What are the results of nondisjunction, and why are these chromosome alterations

Nondisjunction can occur when homologous chromosomes fail to separate in meiosis I or when sister chromatids fail to separate in meiosis II. This can result in either too few or too many chromosomes in the gametes. If such a gamete unites with a normal gamete during fertilization, then the zygote will have an excess or deficit of chromosomes. An example of an excess number of autosomal chromosomes is Down syndrome (Trisomy 21). When nondisjunction of sex chromosomes occurs, gametes end up with too many sex chromosomes or none at all. When a gamete with an abnormal number of sex chromosomes unites with a normal gamete at fertilization, the resulting zygote has an abnormal number of sex chromosomes. Examples include Klinefelter syndrome (XXY) and Turner syndrome (XO). These are all detrimental and the effects will vary depending on which chromosome is missing or which is extra, but all seem to be problematic because of the extra genetic material or because of the missing genes.