Genetic disorders that cannot be inherited are caused by mechanical errors in meiosis. Sometimes, sister chromatids do not separate during anaphase I. This is called nondisjunction, which causes multiple sister chromatids to stay in one daughter cell, and fewer to stay in the other. The condition of a gamete having more chromosomes than normal is called polysomy. When a gamete has only one copy of a chromosome, it is called monosomy. If fertilization occurs with aberrant gametes, it is called aneuploidy.
Turner’s syndrome, the only total monosomy which can be survived, involves monosomy of the X chromosome in women. Women with Turner’s syndrome are unable to have children, and some have an unusual abnormality called neck webbing. Cri du chat (French for cry of the cat) syndrome, a partial monosomy of chromosome 5, also known as deletion, is named for the malformation of the larynx that causes afflicted babies to cry like meowing cats. One of the more well known chromosomal disorders is Down’s syndrome. Down’s syndrome occurs when a gamete is fertilized with three copies of chromosome 21. Three copies of one chromosome in one gamete is called trisomy. One in one thousand girls are trisomic for the X chromosome. Unlike other chromosomal conditions, women with an extra X chromosome don’t exhibit any abnormalities.