Cytogenetics Flashcards
Metacentric chromosome
Centromere is in the middle p=q
Submetacentric chromosome
Off-center centromere
Acrocentric chromosome
Centromere near the end (ends are called “satellite DNA” on p arm
Short arm
p
Long arm
q
What is euploidy?
abnormal number of copies of all chromosomes
What is triploidy?
3n
- usually SAB in 1st trimester
- 17% of SABs
- Usually 2 sperm cells fertilize 1 egg
What is tetraploidy?
4n
few cases, nonviable
1-2% of SABs
Possible due to failure of cells to cleave during division
What is the incidence of Down Syndrome?
1 in 185 births
What causes Down Syndrome?
95% T21 (90% maternal meiosis I, 10% paternal meiosis II)
4% Robertsomian translocation
Usually with 14 or 22
Rare: 21q21q translocation - isochromosome
Rare: partial T21 (part of q arm)
Rare: mosaic T21
What are the psuedoautosomal regions on the X and Y chromosomes?
Regions on X and Y that exhibit autosomal inheritance
Site of recombination - failure can lead to nondisjunction
What is uniparental disomy (UPD), and what types are there?
Both chromosomes from the same parent
Isodisomy: Chromosomes are identical and inherited from sister chromatids
Heterodisomy: inheritance of both homologs from one parent
This often happens from trisomy rescue
What is a marker chromosome? What are the most common types?
small chromosomes in addition to the normal amount
Extra chromosome 15
Ring chromosomes - marker chromosome with no telomeres (usually mosaic)
What is an isochromosome?
one arm is duplicated → mirrored chromosome
Opposite arm may be deleted
Trisomy of duplicated arm, monosomy of deleted arm
What is a Robertsonian translocation?
Most common rearrangement in humans
Fusion of two acrocentric chromosomes near the centromere
Loss of short arms is fine - little genetic material