L19/L20: Chromosome Anomalies

Question 1

What is aneuploidy?

What is polyploidy?

Answer 1

Abnormal number of chromosomes

More than the normal number of chromosomes

Question 2

What is the difference between trisomy and triploidy?

Answer 2

Trisomy - three of one autosome e.g. trisomy 21

Triploidy - three of every autosome

Question 3

What are the 2 types of translocations?

Answer 3

robertsonian

reciprocal

Question 4

Describe a robertsonian translocation

Answer 4

Involved any of the acrocentric chromosomes (13, 14, 15, 21, 22)
Breakage of two afrocentric chromosomes at or close to their centromeres, with subsequent fusion of their long arms - short arms are lost
Occurs due to error during chromosome pairing in meiosis
Usually inherited from a parent

Question 5

Describe a reciprocal translocation

Answer 5

Breakage of two non-homologous chromosomes with exchange of the fragments

Question 6

In Triploidy what differences does it make if the extra chromosome is a maternal or paternal one?

Answer 6

XXY = small underdeveloped placenta, large baby 
XYY = large cystic placenta, small baby (more lethal)

Question 7

What are these conditions called?

• 47,XY,+21
• 69,XXY
• 47,XXX
• 45,X
• 47,XXY

Answer 7

• trisomy 21, down’s syndrome
• triploidy,
• triple X syndrome
• turners syndrome
• kleinfelters syndrome

Question 8

What is gonadal mosaicism?

Answer 8

Two poplulations of cells in the gonads
Very rare
Suggested by recurrence of otherwise sporadic event

Question 9

What are the 3 different patterns of chromosomes that can cause Down Syndrome?

Answer 9

• 95% trisomy 21
• 4% Robertsonion translocation
• 1% Mosaicism, milder features

Question 10

What is genomic imprinting?

Answer 10

Some human genes behave differently depending on their parental origin
They must carry some sort of ‘imprint’ - epigenetic mechanism (possibly methylation)

Question 11

What are the acrocentric chromosomes?

Answer 11

13, 14, 15, 21, 22

They have a very small short arm and the centromere is at the top

Question 12

How can chromosomes be visualised?

Answer 12

Karyotype is prepared from peripheral lymphocytes.
Only visible when chromatin is condensed, so need to freeze at metaphase.
Chromosomes paired and arranged.

Question 13

What is a telomere?

Answer 13

DNA and protein cap, ensures replication to tip, tether to nuclear membrane

Question 14

What are the light bands and dark bands visible on chromosmes?

Answer 14

light bands
- replicate early in S phase
- less condensed chromatin
- transcriptionally active
- gene and GC rich
dark bands
- replicate late
- contain condensed chromatin
- AT rich
- less transcriptionally active

Question 15

What is a centromere?

Answer 15

joins sister chromatids
essential for chromosome segregation at cell division
point at which chromatics attach to mitotic spindle

Question 16

Which arm is q/p/

Answer 16

p = short arm
q = long arm

Question 17

What are the most frequent numerical abormalities in live-born? What are their karyotypes?

Answer 17

Down syndrome: 47,XX,+21
Edwards syndrome: 47,XX,+18
Patau syndrome: 47,XX,+13
Turner syndrome: 45,X
Klienfelter syndrome: 47,XXY
Triplpoidy: 69 chromosomes

Question 18

What are the clinical features of down syndrome?

Answer 18

facial appearance
marked hypotonia as newborn
single palmar crease
mental handicap
congential heart malformations

Question 19

How does having three copies of the genes on chromosome 21 cause features of downs?

Answer 19

gene dosage effect:
- features caused by 1.5x gene product
amplified developmental instability
- features causes by overall effect of imbalance on development

Question 20

What the differences in chromosome numbers of gametes if nondisjunction occurs during meiosis 1 and meiosis 2?

Answer 20

meiosis 1
- 2 disomic, 2 nullisomic
meiosis 2
- 1 disomic, 1 nullisomic, 2 monosomic (normal)

Question 21

What are the lineal features of edwards, pataus, klinefelter, and turner syndrome?

Answer 21

edwards: multiple malformations, heart and kidneys, clenched hands with overlapping fingers, cleft palate
patau’s: midline malformations, cleft lip, incomplete lobation of brain, congenital heart disease
Klinefelter: infertility, atrophic testes do not produce sperm, poorly developed secondary sexual characteristics, tall
Turner: short stature, primary ammenhorea, streak ovaries, coarctation of aorta

Question 22

What is somatic mosaicism?

Answer 22

2 populations of cells with a chromasomal anomaly in the body due to a mutation that occurs during mitosis