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Genetic Counseling > Cytogenetics > Flashcards

Cytogenetics Flashcards

1
Q

What is a contiguous gene syndrome?

A

The phenotype is attributable to haploinsufficiency for multiple contiguous genes within the deleted region.

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2
Q

Name some examples of contiguous gene syndromes.

A

Williams syndrome, DiGeorge syndrome, Smith-Magenis syndrome, Azoospermia

T&T p96 has a good table

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3
Q

What are acrocentric chromosomes?

A

Chromosomes 13, 14, 15, 21 & 22

short arm is made up of stalks and satellites - repetitive DNA

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4
Q

What is aneuploidy?

A

loss (monosomy) or gain (trisomy) of one or more chromosomes

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5
Q

What is polyploidy?

A

gain or one or more haploid sets of chromosomes

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6
Q

What is an isochromosome?

A

chromosome comprised of two copies of the same arm in mirror image of each other

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7
Q

What is the most common mechanism by which trisomy can arise?

A

Nondisjunction

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8
Q

Nondisjunction accounts for what percentage of Down syndrome?

A

95%

maternal meiosis I error is by far the most common

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9
Q

What is the recurrence risk of Trisomy 21 in women <30 years?

A

1-1.4%

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10
Q

What is the recurrence risk of Trisomy 21 in women >35 years?

A

Age related risk

*double check that it isn’t higher

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11
Q

What is a Robertsonian translocation?

A

translocation involving the acrocentric chromosomes and breakage in short arms/stalk/satellite or at centromere

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12
Q

A woman is a 21:21 Robertsonian translocation carrier. What is her recurrence risk for a child with Down syndrome?

A

100%
21:21 translocations are rare so most cases are de novo in the child with a very low recurrence risk (if mom’s karyotype is normal)

(monosomy 21 is lethal which could explain miscarriage or trouble getting pregnant)

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13
Q

What percentage of Down syndrome is caused by translocation?

A

~4%

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14
Q

What is the most common Robertsonian translocation involving chromosome 21?

A

14:21

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15
Q

What is the risk for a woman carrying a 14:21 translocation to have a baby with Down syndrome?

A

10-15%

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16
Q

What is the risk for a man carrying a 14:21 translocation to have a baby with Down syndrome?

A

1-3%

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17
Q

What percentage of Trisomy 13 phenotype is due to Robertsonian translocation?

A

20% (higher than Down syndrome)

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18
Q

What is the most common Robertsonian translocation from chromosome 13?

A

13:14

most common Robertsonian translocation in gen pop (1/1300)

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19
Q

What is the recurrence risk for Patau syndrome (tri 13) for translocation carriers?

A

der(13;14) carriers: <2% for males and females

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20
Q

How often does triploidy occur?

A

1-3% of all conceptions

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21
Q

True/False: Triploidy is one of the most common chromosome abnormalities observed in first trimester miscarriage

A

TRUE

only a small % survive to full term

22
Q

What is diandry?

A

two sets of paternal chromosomes in triploidy- moderate growth retardation of fetus, enlarged abnormal placentas

23
Q

What is digyny?

A

two sets of maternal chromosomes in triploidy - severe growth retardation of fetus, disproportional body parts with macrocephaly and small truck and limbs, placenta small and non-cystic

24
Q

What percentage of recognized pregnancies lost in 1st trimester are chromosomally abnormal?

A

50-60%

25
Q

What is the most common trisomy associated with miscarriage?

A

Trisomy 16

26
Q

What is the single most frequent abnormality in miscarriage?

A

Turner syndrome, 45X

trisomy is the most common group of abnormalities in miscarriage

27
Q

What type of collection tube is needed for cytogenetics?

A

Green top (contains heparin) - need cells to be alive for cell division to analyze chromosomes

28
Q

What is the risk for birth of an abnormal child of a balanced translocation carrier?

A

5-30%

plus higher risk of miscarriage or infertility

29
Q

What is the risk for a de novo balance translocation rearrangement to cause birth defects? Why?

A

5-10%; breakpoint of translocation could disrupt an active gene (inactivate it), regulatory elements moved too far away, submicroscopic loss or gain of DNA

30
Q

What are some limitations of CMA?

A

cannot detect balance translocations, triploidy, low level mosaicism

31
Q

What is genomic imprinting?

A

expression of an imprinted gene depends on the sex of the transmitting parent (ie: Prader Willi syndrome and Angelman syndrome)

32
Q

What is a maternally imprinted gene?

A

gene inherited from mom is not supposed to be active; need expression of that gene inherited from father for normal developement; without paternal expression in 15q12 Prader Willi syndrome occurs (missing paternal)

33
Q

What is paternally imprinted gene?

A

gene inherited from father is not supposed to be active; need expression of that gene inherited from mother for normal developemnt; without maternal expression in 15q12 Angelman syndrome occurs (missing maternal)

34
Q

What is uniparental disomy?

A

both homologous chromosomes are inherited from the same parent; normal chromosome number but not bi-parental inheritance; other parent’s chr is not represented

35
Q

What are three proposed mechanisms of UPD?

A
  1. trisomy rescue
  2. monosomy rescue
  3. gamete complementation - not common; disomy egg and null sperm
36
Q

What is trisomy rescue?

A

lethal trisomy due to non-disjunction, embryo will expel one of the three copies of the trisomic chromosome from a cell; UPD when both chr from same parent are retained

37
Q

What does methylation do to gene expression?

A

Turns gene off

38
Q

What percentage of Turner syndrome conceptions spontaneously abort by 28 weeks?

A

99%

39
Q

The Philadelphia chromosome (translocation of chromosomes 9 & 22) is associated with an increased risk for what condition?

A

CML (chronic myelogenous leukemia); fusion of two genes in translocation creates a new gene, this protein transforms normal cells into cancer cells

40
Q

What is the most common cause of Prader Willi Syndrome?

A

70% caused by deletion of 15q12 on the paternally inherited chromosome
25-30% result from maternal UPD
~5% result from a mutation in the imprinting center of the transmitting father (unable to unmethylate his 15q12 in his gametes– no ‘resetting’)–> 50% recurrence risk!

41
Q

What is the most common cause of Angelman syndrome?

A

70% caused by maternal deletion of 15q12
10% mutation in maternal copy of UBE3A–> if inherited, potential for 50% RR
10-15% unidentified etiology
~5% paternal UPD
~5% result from a mutation in the imprinting center of the transmitting mother (unable to unmethylate her 15q12 in her gametes– no ‘resetting’)–> 50% recurrence risk!

42
Q

What percentage of children with T21 have cardiac anomalies?

A

40%

43
Q

What would you expect the phenotype of a 45, XX, der(21;21) individual to be?

A

Normal carrier of a 21;21 robertsonian translocation. –> all children will have T21.

44
Q

What percentage of children with T13 have cardiac defects?

A

80%

45
Q

What percentage of T18 conceptions end in SAB?

A

~95%

46
Q

Most triploid pregnancies lost to 1st trimester miscarriage represent _____________

A

Diandry

47
Q

Most triploid pregnancies that survive to term represent _________

A

Digyny

48
Q

What tends to be more viable for the offspring of balanced translocation carriers– partial monosomy or partial trisomy?

A

Partial trisomy. Monosomies generally less viable than trisomies.

49
Q

CMA shows monosomy for the 5p region… what findings do you expect to see in this child and what is the most likely RR to the parents?

A

5p minus= Cri du Chat
Expected findings: microcephaly, down-slanting eyes, epicanthic folds, hypertelorism, low set ears, hypotonia, micrognathia, cat like cry, ID… major malformations uncommon.
89-90% are de novo in origin. So MOST likely RR is <1%.
However, 10-11% are the result of a parental structural rearrangement of chromosome 5 (either translocations involving 5p or peri/paracentric inversions of parental chromosome 5)

50
Q

Other than heart defects, what might comprise the 22q phenotype?

A

(all variable) cleft palate, immunodeficiency (related to hypoplasia of the thymus), neonatal hypocalcemia (related to parathyroid hypoplasia), ID/LD, psych issues

51
Q

What percentage of Turner syndrome is 45,X?

A

50%

(25% are mosaic with 45,X/46,XX as the most common complement)
(25% result from a structurally abnormal X such as del Xp, del Xq, ring X, or isochromosome X)

52
Q

Why is the likelihood of maternal cell contamination decreased after culturing?

A

Blood cells do not attach to culture vessels and are NOT induced by mitogens to divide

Genetic Counseling (14 decks)

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