A trisomy is a type of polysomy in which there are three instances of a particular chromosome, instead of the normal two.[1] A trisomy is a type of aneuploidy (an abnormal number of chromosomes).

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)
Of 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.

Here we will also see what are the differences between Down’s Syndrome , Edward’s Syndrome and Patau’s Syndrome.
Why Down’s Syndrome is more frequent and why Patau’s Syndrome is very rare ?
Find out in this video.