Aneuploidy is a numerical chromosomal disorder originated from nondisjunction during cell division (Dellarco 1985). Aneuploid cells exhibit altered chromosome numbers varying from nulisomy (absence of a chromosome pair), monosomy (only one member of a chromosome pair), trisomy (three homologous chromosomes instead of two), tetrasomy (four homologous chromosomes instead of two), etc. In an aneuploid complement, one or more chromosome pairs can be involved. In human beings aneuploidy is the main cause of pregnant loss, congenital malformation and mental retard (Hook 1983; Epstein 1988). Although the consequences of meiotic non-disjunction are clear, the relation between mitotic non-disjunction and neoplastic transformation susbsists uncertain despite there is increased evidence indicating that it is a significant factor during somatic malignancy development (Kondo 1984; Tsutsui 1983; Cavenee 1983). In addition, aneuploidy is frequently associated with infertility and abortion in domestic livestock (Halnan 1989). <i>(Párrafo extraído del texto a modo de resumen)</i>Instituto de Genética Veterinaria