2. Cytogenetics Flashcards
Cytogenetics
The study of chromosome number, structure and function
Chromsome territory model
If chromosomes occupied distinct territories, localised damage would affect only a small subset of chromosomes
Random organisation model
If the chromatin fibres of each chromosome were randomly distributed throughout the nucleus many chromosomes would be damaged
Structural chromosome defects grouped on the basis of..
- Number of chromosome breakages
- Number of chromosomes involved
Deletions: What is it and what is the result
- Occurs when a chromosome breaks and a piece is lost
- Results in partial monosomy for that chromosome
Types of deletions
Terminal and interstitial
Example of disease where terminal deletion has occurred
Cri-du-chat (5p-), partial deletion of short arm on chromosome 5
Example of disease where interstitial deletion has occurred
Williams syndrome, deletion of elastin genes on long arm of chromosome 7
Translocation
Chromosome break and the fragments rejoin to other chromosomes
Loss of genetic material during translocation?
No, but breakpoint can disrupt a critical gene or juxtapose pieces of two genes to create a fusion gene
Translocations cause reductions in fertility?
Yes
Types of translocations
Reciprocal, Robertsonian (centric fusion), insertional
Reciprocal translocation
Exchange of material between nonhomologous chromosomes
Robertsonian translocation
Takes place between acrocentric chromosomes,. Break takes place at or near centromere
Inversion
Occur after a breakage -> chromosome fragment is inverted and rejoined (not lost!)
Types of inversions
- Paracentric (centromere. not involved)
- Pericentric (centromere involved)
Insertion
Insertion of one part of a chromosome into another chromosome
Ring chromosome
A break removes both telomeres, repaired by creating a ring
Isochromosomes
Split the wrong way in mitosis or meiosis 2: The chromosome consists of two copies of either the long (q) arm or the short (p) arm.
Prognosis of isochromosome
Poor, except for iXq (isochromosome of the long arm of the X)
Euploidy
Normal number of structurally normal chromosomes
Aneuploidy
Less or more than normal diploid number
Monosomy
Lack of one pair of chromosomes
Trisomy
A specific instance of polysomy (3 of a certain chromosome)
Trisomy 21
Downs syndrome
Trisomy 13
Patau syndrome
Trisomy 18
Edwards syndrome
Triploidy
Three of every chromosome
69 human chrs
Why does triploidy occur?
- Fertilisation by 2 sperms
- Fertilisation by a diploid sperm
- Fertilisation of a diploid ovum
Only one sex chromosome: X
Turner syndrome
Chromosomes XXY
Klinefelter syndrome
Chromosomes XYY
Jacobs syndrome
Uniparental disomy
When a person receives two copies of a chromosome, or of part of a chromosome, from one parent and no copy from the other parent.
Mosaic
An individual with more than one cytogenetically distinct population of cells with the same origin
In pregnancy; microchimerism
The presence of a small number of cells that originates from another individual and are therefore genetically distinct from the cells of the host individual
How frequent are chromosomal abnormalities?
1/200 newborn children
