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MTM > Chromsome Disorders > Flashcards

Chromsome Disorders Flashcards

1
Q

How many pairs of chromosomes?

A

22 autosomal
1 sex
Plus mitochondrial genome

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2
Q

What is a gene?

A

Specific stretch of DNA where sequence contains genetic instructions

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3
Q

How are genes arranged on chromosomes?

A

Linear order

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4
Q

Chromosomes have a short arm and long arm. True/ False

A

True

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5
Q

Function of the centromere:

A
  • joins sister chromatids
  • essential for chromosome segregation at cell division
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6
Q

What are light bands of chromosomes?

A

Replicate early in S phase
Less condensed chromatin
Transcriptionally active
Gene and GC (bases) rich

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7
Q

What are dark bands on chromosomes?

A

Replicate late
Contain condensed chromatin
AT (base) rich but not gene rich

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8
Q

Telomere on chromosome?

A

DNA and protein cap
Ensures replication to tip
Tether to nuclear membrane

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9
Q

How chromosomes are examined?

A

Karyotype
Florescent in situ hybridisation (FISH)
Array CGH

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10
Q

How karyotypes are produced:

A
  • 5ml of venous blood
  • Separate off rbc
  • add culture medium to white cell suspension
  • incubate
  • colchicine added, separate off white cells
  • hypotonic saline added
  • cells fixed
    …..
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11
Q

How are specific chromosomes recognised?

A

Via their banding pattern

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12
Q

How to examine karyotypes?

A
  1. Count number of chromosomes and check if any extra
  2. Look for copy no. Variants or sections of dna missing
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13
Q

What types of chromosome abnormalities are there?

A
  1. Chromosome number
  2. Chromosome structure
  3. Classified according to which cells they disrupt:
    - constitutional (all cells of the body)
    - somatic (only certain cells)
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14
Q

How to name and classify abnormalities?

A
  1. Total number of chromosomes
  2. Sex chromosomes constitution
  3. Abnormalities/ variants

E.g. trisomy syndrome
47, XX, +21
Extra chromosome and male

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15
Q

Types of abnormalities of chromosome number:

A
  1. Aneuploidy - changes in a single chromosome number (haploid set)
  2. Polyploidy - changes in overall chromosome number (more than 2 complete sets of chromosome)
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16
Q

Trisomy?

A

Aneuploidy - Additional chromosome

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17
Q

Monosomy?

A

Aneuploidy - missing chromosome

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18
Q

Triploidy?

A

Polyploidy - additional set of chromosomes in the cell for total of 69 rather than normal 46

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19
Q

Tetraploidy?

A

Polyploidy - when an affected individual has four copies of each chromosomes instead of 2

96 chromosomes

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20
Q

Trisomy 21?

A

Known as “Down syndrome”
Extra copy of chromosome 21. 3 in total.

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21
Q

How does a trisomy occur?

A

Non dysjunction during meiosis II
Sister chromatids don’t separate equally

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22
Q

What is disomic nullisomic?

A

Produced after non disjunction
Incorrect number of chromosomes in gametes

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23
Q

What are oocytes?

A

All women born with them
Each remains in maturation arrest at the crossing over stage until ovulation
Each month 1 oocyte ovulated and completes meosis

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24
Q

Parental origin of meiotic error leading to trisomies (associated with increase in maternal age):

A

Older the woman the older the oocytes
Accumulating effects on the primary oocytes during this phase may damage the cells spindle formation and repair mechanisms
- predisposing them to non dysjunction

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25
Q

Which has more severe consequences?
Reducing the gene copy
Increasing the gene copy

A

Reducing the gene copy

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26
Q

Most frequent numerical autosomal anomalies in liveborns:

A

Down syndrome (trisomy 21: 47,XX,+21)
Edwards syndrome (trisomy 18: 47,XX,+18)
Patau syndrome (trisomy 13: 47,XX,+13)

27
Q

Most frequent numerical sex chromosome anomalies in liveborn:

A

Turner syndrome female: 45,X
Klinefelters syndrome male: 47,XXY

28
Q

Most frequent numerical anomalies in liveborns including all chromosomes?

A

Triploidy 69 chromosomes

29
Q

Features of trisomy 21:

A

Epicanthic folds
Depressed nasal bridge
Profound neonatal hypotonia
Exomphalos
Single palmar crease
Brushfield spots
Up slanting palpebral fissures
Cataract

30
Q

3 types of genetic basis (causes) of Down syndrome:

A
  • 95% people ave 3 separate copies of chromosome 21 (trisomy 21)
  • 4% have extra copy of chromo 21 due to Robertsonian translocation
  • 1% have mosaicism with normal and trisomy 21 cell lines. Occurs after fertilisation. Non dysjunction happens in mitosis after development.
31
Q

What is edwards syndrome?

A

Trisomy 18
1 in 3000 births
Multiple malformations (especially heart, kidneys)

32
Q

How does 3 copies of chromosome 21 cause the features of Down syndrome?

A

No mutations so protein still the same
- “Gene dosage effect” - features of syndrome caused by 1.5 times amount of specific gene product from chromo 21

  • “Amplified developmental instability” - features of syndrome cause by overall effect of imbalance on development
33
Q

Patau syndrome?

