L19/L20: Chromosome Anomalies Flashcards
What is aneuploidy?
What is polyploidy?
Abnormal number of chromosomes
More than the normal number of chromosomes
What is the difference between trisomy and triploidy?
Trisomy - three of one autosome e.g. trisomy 21
Triploidy - three of every autosome
What are the 2 types of translocations?
robertsonian
reciprocal
Describe a robertsonian translocation
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
Describe a reciprocal translocation
Breakage of two non-homologous chromosomes with exchange of the fragments
In Triploidy what differences does it make if the extra chromosome is a maternal or paternal one?
XXY = small underdeveloped placenta, large baby XYY = large cystic placenta, small baby (more lethal)
What are these conditions called?
- 47,XY,+21
- 69,XXY
- 47,XXX
- 45,X
- 47,XXY
- trisomy 21, down’s syndrome
- triploidy,
- triple X syndrome
- turners syndrome
- kleinfelters syndrome
What is gonadal mosaicism?
Two poplulations of cells in the gonads
Very rare
Suggested by recurrence of otherwise sporadic event
What are the 3 different patterns of chromosomes that can cause Down Syndrome?
- 95% trisomy 21
- 4% Robertsonion translocation
- 1% Mosaicism, milder features
What is genomic imprinting?
Some human genes behave differently depending on their parental origin
They must carry some sort of ‘imprint’ - epigenetic mechanism (possibly methylation)
What are the acrocentric chromosomes?
13, 14, 15, 21, 22
They have a very small short arm and the centromere is at the top
How can chromosomes be visualised?
Karyotype is prepared from peripheral lymphocytes.
Only visible when chromatin is condensed, so need to freeze at metaphase.
Chromosomes paired and arranged.
What is a telomere?
DNA and protein cap, ensures replication to tip, tether to nuclear membrane
What are the light bands and dark bands visible on chromosmes?
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
What is a centromere?
joins sister chromatids
essential for chromosome segregation at cell division
point at which chromatics attach to mitotic spindle
Which arm is q/p/
p = short arm q = long arm
What are the most frequent numerical abormalities in live-born? What are their karyotypes?
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
What are the clinical features of down syndrome?
facial appearance marked hypotonia as newborn single palmar crease mental handicap congential heart malformations
How does having three copies of the genes on chromosome 21 cause features of downs?
gene dosage effect:
- features caused by 1.5x gene product
amplified developmental instability
- features causes by overall effect of imbalance on development
What the differences in chromosome numbers of gametes if nondisjunction occurs during meiosis 1 and meiosis 2?
meiosis 1
- 2 disomic, 2 nullisomic
meiosis 2
- 1 disomic, 1 nullisomic, 2 monosomic (normal)
What are the lineal features of edwards, pataus, klinefelter, and turner syndrome?
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
What is somatic mosaicism?
2 populations of cells with a chromasomal anomaly in the body due to a mutation that occurs during mitosis
