How common are variations in the number of X and/or Y chromosomes?
It is estimated that between 10% and 30% of all fertilized human eggs have either more or less than 46 chromosomes, resulting in approximately one-third of all miscarriages, and in turn the leading known cause of pregnancy loss. (3) Most people have heard about Downs syndrome, which results from an extra copy of chromosome 21, and occurs in approximately 1 out of 1000 live births, or one of several other chromosomal variations which are even less common, occurring in as few as 1 out of 10,000 births. Ironically, although far less well known – probably because they are the least severe and have little or no dysmorphism – the most common chromosomal variations in full-term births occur on the X and/or Y chromosomes. For instance, with respect to the X chromosome, Turner syndrome refers to females who are born with only 45 chromosomes. Girls who are born with an extra X chromosome are referred to as Triple X or Trisomy X. Boys, who are born with an extra X chromosome are 47,XXY and at ris
