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Cytogenetics Flashcards

1
Q

Homologous chromosomes/homologues

A

Members of a pair of chromosomes that carry the same genes in the same sequence; one inherited from mom and one from dad

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2
Q

Alleles

A

Slightly different forms of the same gene

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3
Q

Chromatin

A

Genomic DNA complexed with proteins

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4
Q

Sister chromatids

A

Created in the S phase of the cell cycle when chromosomes replicate; each contains an identical copy of the original DNA strand

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5
Q

Telomeres

A

Specialized repetitive DNA sequences that ensure the integrity of the chromosome during cell division

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6
Q

Centromere

A

A region of DNA that hold two sister chromatids together

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7
Q

Daughter chromosomes

A

Separated sister chromatids

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8
Q

Recombination: when does it happen and what happens?

A

Occurs in meiosis I when homologous segments of DNA are exchanged between non-sister chromatids of a pair of homologous chromosomes

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9
Q

Meiosis I separates…

A

homologous chromosomes

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10
Q

Meiosis II separates…

A

Sister chromatids

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11
Q

Nondisjunction

A

When a pair of homologous chromosomes fails to separate during anaphase of meiosis I

The most common chromosomal mechanism of aneuploidy

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12
Q

Percentage of parents with karyotype alterations that impact fertility

A

1%

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13
Q

Acrocentric chromosomes (#s)

A

13, 14, 15, 21, 22

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14
Q

Live birth incidence of Down syndrome

A

1 in 691

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15
Q

Trisomy 21 (Down syndrome) features

A

Brushfield spots, hypotonia, flat occiput, epicanthic folds, upslanted palpebral fissures, small low set ears, low nasal bridge, maxillary hypoplasia, narrow palate, broad neck (often webbed), CHD (50%, AVSD most common), umbilical hernia, clinodactyly of fifth finger, single palmar crease, sandal toe gap, cryptorchisism (occasional), DD, duodenal atresia, tracheoesophageal fistula, increased risk of leukemia

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16
Q

Pallister Killian syndrome features

A

Hypotonia, ID, sparse hair, coarse facies, hypopigmented areas of the skin, depressed nasal bridge, down turned mouth, thin upper lip, microcephaly, ptosis

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17
Q

Trisomy 18 (Edwards syndrome) features

A

FTT, prominent occiput, HYPERtonia, micrognathia, rocker bottom feet, clenched fists, CHDs (99%), low set ears, hypoplastic nails, renal anomalies

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18
Q

Trisomy 13 (Patau syndrome) features

A

scalp defects, CL/P, CHD (88%, often VSD and PDA), holoprosencephaly, iris coloboma/eye abnormalities, micrognathia, polydactyly, polycystic kidneys, clenched fists, rocker bottom feet

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19
Q

Turner syndrome features

A

Horseshoe kidney, webbed neck, coarctation of the aorta, widely spaced nipples, short stature, rarely fertile, IQ is typically in normal range

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20
Q

Kleinfelter syndrome (XXY) features and incidence

A

Tall stature, gynecomastia, infertile, obesity common, IQ typically normal or slightly below

1/500 incidence

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21
Q

47, XXX features

A

Speech delay, slightly lower IQ than siblings, increased risk of infertility, chromosomally normal offspring

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22
Q

47, XYY features

A

slightly lower IQ than siblings, ?impulsivity, chromosomally normal offspring

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23
Q

% of live births with cytogenetic disorders

% of stillbirths with cytogenetic disorders

% of 1st trimester SABs with cytogenetic disorders

A

~0.7%

~10%

50%

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24
Q

Clinical indications for chromosome analysis

A 
Problems with early growth/dev.
Stillbirth and neonatal death
Fertility problems
Fam hx of chromosome abnormality
Neoplasia (analysis of the tumor itself)
AMA pregnancy
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25
Q

Monosomy prognosis

A

More deleterious than trisomies - generally not viable except for Turner

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26
Q

Pericentric inversions: definition and possible gametes

A

Include the centromere

Gametes formed may be normal, balanced, or unbalanced

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27
Q

Paracentric inversion: definition and possible gametes

A

Does not involve the centromere

Gametes formed may be normal, balanced, or inviable

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28
Q

Triploidy: how common is it in conceptions? What is the most common mechanism? What do we see if the extra set of chromosomes is maternal? Paternal?

