Lecture 3

Question 1

What are cytogenetics?

Answer 1

Study of structure and function of chromosomes.

Question 2

What is a karyotype?

Answer 2

Preparation of chromosomes arranged in size order.

Question 3

How do you make a karyotype?

Answer 3

• Add a few drops of blood with phytohaemagglutinin into a culture flask to stimulate mitosis.
• Incubate for 2-3d, before adding colcemid to arrest mitosis at metaphase.
• Then pellet cells in tube by centrifugation. Add hypotonic solution to lyse cells.
• Then add to tube containing fixative.
• Drop cells onto microscopic slide, stain with Giemsa.
• Examine and photograph metaphases, before arranging into a karyotype.

Question 4

What are the bands on chromosomes called?

Answer 4

G bands.

Question 5

3 ways to identify a chromosome?

Answer 5

Length,
Banding pattern,
Placement of centromere.

Question 6

3 descriptions of placement of centromere, and meaning?

Answer 6

Metacentric - centromere in centre.
Sub-metacentric - centromere off-centre.
Acrocentric - centromere very close to end.

Question 7

2 uses for karyotyping?

Answer 7

Detect changes in:

• chromosome number
• chromosome structure.

Question 8

What is polyploidy?

Answer 8

Extra whole sets of chromosomes (e.g. triploid).

Question 9

Is polyploidy common in animals and plants?

Answer 9

Rare in animals, common in plants.

Question 10

What is a wild type banana’s karyotype?

… And a cultivated seedless banana’s karyotype?

Answer 10

Wild = 2n=22.
Cultivated = 3n=33.

Question 11

What is aneuploidy?

Answer 11

Some additional or missing chromosomes.

Question 12

What is monosomy and trisomy?

Answer 12

Monosomy = 2n-1.
Trisomy = 2n+1.

Question 13

How would trisomy 18 be written, assuming a woman?

Answer 13

47,XX,+18.

Question 14

What is the cause of aneuploidy?

Answer 14

Non-disjunction of homologous chromosomes in Meiosis I, and of sister chromatids in meiosis II.

Question 15

How common is aneuploidy in human conceptions, and what does it typically lead to?

Answer 15

50%.

Usually leads to embryonic death or spontaneous abortion.

Question 16

What is the only autosomal aneuploidy permitting survival to adulthood?

Answer 16

Down’s syndrome.

Question 17

Are aneuploidies of the sex chromosomes more common than Down’s?

Answer 17

Yes.

Question 18

What is Trisomy 21?

Answer 18

Down’s syndrome.

Question 19

5 characteristics of Down’s?

Answer 19

Characteristic facial features.
Short.
Sterile.
Mental retardation.
Heart defects

Question 20

What diseases are Down’s syndrome affected people more susceptible to?

Answer 20

Leukaemia and Alzheimer’s.

Question 21

What is the chance of producing a Down’s baby at 40?

Answer 21

1 in 100.

Question 22

How do you screen for chromosomal abnormalities in foetuses with suspected Down’s?

Answer 22

Amniocentesis and karyotyping.
Blood tests to detect proteins associated with a Down’s foetus.
Ultrasound scans to measure size of nuchal pad.

Question 23

What is the only viable monosomy, and how is it written?

Answer 23

Turner’s syndrome.

45,XO.

Question 24

How common is Turner’s?

Answer 24

1 in 2500 births.

Question 25

4 characteristics of Turner’s?

Answer 25

Phenotypically female.
Sterile - lack of maturation of sex organs.
Normal intelligence.
Oestrogen replacement therapy leads to development of secondary sex characteristics.

Question 26

What is 47, XXY?

How common is it?

Answer 26

Klinefelter’s syndrome.

1:500 - 1000 live males.

Question 27

5 characteristics of Klinefelter’s?

Answer 27

Male, but some female characteristics.
Tall.
Sterile.
May have sub-normal intelligence.
Treated with HRT - testosterone.

Question 28

What determines gender?

Answer 28

Presence of absence of Y.

Question 29

4 types of chromosomal mutation?

Answer 29

Deletion, Duplication, Inversion, Reciprocal translocation.

Question 30

What can be given an address on a chromosome?

Also, what do p and q refer to?

Answer 30

Each G band.
p = short arm.
q = long arm.

Question 31

2 diseases arising from deletions in chromosome structure? (and which part of which chromosome)

Answer 31

Cri-du-chat syndrome. ()

Prader-Willi syndrome/Angelman syndrome. (15q1.12)

Question 32

How common is Cri-du-chat?

Answer 32

1 in 50,000.

Question 33

5 characteristics of Cri-du-Chat?

Answer 33

Babies cat-like cry.
Defects in glottis and larynx.
Wide face with saddle nose.
Physical and mental retardation.
Ranges in severity, depending on extent of condition.

Question 34

How common is Prader-Willi?

Answer 34

1 in 15,000.

Question 35

6 symptoms of Prader-Willi?

Answer 35

Poor suckling reflex in infants.
Uncontrollable eating in later life.
Obesity.
Diabetes.
Poor sexual development in males.
Only occurs when affected chromosome inherited from father.

Question 36

How does Angelman’s differ from Prader-Willi genomically?

Answer 36

15q1. 12 missing from maternal chr. 15 - Angelman’s

15q1. 12 missing from paternal chr. 15 - Prader-Willi.

Question 37

2 issues caused by Translocations? (and which chromosomes?)

Answer 37

Familial Down’s syndrome t(14;21).

Chronic myelotic leukaemia (CML) t(22;9).

Question 38

Proportionately, how common is Down’s syndrome?

Also, what is the chance of producing a second affected child?

Answer 38

5%.

1 in 3.

Question 39

Is CML heritable?

How common is it?

Answer 39

No, spontaneous.

1 in 50,000 to 100,000.

Question 40

What chromosome do 95% of people with CML have?

What gene is produced with the formation of this chromosome, and what does this gene do?

Answer 40

Philadelphia chromosome.

BCR-ABL fusion gene - oncogene, stimulating overproduction of WBCs.

Question 41

At what age range is CML most common?

Answer 41

Middle aged/elderly.

Question 42

What percentage of all cases of Leukaemia does CML account for?

Answer 42

15-20%.