Karyotypes are pictures of homologous chromosomes lined up together during Metaphase I of meiosis. The chromosome micrographs are then arranged by size and pasted onto a sheet. Click here for a larger picture . A gamete lacking a chromosome cannot produce a viable embryo. Ninety-five percent of Down's cases result from nondisjunction of chromosome 21.
Occasional cases result from a translocation in the chromosomes of one parent.
Sex-chromosome abnormalities may also be caused by nondisjunction of one or more sex chromosomes. Males with more than one X are usually underdeveloped and sterile. XXX and XO women are known, although in most cases they are sterile. Human sex chromosome abnormalities. Image from W.H. Freeman and Sinauer Associates, used by permission.