A
  • Present in 1-3% of recognized conceptions
  • Most frequently the result of dispermy
  • Extra set of paternal chromosomes = partial hydatidiform mole
  • Extra set of maternal chromosomes - SAB earlier in pregnancy
29
Q

Types of chromosome structural rearrangements (7)

A 
Terminal deletion
Interstitial deletion
Unequal crossing over
Ring chromosomes 
Isochromsomes
Robertsonian translocation 
Insertion
30
Q

Most common robertsonian translocations

A

45% 14, 13
36% 14, 21
6% 22,13

31
Q

Inv(9)(p11q12) prognosis

A

The most common inversion, seen in up to 1% of all individuals tested in cyto labs

No known deleterious effects; considered a normal variant

32
Q

Alternate segregation

A

Produces gametes with normal or balanced chromosome complement

33
Q

Adjacent 1 segregation

A

Homologous centromeres go to separate daughter cells

All gametes are unbalanced

34
Q

Adjacent 2 segregation

A

RARE

Homologous centromeres go to the same daughter cell

All gametes unbalanced

35
Q

How often does 3:1 segregation occur in translocation carries?

A

Observed in 5-20% of sperm from balanced translocation carriers

36
Q

Robertsonian translocation carrier potential gametes: How many are viable? what is the risk of a carrier to transmit the translocation to a child?

A

Causes 6 types of potential gametes, but only 3 are viable (normal, balanced, and unbalanced +21, 18, or 13)

Carrier females: 10-15% risk of transmitting the translocation; carrier males: only a few % risk of transmitting the translocation –> VIABLE GAMETES DO NOT OCCUR IN EQUAL PROPORTIONS

37
Q

Insertion: definition, how common are insertions?, what is the risk of producing an abnormal child?

A

A no reciprocal translocation when one segment is removed and inserted into a different chromosome

Relatively rare because it requires three chromosome breaks

Risk of producing an abnormal child up to 50%

38
Q

Imprinting: when does it occur? what is it’s purpose?

A

Occurs during gametogenesis

Marks certain genes as having come from mother or father

39
Q

Prader-Willi syndrome features and mechanisms

A

Characterized by obesity, obsessive eating habits, small hands and feet, short stature, hypogonadism, and ID

70% have deletion of paternal 15q11-13
30% have maternal UPD
5% have imprinting center defect

40
Q

Angelman syndrome features and mechanisms

A

Characterized by dysmorphic facies, short stature, severe ID, spasticity, and seizures

70% have deletion of maternal 15q11-13
11% have mutation in maternal UBE3A gene
3-5% have paternal UPD
5% have imprinting center defect
10-15% Cause unknown
41
Q

Uniparental disomy: definition, difference between isodisomy and heterodisomy

A

Presence of two chromosomes inherited from only one parent

Isodisomy if the chromosome is a duplicate
Heterodisomy if both homologues from one parent are present

42
Q

Beckwith-Wiedemann syndrome features and mechanism

A

Macrosomia, enlarged tongue, omphalocele, neonatal hypoglycemia, increased risk of tumors (Wilms tumor, hepatoblastoma)

Due to paternal UPD of 11p15

43
Q

Complete mole: cause, origin of chromosomes, U/S findings

A

Result of an abnormal pregnancy that arises when a sperm fertilizes an egg with no nucleus and its chromosomes double

46, XX karyotype of all paternal origin

“Cluster of grapes” on U/S with no fetal development

44
Q

Ovarian teratoma

A

Benign tumor that arises from abnormal pregnancy where egg duplicates chromosomes

46, XX all of maternal origin

Little to no extraembryonic development (I.e. Placenta)

45
Q

Partial mole: karyotype, parent of origin

A

Triploid
2/3 have extra paternal chromosome set
1/3 have extra maternal chromosome set

46
Q

Cri du Chat features and mechanism

A

Caused by terminal or interstitial deletion of 5p; critical region is 5p15

Characteristic cry in infants, microcephaly, hypertelorism, epicanthal folds, low set ears, micrognathia, moderate to severe ID, heart defects

