Normally, during meiosis, the sex cells divide properly so that a zygote either contains an XY or XX chromosome configuration. However, occasionally the cell division will occur when more or less chromosomal material ends up in the sex cells -- so that the cell contains more than one X chromosome or more than one Y chromosome, or only one X chromosome.  Any other configuration is fatal and the embryo will spontaneously abort. However, those individuals with "nonstandard" genetic makeup may have either no or diminished sexual characteristics, or traits of both male and female.  See the link for specific diseases: