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Genetic Counseling Boards Prep 2023 > Molecular and cytogenetics > Flashcards

Molecular and cytogenetics Flashcards

1
Q

What is the detection rate for G banded chromosomes?

A

4% detection rate

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2
Q

What is the detection rate for CMA?

A

15-20%

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3
Q

Which type of microarray can detect regions of homozygosity?

A

SNP array

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4
Q

What types of chromosome analyses can detect low level mosaicism?

A

Karyotype and SNP, not oligo arrays

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5
Q

What is CMA a first line test for?

A

multiple congenital anomalies, non-syndromic delays/ID, autism, prenatal: US anomalies, multiple losses, POC

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6
Q

What is the coefficient of inbreeding (F) for cousins?

A

1/16

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7
Q

What percent of the genome of cousins is identical by descent?

A

6.25%

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8
Q

What size of microarray deletions/duplications are likely to be benign?

A

<500kb 90-95% benign

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9
Q

What size of a ROH is common in the general population (not in-bred)?

A

<3 mb

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10
Q

What microarray results suggest a first or second degree parental relationship?

A

> 10% identical by descent with multiple regions of homozygosity of 2-5 mb

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11
Q

What is the diagnostic yield of WES?

A

25-50% (~30%, ~40% for WGS)

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12
Q

In what scenarios is WES recommended?

A

non-specific phenotypes, genetic heterogeneity (many genes), diagnostic odyssey/ other negative tests

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13
Q

How often are secondary findings found in WES?

A

1-6%

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14
Q

What regulates the cell cycle?

A

cyclins, and cyclin dependent kinases

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15
Q

What is nondisjunction and when does it occur?

A

Mistake in separation of homologs in meiosis I, or between sister chromatids in meiosis II

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16
Q

What are the DNA mismatch repair enzymes?

A

MLH1, MSH2, MSH6, PMS2

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17
Q

What genes are involved with nucleotide excision repair?

A

the XP related genes

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18
Q

What are the CGG repeat ranges for FMR1 related disease?

A

normal: 5-44, intermediate: 45-54, pre-mutation 55-200, Fragile X syndrome: >200

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19
Q

What additionally regulates stability of the FMR1 regulatory sequence?

A

stable alleles have AGG repeat every 9-10 CGG repeats

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20
Q

What types of blot are for DNA, RNA, proteins?

A

SNOW DROP:
Southern blot- DNA
Northern blot- RNA
Western blot- Protein

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21
Q

What type of chromosome analysis requires actively dividing cells?

A

karyotype (and metaphase FISH)

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22
Q

What is the difference between interphase and metaphase FISH?

A

interphase: can tell absence or presence but not location, faster than metaphase

metaphase: requires cell culturing, can show locations

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23
Q

What are the recommendations for chromosome testing order in pregnancy ?

A

FISH first
if abnormal: karyotype next
if FISH normal: CMA next

note: prenatal FISH not considered diagnostic

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24
Q

What is the risk of a child having an unbalanced translocation when a parent has an autosomal reciprocal translocation?

A

1-50%, depends

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25
Q

What is the most common Robertsonian translocation (75%)?

A

13q;14q

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26
Q

What is the risk for trisomy 21 with a 21q;21q isochromosome carrier?

A

100%

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27
Q

What is the reason that T21, T18, and T13 are the most common liveborn trisomies?

A

These chromosomes have the lowest gene content

28
Q

What are the percent causes of T21 (maternal vs paternal, meiosis I vs II)?

A

85-90% maternal
(75% MI, 25% MII)
3-5% paternal
4-5% Robertsonian
3-5% mitotic

29
Q

What is the risk of a liveborn infant with T21 at age 35?

A

1/385

30
Q

What are the percent causes of T18 (maternal vs paternal, meiosis I vs II)?

A

97% maternal
(31% MI, 69% MII)

31
Q

How many cells are typically analyzed in a chromosome analysis?

A

~20, 50-100 if concerned for mosaicism

32
Q

What is a Philadelphia chromosome?

A

translocation between chromosome 9 and 22

33
Q

What are cells treated with to test for chromosome breakage syndromes and how does this work?

A

mitomycin C, causes chromatids to break and join to make tetraradials

34
Q

What is the resolution of FISH?

A

50-250 kb

35
Q

What percent of unbalanced translocations are inherited?

A

75% inherited, 25% de novo

36
Q

How are chromosomes labeled at the centromere?

