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MCBG > Session 16: Chromosome Abnormalities, Cytogenetics > Flashcards

Session 16: Chromosome Abnormalities, Cytogenetics Flashcards

1
Q

What are some reasons for referral to a cytogeneticist?

A

Prenatal diagnosis
Birth defects
Abnormal sexual development
Infertility

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2
Q

What two methods can be used for prenatal diagnosis methods?

A

Chorionic Villus Sampling (CVS) (Sample from villi of chorion)
Amniocentesis (Sample from amniotic fluid)

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3
Q

Is there a slightly greater risk of miscarriage in using CVS or amniocentesis in prenatal diagnosis?

A

CVS

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4
Q

What is polyploidy?

A

When cells gain a whole set of chromosomes (i.e. 3n) which is usually the result of polyspermy

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5
Q

What is aneuploidy?

A

The loss or gain of individual chromosomes, resulting in abnormal chromosome number

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6
Q

How does aneuploidy arise?

A

From meiotic non-disjunction, when one or more homologous chromosomes fail to separate at anaphase resulting in abnormal distribution (monosomy or trisomy)

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7
Q

If non-disjunction occurs during meiosis I, what will be the outcome for the gametes?

A

They will all have chromosomal abnormalities
50% monosomy
50% trisomy

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8
Q

If non-disjunction occurs during meiosis II, what will be the outcome for the gametes?

A

Half of the gametes will be “normal”
Half of the gametes will be abnormal: 25% monosomy
25% trisomy

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9
Q

If non- disjunction occurs during mitosis, what will be the outcome for the gametes?

A

They will have mosaicism: populations of somatic cells will be composed of cells that are genetically different to others

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10
Q

What are the pseudoautosomal regions?

What happens to these during meiosis?

A

PAR1 (terminal region of q arm)
PAR2 (terminal region of p arm)
They pair and recombine during meiosis

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11
Q

Can an individual have monozomy and be viable?

A

Yes, usually not viable but can be in Turner syndrome

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12
Q

What is the SRY?

A

A region on the Y chromosome that lines up with an X chromosome during meiosis

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13
Q

What are the two types of translocation?

A

Reciprocal

Robersonian

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14
Q

What types of reciprocal translocation are there?

A 
Alternate (balanced) 
Adjacent 1 (unbalanced) 
Adjacent 2 (unbalanced) 
3:1 non disjunction (unbalanced)
4:0 non disjunction (unbalanced)
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15
Q

Does reciprocal translocation occur during meiosis I or meiosis II? Why?

A

Meiosis I- this is when crossing over occurs

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16
Q

During meiosis I, chromosomes form what, which then segregates to give unbalanced or balanced arrangements of chromosomes?

A

A quadrivalent

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17
Q

What is a Robertsonian Translocation?

A

When the long arms of two acrocentric chromosomes fuse together at a common centromere

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18
Q

What does Robersonian translocation lead to?

A

Either monosomy or trisomy

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19
Q

What three events can lead to aneuploidy?

A

Non-disjunction
Anaphase lag
Robertsonian translocation

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20
Q

What can FISH be used to identify?

A

Microdeletions/duplications
To identify chromosome of origin
To identify individual chromosomes in arrangement

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21
Q

What can aCGH be used to detect?

A

Unbalanced translocations

22
Q

Is adjacent 1 or adjacent 2 segregation more common?

A

Adjacent 1

Adjacent 2 is very very rare and is not really seen in live newborns

23
Q

Will an individual with a 47, XXY karyotype have affected siblings?

A

No, it is unlikely as the gametes from the parent are unlikely to have the same fault during meiosis again

24
Q

What is non-invasive prenatal testing (NIPT)?

A

Prenatal testing that involves taking a blood sample from the mother and does not require a sample to be taken directly from the pregnancy

25
Q

What can NIPT currently be used to look for?

A

Downs syndrome
Fetal sex
Single gene disorders

26
Q

What can NIPT not be currently used to detect?

Why?

