The proper development and eventual function of the gonad depends on the presence of germ cells, the appropriate sex chromosome constitution, and appropriate gonadal ridge somatic cells. Errors in meiotic division can cause aneuploidy and abnormal sex chromosomes. These occur by nondisjunction, anaphase lag, translocation, breakage, rearrangements, or deletions. Mitosis can also be marred by nondisjunction and anaphase lag leading to mosaicism. Two or any more different cell lines can persist and appear in different tissues. Finally, abnormal gonadogenesis may occur as a result of structural or disease related catastrophes leading to loss of fetal gonadal function.
Bilateral Dysgenesis of the Testes (Swyer Syndrome)

Affected individuals have an XY karyotype but normal (infantile) female external and internal genitalia. There are fibrous bands in place of the gonads yielding primary amenorrhea and lack of secondary sexual development at puberty. It is a matter of prudent practice to avoid the possibility of virilization or neoplasm; therefore, removal of these band areas is advocated as soon as the diagnosis is made. Presumably, testes failureed to develop or were eliminated (testicular regression) before internal or external genital differentiation. Estrogen and progestin sequential therapy helps female secondary sex development.