A trisomy is caused by due to having one additional chromosome than the normal individual; person having trisomy will be characterized by 47 chromosomes rather than the usual 46. It is a type of polysomy in which there are three copy of a particular chromosome, instead of the normal two. 


In humans chromosome occur in pair form in each cell, with one chromosome inherited from each parent. In such organisms, a process called meiosis creates cells called gametes (eggs or sperms) that have only one set of chromosomes. Humans male have 46 chromosomes (i.e. 23 pairs of chromosomes) and Human female gametes have also 23 chromosomes.

If the chromosome pairs fail to separate properly during cell division, the egg or sperm may end up with a second copy of one of the chromosomes. If such a gamete results in fertilization and an embryo, the resulting embryo may also have an entire copy of the extra chromosome.

The most common types of autosomal trisomy that survive to birth in humans are:

  • Trisomy 21 (Down syndrome)
  • Trisomy 18 (Edwards syndrome)
  • Trisomy 13 (Patau syndrome)
  • Trisomy 9
  • Trisomy 8(Warkany syndrome 2)
  • Trisomy 22

Among these, Trisomy 21 and Trisomy 18 are the most common. In rare cases, a fetus with Trisomy 13 can survive, giving rise to Patau syndrome. Autosomal trisomy can be associated with birth defects, intellectual disability and shortened life expectancy. The chances of having a baby with Downs syndrome increase with increasing age of the parents. The couples in age group 35 plus are also advised to have prenatal diagnosis for Down syndrome.


Trimosy can be diagnosed by conventional cytogenetic analysis or by the PCR amplification of short tandem repeats (STR) analysis. Prenatal DNA testing is advised to patients who already have a child with trisomy.