Aneuploidy is an error in cell division that results in the "daughter" cells having the wrong number of chromosomes. In some cases there is a missing chromosome, while in others an extra.
Monosomy (2n-1)
This state occurs when an individual has only 1 of a particular type of chromosome. Turner syndrome (XO) is a chromosomal condition that affects development in females. The most common feature of Turner syndrome is short stature, which becomes evident by about age 5. An early loss of ovarian function is also very common. The ovaries develop normally at first, but egg cells usually die prematurely and most ovarian tissue degenerates before birth. Many affected girls do not undergo puberty unless they receive hormone therapy, and most are unable to conceive (infertile). A small percentage of females with Turner syndrome retain normal ovarian function through young adulthood.
Trisomy (2n+1)
Extra copies of the autosomal chromosomes also usually results in death. One extra copy is called "trisomy." Trisomy (2n+1) is a condition that occurs when an individual has an extra chromosome in a particular chromosome set. Trisomy of three of the smallest chromosomes (numbers 13, 18, 21) usually doesn't result in fetal death, but instead results in severe birth defects and in most cases, early childhood death. They can survive to adulthood, but have significant health problems and developmental delays and are almost always mentally retarded.
Extra copies of the sex chromosomes also cause developmental errors, although the effects are not fatal. Cases that result in an embryo with two X and one Y chromosome (XXY) develop Klinefelter syndrome: a sterile male with many female body characteristics and, in some cases, mental retardation. An individual with three X chromosomes (XXX) develops into a sterile female (Poly-X females). Extra copies of the Y chromosome (XYY) result in outwardly normal males (Jacob Syndrome). There is some statistical indication that XYY men are more likely to wind up in jail, suggesting an impact on neuro-cognitive development.
Human Karyotype of Trisomy in the Autosomes
Patau Syndrome
Edward Syndrome
Down Syndrome
Human Karyotype of Trisomy in the Sex Chromosomes