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Lecture 3 Chromosomal Abnormalities Flashcards

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1
Q

Cytogenetics

A

The study of the structure and function of chromosomes

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2
Q

What is cytogenetics used for ?

A

Used for the screening and diagnosis of inherited chromosomal orders

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3
Q

Karyotyoe

A

A preparation of chromosomes arranged in size order

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4
Q

What are autosomes?

A

Chromosomes 1-22
They are not the sex chromosomes

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5
Q

Steps in making a karyotype? (11)

A
  1. Draw 10-20ml blood
  2. Add a few drops of blood to culture flask containing growth medium
  3. Add one phytohaemagglutin to stimulate mitosis
  4. Incubate 2-3 days
  5. Treat with colcemid for 1-2 hrs to stop mitosis in metaphase
  6. Transfer cells to tube
  7. Pellet cells by centrifugation. Add hypotonic solution to lyse cells
  8. Transfer cells to tube containing fixative
  9. Drop cells onto microscope slide and stain with Giemsa
  10. Examine with microscope. Photograph metaphases
  11. Arrange chromosomes into a karyotype
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6
Q

What are chromosomes stained with when making a karyotype?

A

Giemsa
Creates G bands which help us to recognise the chromosomes

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7
Q

3 ways to identify a chromosome?

A
  1. Length
  2. Banding pattern
  3. Placement of centromere
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8
Q

What are the 3 different placements of centromere called ?

A
  1. Metacentric - centromere in centre
  2. Sub-metacentric - centromere off-centre
  3. Acrocentric - centromere very close to end
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9
Q

What is karyotyping used for ? (2)

A
  1. To detect changes in chromosome number
  2. To detect changes in chromosome structure
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10
Q

Polyploidy

A

Extra whole sets of chromosomes

E.g
diploid=2n
but polyploid: triploid=3n and tetraploid=4n

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11
Q

Is polyploidy rare in animals or in plants?

A

Rare in animals
Common in plants e.g. bananas

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12
Q

Aneuploidy

A

Some additional or missing chromosomes

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13
Q

2 types of aneuploidy ?

A
  1. Monosomy
  2. Trisomy
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14
Q

Monosomy

A

One chromosome is missing
2n-1

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15
Q

Trisomy

A

One extra chromosome
2n+1

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16
Q

How would you write the karyotype of trisomy 18 in female ?

A

47, XX, +18

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17
Q

What causes aneuploidy?

A

Non-disjunction

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18
Q

What happens when non-disjunction occurs in Meiosis I ?

A

Non-disjunction of homologous chromosomes

2 trisomy gametes
2 Monosomy gametes

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19
Q

What happens when non-disjunction occurs in Meiosis II ?

A

Non-disjunction of sister chromatids

1 trisomy gamete (n+1)
1 Monosomy gamete (n-1)
2 normal gametes

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20
Q

How common is aneuploidy?

A

Very common
Occurs in 50% of human conceptions

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21
Q

What do most aneuploidies lead to?

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A

Embryonic death or spontaneous abortion

22
Q

What is the only autosomal aneuploidy that permits survival into adulthood?

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A

Down’s Syndrome

23
Q

Aneuploidies of sex chromosomes are more common

24
Q

What is Down’s Syndrome?

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A

Trisomy 21
47,XX, +21

25
Characteristics of Down’s syndrome ? (5)
1. Characteristic facial features 2. Short stature 3. Some level of learning disability 4. Heart defects 5. Susceptibility to leukaemia & Alzheimer’s disease
26
What does the incidence of Down’s syndrome increase with?
Maternal age By 40 ~1:100 chance
27
3 ways we can screen for chromosomal abnormalities ?
1. Blood tests to detect specific proteins associated with a Down’s foetus 2. Ultrasound scans- to measure the size of the nuchal pad at the nape of the foetal neck associated with Down’s syndrome 3. Amniocentesis and karyotyping
28
What is Turner’s syndrome?
45, XO The only viable human Monosomy 1 in 2500 live female births Only have one X chromosome
29
Characteristics of Turner’s syndrome? (3)
1. Phenotypically female 2. Sterile due to lack of maturation of sex organs 3. Oestrogen replacement therapy leads to development of secondary sex characteristics
30
What is Klinefelter’s syndrome ?
47, XXY - One of the most common genetic abnormalities - 1:500-1000 live male births - either an extra X or extra Y chromosome
31
Characteristics of Klinefelter’s syndrome (4)
1. Essentially male, but some female characteristics 2. Tall stature 3. Sterile 4. Treated with hormone replacement therapy (testosterone)
32
What determines biological sex ?
Presence or absence of Y chromosome
33
4 different types of change in chromosome structure
1. Deletion 2. Duplication 3. Inversion 4. Reciprocal translocation
34
What letter is given to short arm of chromosome?
P (petite)
35
What letter is given to long arm?
q
36
2 (/3) syndromes caused by deletions in chromosome structure?
1. Cri-du-Chat Syndrome (chr 5) 2. Prader-Willi Syndrome/Angeman syndrome (chr 15)
37
What is Cri-du-chat syndrome?
‘5p minus syndrome’, ‘ Le Jeune’s syndrome’ ~1in 50,000 live births Deletion of part of the short arm of chromosome 5
38
Characteristics of Cri-du-Chat syndrome (5)
1. Babies have cat-like cry 2. Defects in glottis and larynx 3. Wide face with saddle nose 4. Physical & intellectual disability 5. Range of severity, depending on the extent of deletion
39
What is Prader-Willi syndrome ?
Deletion in long arm of chromosome 15 15q1.12 ~1:15,000 live births
40
Characteristics of Prader-Willi’s syndrome
- poor suckling reflex in infants - uncontrollable eating in later life - obesity - diabetes - poor sexual development in males
41
When does prader willi syndrome only occur
Only occurs when the affected chromosome is inherited from the father- due to genomic imprinting
42
Angelman’s syndrome
Arises when Same segment in prader willi syndrome is missing from maternally derived chromosome 15
43
Characteristics of angleman’s syndrome?
- a happy demeanour - inappropriate outbursts of laughter - intellectual disabilities - severe speech problems - stiff limb movements - Seizures
44
Genomic imprinting
Prader willi- normal expressed copy of gene A is missing from father Angelman - lack expressed copy of genes B from mother
45
2 syndromes caused by translocations
1. Familial Down’s syndrome 2. Chronic myelitis leukaemia
46
Familial Down’s syndrome
5% of Down’s cases Translocation carrier t(14;21) One of the chromosome 21’s is attached to one of the chromosome 14s
47
Familial Down’s syndrome
5% of Down’s cases Translocation carrier t(14;21) One of the chromosome 21’s is attached to one of the chromosome 14s
48
What is chance of parents have a second affected child if first has familial Down’s syndrome?
1in 3 chance they will have Down’s syndrome child
49
Chronic myelocytic leukaemia (CML)
A 22-9 translocation Spontaneous Not hereditable Occurs in 1 per 50-100,000 of the population Very high white blood cell count
50
What % of people with CML have the Philadelphia chromosome?
95% Translocated chromosome 22
51
CML what happens during reciprocal translocation?
Translocation creates the BCR-ABL fusion gene This is an oncogene which stimulates over production of white blood cells

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