A

Chromosomal translocations

Need to be able to grow cell and get them in metaphase to determine this

27
Q

If a man has a Robertsonian 21;21 translocation karyotype, will he be clinically normal? Will his children be abnormal?

A

Yes he will be clinically normal

He can only father embryos wth trisomy 21 or monosomy 21

28
Q

Non-disjunction in one of the early mitotic divisions of the zygote will result in what?

A

Chromosomal mosaics

29
Q

Why can Array Comparative Genome Hybridisation (aCGH) not be used to detect balanced rearrangements?

A

aCGH is used to assess gene content and therefore will not give us additional information about balanced rearrangements as the rearrangement contains the same amount of genetic information

30
Q

Which cohort of patients, who require cytogenetic investigation would you expect to have a balanced rearrangement?

A

Patients who have been trying to conceive but have not been able to due to the unbalanced translocation in the foetus leading to it not being viable

31
Q

What test can be used to detect anything phenotypically abnormal a child’s development and learning?

A

aCGH

32
Q

What would be the preferred investigative technique in a cytogenetic laboratory for a newborn baby with a heart abnormality?

A

aCGH

33
Q

What would be the preferred investigative technique in a cytogenetic laboratory for a child with learning difficulties?

A

aCGH

34
Q

What would be the preferred investigative technique in a cytogenetic laboratory for a women who was having recurrent miscarriage?

A

FISH karyotyping

35
Q

What would be the preferred investigative technique in a cytogenetic laboratory for sperm and egg donation?

A

FISH karyotyping

36
Q

What would be the preferred investigative technique in a cytogenetic laboratory for a child with developmental delay and an apparently balanced chromosome inversion that is de novo?

A

aCGH

We want to know whether it is unbalanced or not and aCGH will tell us that i.e. it will only tell us if it is unbalanced

37
Q

During what phase of mitosis do chromosomes need to be in order to do chromosome analysis on them?

A

Metaphase

38
Q

In a 3:1 reciprocal translocation, what can be seen in tertiary trisomy?

A

2 normal chromosomes

One derivative

39
Q

In a 3:1 reciprocal translocation, what can be seen in tertiary monosomy?

A

One derivative - This is very rare

40
Q

In a 3:1 reciprocal translocation, what can be seen in interchange trisomy?

A

2 derivative chromosomes

One normal

41
Q

What happens during pregnancy if there is a reciprocal translocation involving chromosomes 13, 18 or 21?

A

The baby may be compatible to survive to term

42
Q

In a 3:1 reciprocal translocation, what is interchange monosomy?

A

Where there is one normal chromosome

This has never been seen

43
Q

What are the advantages of aCGH?

A

It examines the entire genome at high resolution
It is targeted against known genetic conditions and sub telomere regions
1 array is equivalent to many thousands of FISH investigations
Can be automated
Detailed information on duplicated/deleted regions
Better phenotype/genotype correlation

44
Q

What are the disadvantages of aCGH?

A

More expensive than karyotyping
Will not detect balanced rearrangements
Mosaicism may be missed

45
Q

Deletions and duplications arise through what?

A

Uneven pairing and recombination during meiosis

46
Q

Where can deletions and duplications arise?

A

At the ends of the arms (terminal)

Within the chromosome arm itself (interstitial)

47
Q

Deletions and duplications always result in balanced or unbalanced?

A

Unbalanced

48
Q

Can G banding be used to see microdeletions and microduplications?
If not, what would you use instead?

A

Not always

Would use FISH

49
Q

Can aCGH be used to detect mutations (nucleotide level)?

A

No, not sensitive enough to see at this small level

50
Q

What is uniparental disomy (UPD)?

A

Presence of two chromosomes from one parent

i.e. two chromosomes from one parent rather than one from each

51
Q

What is meant by idodisomy?

At what stage of meiosis does this occur?

A

The presence of two identical chromosomes from one parent

Meiosis II

52
Q

What is meant by heterodisomy?

At what stage of meiosis does this occur?

A

The presence of two homologous chromosomes from one parent

Meiosis I

MCBG (9 decks)

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