Thought to be from postzygotic loss of one copy of the X chromosome
(15%)
Some have normal testicular development

107
Q

Variants of Klinefelter?

A

48, XXYY
48, XXXY
49, XXXX7 etc

108
Q

Significance of additional X variants in Klinefelter?

A

increasingly more severe phenotype and retardation

109
Q

Mechanism of tetraploidy (3)

A

2 sperm (n) fertilize (n) egg

n sperm fertilize 2n egg

2n sperm fertilize n egg

110
Q

tetraploidy mechanism?

A

Dna duplication but no cell division (endomitosis)

111
Q

Triploid = 3n
How often do we see?
Survive to birth?
Most common cause

A

1-3% karyotypes
may survive to birth but not long after
most due to two sperm fertilizing one egg

112
Q

How many chromosomes in triploid?

A

69, XXX

69, XXY

113
Q

What is mosaicism?

A

Two or more different karyotypes from the same individual

114
Q

What is the most common cause of mosaicism?

A

Nondisjunction early in embyronic develoment

115
Q

Germ line mosaicism, result of?

A

nondisjuction in germ cell precursor

116
Q

pseudomosaicism

A

occurs in lab / not patient

117
Q

Broadly: polyploidy

A

Extra copies of all chromosomes e.g. 3n or 4n

118
Q

Broadly: aneuploidy

A

loss or gain of certain chromosomes e.g. 21 or 45x

119
Q

Broadly: mosaicism

A

A zygote containing two cell lines which differ in chromosome number

120
Q

Chromosomal abnormalities make up __% spontaneous 1st trimester abortions

A

50

121
Q

Chromosomal abnormalities make up ___% live born infants with congenital abnormalities

A

3

122
Q

What % of sperm are abnormal?

A

1-3%

123
Q

What percent of ova are abnormal?

A

> 3% with increasing abnormality with age

M2M2 (27 decks)

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