Cytogenetics 1 Flashcards
Earliest you can get amniocytes from amniotic fluid for gross chromosome morphology studies
16-20 weeks gestation
to early risks maternal cell contamination
What can be done if you want to test prenatal cytogenetics before 16-20 weeks?
Chorionic villus sampling at 9-11 weeks
Abnormal chromosome dosage in a cell
Aneupoidy (trisomy and monosomy; partial addition or loss of matrial)
One or more complete sets of chromosomes
Euploidy (diploid, triploid, tetraploid; multiples of 23)
Mechanism and probability for polyploidy (triploid)
3 % of pregnancies
Dispermy (66%)
Meiotic failure (gamete stayed at 2n rather than 1n)
- Spermatogenesis (24%)
- oogenesis (10%)
Endomitosis
Failure to complete first zygotic division. (DNA replicated to become 4C, but cell division did not take place).
Very rare, always lethal results in tetraploidy.
Additional sex chromosomes
47 xxx
47 xxy
47 xyy
All experience relatively minor problems and live normal lives
Lacking sex chromosomes
45y is never viable
45x is turner syndrome
- 99% abort
Turners that survive:
- Normal intelligence
- Infertile
- Minor physical diagnostic characteristics
Autosomal aneuploidy
Nullisomy (missing pair of homologs), monosomy (missing one chromosome), and trisomy are usually lethal (except 13, 18, 21).
Structural Abnormalities: Unbalanced Chromosomal Rearrangements
Most often associated with:
- inversions
- translocations (reciprocal, robertsonian)
- insertions
Usually a normal phenotype but pose a risk to future generations because crossing over is not aligned correctly
Structural Abnormalities: Balanced Chromosomal Rearrangements
Most have normal phenotype unless the breakpoints disrupt a gene or some region of sequence gets deleted during the process of breaking and rejoining (becomes unbalanced)
Reciprocal Translocation Consequences for Gamete Formation
Pairing of translocation products to form quadrivalents
Major segregation products of meiosis are:
- alternate (normal/balanced)
- adjacent 1 (two unbalanced)
- adjacent 2
Robertsonian Translocation
Translocation between two acrocentric chromosomes resulting in loss of the short arms of both chromosomes, but does not affect the DNA content of long arms
During meiosis form trivalent configuration
- seg products produces normal/balanced gametes
- monsomies and trisomies
Structural Abnormalities: Inversions
Paracentric:
50% balanced
50% inviable (one acentric and other dicentric)
Pericentric:
50% balanced
50% unbalanced