trisomy
Last reviewed 01/2018
Trisomy is having three copies of a chromosome. It is caused by a failure of disjunction during meiosis, with unequal separation of chromosomes between the gametes. Nearly all types of trisomy are rapidly fatal, causing early abortion. Of the three trisomies which survive to birth, only trisomy 21 - Down's syndrome - tend to survive to older than one year.
Incidence of all trisomies increases with maternal age, suggesting some relationship with the amount of time the oocyte spends in suspended meiosis.