A

p10/q10

37
Q

What is the risk of T21 or any chromosome aneuploidy at age 40?

A

T21 1/85
any 1/62

38
Q

What percent of early losses or still births are aneuploid?

A

50% of early losses
6-12% still births

39
Q

What percent of fetal T21 results in a live birth?

A

20%, 80% lost

40
Q

What is the recurrence risk of T21?

A

1-1.5%, higher in younger women
(age related risk +1% >35)

41
Q

How does a ring chromosome form?

A

Terminal deletion of long and short arms, then fusion

42
Q

Are large or small percentric inversions more likely to result in a livebirth?

A

Large - cause small terminal del/dups, result in viable offspring with aneuploidy

43
Q

What fraction of small supernumerary chromosomes are familial?

A

1/3

44
Q

What is the likelihood of T21 if a mother or father is a carrier of a Robertsonian translocation?

A

maternal carrier: 10-15% risk
paternal carrier: 3-5% risk (also seen 2% quoted)

45
Q

What percent of X chromosome genes escape X inactivation?

A

15%

46
Q

What percent of female carriers of XL conditions have symptoms?

A

10-15%

47
Q

Which of the following sex chromosome aneuplodies are associated with advanced maternal age: XXY, XXX, X0

A

associated with advanced maternal age: XXY,
not: XXX, X0

48
Q

What percent of individuals with Turner syndrome have a 45, X karyotype?

A

45-50% monosomy X
15-25% mosaic
20% structural

49
Q

What percent of individuals with Turner syndrome go through puberty?

A

10%

50
Q

How many chromosomes with how many sister chromatids are present at:
the start of meiosis?
the end of meiosis I?
the end of meiosis II?
the end of mitosis?

A

the start of meiosis: 46 chromosomes, each with 2 sister chromatids
the end of meiosis I: 23 chromosomes, each with 2 sister chromatids
the end of meiosis II: 23 chromosomes, each with 1 sister chromatid
the end of mitosis: 46 chromosomes, each with 1 sister chromatid

51
Q

What are the consequences (percentages) of nondisjunction in :
mitosis?
meiosis I?
meiosis II?

A

mitosis: 50% trisomy, 50% monosomy
meiosis I: 50% trisomy, 50% monosomy
meiosis II: 50% normal, 25% trisomy, 25% monosomy

52
Q

What is the parent of origin for monosomy x?

A

80% paternal, 20% maternal

53
Q

What is the difference between isodisomy and heterodisomy and when do the corresponding errors occur?

A

Isodisomy: two of the same homolog from the same parent
(meiosis II error or monosomy rescue)

Heterodisomy: two different homologs from the same parent
(meiosis I error or trisomy rescue)

54
Q

What percentage of SAB are due to triploidy?

A

6%

55
Q

Is diandric or digynic triploidy more common?

A

Diandric (85%)
digynic (15%)

56
Q

What is the most common karyotype in chromosomal abnormal pregnancies losses?

A

45, X

57
Q

What are the causes of Patau syndrome?

A

80% T13, 20% 13;14 translocations

58
Q

What are the risks of a serious congenital anomaly in a de novo:
Robertsonian translocation?
Reciprocal balanced translocation?
Inversion?

A

Robertsonian translocation: 3.7%
Reciprocal balanced translocation: 6.1%
Inversion: 9.4%

RBI 3-6-9

59
Q

For the carrier of a balanced translocation carrier, what are the possible outcomes for adjacent-1 segregation?

A

all unbalanced gametes

60
Q

For the carrier of a balanced translocation, what are the possible outcomes for adjacent-2 segregation?

A

usually lethal, all unbalanced gametes (more unbalanced than adj-1)

61
Q

For the carrier of a balanced translocation carrier, what are the possible outcomes for alternate segregation?

A

50% normal, 50% balanced carrier

62
Q

What abnormality is seen on chromosome breakage studies for individuals with Fanconi anemia?

A

tetraradials

63
Q

What abnormality is seen on chromosome breakage studies for individuals with Bloom syndrome?

A

increased sister chromatid exchange

64
Q

What abnormality is seen on chromosome breakage studies for individuals with ataxia telangiectasia?

A

can have 7;14 rearrangements

65
Q

What are the possible outcomes of a paracentric inversion ?

A

balanced-normal, balanced with inversion, inviable with 2 centromeres, inviable with no centromeres

Genetic Counseling Boards Prep 2023 (13 decks)

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