Chromosomal abnormality

Three copies of chromosome 21

1. Non-disjunction (esp mothers > 40 years)
   • Non-disjunction of a gamete: one progeny gets TWO chromosomes (and the other one gets none) which is then fertilised by a normal gamete from the other parent enduing up with THREE copies of chromosomes.
   • Usually maternally derived extra copy
2. Translocation (rare)
   • Parents phenotypically normal but have a balanced translocation
   • Chr 23 +Chr 23 / Ab Cr 22; get's one Chr 23 from mum, one Chr23 from dad and AbCr22+Down's shit
3. Mosaism
   • 1/100 cases
   • Normal ovum
   • Non-disjunction after the blastocysts starts to develop

Associations

• Congenital heart defects: PDA, VSD, ASD
• Neoplasia
• Increased incidence glue ear