A

Trisomy 13
1 in 5000 births
Multiple malformations
Affects midline structures paticularly incomplete lobation of the brain
Cleft lip
Congenital heart disease

34
Q

All autosomal abnormalities have a low survival rate except for:

A

Trisomy 21

35
Q

Klinefelter syndrome?

A

1 in 1000 male births
Infertility
Poorly developed secondary sexual characteristics
Tall stature
Mild learning problems

36
Q

Turner’s syndrome?

A

45, X
25-50 in 100,000 females
Short stature
Primary gonadal failure
Can affect multiple organ systems cardiac, thyroid, renal

37
Q

Are sex or autosomal chromosome abnormalities worse?

A

Sex chromosome abnormalities are worse but less severe than autosomal

38
Q

What is Aneuploidy usually caused by?

A

Caused by the results of meiotic non disjunction related to maternal age
Risk of recurrence is therefore low and will be related to age of the mother

39
Q

What is genetic counselling?

A
40
Q

Types of Aneuploidy (trisomies):

A

Trisomy 21 - Down syndrome
Trisomy 18 - edwards syndrome
Trisomy 13 - Patau syndrome

41
Q

Classification of abnormalities of chromosome structure:

A

Translocations
- reciprocal/ robertsonian
Deletion
Duplication
Inversion
Ring chromosomes
Marker chromosomes
Complex rearrangements

42
Q

What is robertsonian translocation?

A

Breakage of two acrocentric chromosomes at or close to centromeres
With subsequent fusion of their long arms - short arms are lost

43
Q

What are the acrocentric chromosomes?

A

13
14
15
21
22

44
Q

What is Balanced robertsonian translocation?

A

Still two copies of 14 and 21
All genetic material still present but at different positions
Doesnt affect phenotype
So can be a carrier of robertsonian translocation
1 in 1000 individuals

45
Q

What is unbalanced robertsonian translocation?

A

For example two 21 chromosomes and one 14 chromosomes and joined 14+21 chromosome
Down syndrome via robertsonian translocation

46
Q

What are the possibilities for an offspring of a balances robertsonian carrier? E.g. robertsonian translocation of chromosome 14 and 21

A
  1. Normal
  2. Balanced carrier
  3. Monosomy 14
  4. Trisomy 14
  5. Monosomy 21
  6. Trisomy 21 Down’s syndrome
47
Q

What are the lethal chromosomal abnormalities of the offspring of a balanced robertsonian carrier?

A
  1. Trisomy 14
  2. Monosomy 14
  3. Monosomy 21
48
Q

Probabilities of recurrence for robertsonian translocation?

A
  • a carrier for robertsonian 14:21 has a high THEORETICAL probability of having Down’s syndrome child
  • but in reality, lower probability (male carrier: 1%, female Carrier: 10%)
49
Q

What is reciprocal translocation?

A

Breakage of two non homologous chromosomes with exchange of fragments
Can occur btw any non paired chromosomes

50
Q

Possibilities of offspring for balanced reciprocal translocation carriers?

A
  1. Normal
  2. Balanced carrier
  3. Partial trisomy + partial Monosomy
  4. Partial Monosomy + partial trisomy
51
Q

Risks for a balanced carrier of translocation?

A

Risk of children having unbalanced translocation

52
Q

Risks of unbalanced reciprocal translocation carrier?

A

(Partial trisomy and partial Monosomy)
Miscarriage
Or
Some imbalances compatible with survival depends on which chromosomes
Like congenital malformation, developmental delay, metal abnormality

53
Q

The size and position of of the chromosome segments in reciprocal translocation may have an effect on:

A

The pairing of the chromosomes at meiosis
Frequency of diff forms of translocation in the gametes
The likelihood of the conceptus with that abnormality developing to term

54
Q

Consequences of reciprocal translocation depends on:

A

Genes on the translocated segments
Amount of chromosome imbalance

55
Q

Prenatal diagnosis?

A

testing the fetus before birth (prenatally) to determine whether the fetus has certain abnormalities, including certain hereditary or spontaneous genetic disorders.

56
Q

How do large chromosome deletions and duplications occur?

A

Due to unequal crossing over in meiosis following mispairing (often at sites of repeated sequences)

57
Q

Mosaicism?

A

Two populations of cells with different genetic constitutions usually as a result of an error in mitosis
Chromosome abnormality during mitotic cell division

58
Q

Somatic mosaicism?

A

Two populations of cells in body

59
Q

Gonadal mosaicism?

A

Two populations of cells only in the gonads

60
Q

Main points about numerical chromosome abnormalities?

A
  • Gain or loss of complete chromosome
  • Common cause is non disjunction
  • Serious, often lethal consequences
  • Autosomal monosomies are catastrophic
  • common numerical abnormalities in live borns trisomy 21,13,18
  • fewer serious effects from sex chromosome abnormalities
61
Q

Is mosaicism for chromosomal anomaly confined to the gonads?

A

Yes

62
Q

Main points of structural chromosome abnormalities:

A
  • Translocations involve exchange of material btw two chromosomes
  • Robertsonian and Reciprocal
  • Individual clinically normal if no loss or gain of genetic material (translocation carrier)
  • carrier may cause chromosomal abnormal baby, miscarriage, stillbirth, infertility
  • small imbalance: child with multiple congenital abnormalities
  • deletions/duplications occur due to recombination btw mispaired chromosomes
63
Q

What is an inactivated X chromosome called

A

A Barr body

MTM (12 decks)

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