47
Q

Smith Magenis syndrome features and mechanism

A

ID, delayed speech/language, distinctive facial features, sleep disturbances, behavioral problems (“lick and flip”)

Del 17p11.2

48
Q

Charcot-Marie-Tooth mecahnism

A

Dup 17p12

49
Q

Hereditary neuropathy with liability to pressure palsies (HNLPP) mechanism

A

Del 17p12

50
Q

DiGeorge syndrome/velocardiofacial syndrome mechanism and features

A

Del 22q11.2 - contains ~30 genes; haploinsufficiency of TBX1 (involved in development of pharyngeal system) is implicated in the phenotype

Craniofacial anomalies, ID, heart defects (TOF), increased risk of schizophrenia

51
Q

22q11.2 copy number syndromes

A

1 copy - 22q11.2 del/DGS/VCFS
2 copies - normal
3 copies - 22q11.2 duplication syndrome
4 copies - cat-eye syndrome (ocular coloboma, CHDs, craniofacial anomalies, moderate ID)

52
Q

Pseudoautosomal region

A

Region of the Xp and Yp chromosome that are essentially identical and undergo homologous recombination in meiosis I

53
Q

SRY gene: location, function, expression

A
  • Near the pseudoautosomal boundary on the Y chromosome
  • Responsible for male sex determination
  • Expressed only briefly early in development of germinal ridge as the testis-determining factor
54
Q

Azoospermia cause

A
  • Del of AZFa, AZFb, or AZFc regions of Y chromosome; del Yq11.2
  • AZFc contains DAZ genes (Deleted in AZoospermia)
55
Q

X inactivation: definition, % of genes that escape it, role of inactive X

A
  • Done at random in all cells with more than one X
  • At least 15% of genes escape inactivation (most found on Xp)
  • Inactive X: forms a Barr body, expresses XIST RNA
56
Q

XIST

A

Gene in the X inactivation center expressed only from the allele on the inactive X; Produces ncRNA

57
Q

Nonrandom X chromosome inactivation preferentially inactivates… (3 scenarios)

A
  • abnormal X
  • normal X when the other X is involved in a balanced translocation with an autosome
  • abnormal X when the abnormal X is involved in an unbalanced translocation with an autosome
58
Q

%age of T21 pregnancies that end in SAB
%age of T18 pregnancies that end in SAB
%age of Turner pregnancies the end in SAB

A

78% of T21 pregnancies end in SAB
95% of T18 pregnancies end in SAB
99% of Turner pregnancies end in SAB (Accounts for nearly 20% of chromosomally abnormal SABs but less than 1% of chromosomally abnormal live births)

59
Q

What is the most common mechanism for T21?

A

90% caused by error in maternal meiosis (mostly meiosis I)

10% due to error in paternal meiosis (mostly meiosis II)

60
Q

What percentage of T21 is due to a Robertsonian translocation?

A

4% have a robertsonian translocation involving chromosome 21 (usually with 14 or 22)

61
Q

What is the RR for T21?

What is the RR for T13?

A

T21: RR ~1%
T13: RR < 2% even when a parent carries a translocation

62
Q

What is the cause of Pallister Killian syndrome?

A

Mosaic 12p tetrasomy, the two additional copies of 12p are in the form of a small additional metacentric chromosome) - Abnormal cells seen only in the skin

63
Q

What %age of T18 and T13 cases involve a translocation?

A

20%

64
Q

What is the M:F ratio in cases of T18?

A

1:3 M:F ratio

65
Q

What is the most common cause of Turner syndrome?

A

More commonly the result of a paternal meiotic error, 60-80% of the time X is from mom

66
Q

Karyotype(s) in Turner syndrome

A

50% 45,X

50% other abnormal karyotype

67
Q

How does the size of a pericentric inversion relate to the viability of offspring?

A

Larger pericentric inversions are more likely to lead to viable recombinant off spring because the unbalanced sections are smaller

68
Q

What is the risk for a pericentric inversion carrier to have a child with an unbalanced karyotype?

The risk for a paracentric inversion carrier?

A

Pericentric: Overall risk to have a child with an unbalanced karyotype is 5-10%

Paracentric: Very low risk of abnormal karyotype in offspring because unbalanced recombinants are typically acentric or dicentric which are inviable

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