Abnormalities of chromosome number in humans

In humans, the following autosomal trisomies
may occur in liveborn infants: trisomy 13with a
frequency of 1 in 5000 newborns; trisomy 18
with 1 in 3000; and trisomy 21 with about 1 in
650 newborns (1). Additional X or Y chromosomes
occur in about 1 in 800 newborns, much
more frequently than the autosomal trisomies
(2). But unlike the autosomal trisomies, they
usually do not lead to defined clinical pictures.
Triple X (47,XXX) or an additional Y chromosome
(47,XYY) are generally not clinically apparent.
On the other hand, monosomy X (3)
leads to the clinical picture of Turner syndrome