Meiotic Abnormalities: Abnormal Numbers of Chromosomes

摘要: A neuploidy is the loss or gain of individual chromosomes. It can be result nondisjunction in a premeiotic mitotic division germline either parent, first second meiotic an early embryonic (postzygotic) affected individual. Nondisjunction refers to any process that causes two homologous chromosomes go same pole instead segregating opposite poles. Some aberrations leading are described Table 11.1 and illustrated Fig. 11.1. When fail pair form chiasmata homologues fall apart appear as univalents diplotene. Univalents may drift at random poles divide regularly second. Alternatively, they mitotically anaphase I II pole, rarely, one misdivide centromere, just might division. Only small segment XY bivalent forms synaptonemal complex which crossing over takes place (Fig. 17.2). Thus, X Y remain much more often than even smallest autosome pair. Frequencies vary among different individuals, but mean frequency unpaired sex male about 11 per cent (Laurie Hulten, 1985). Multiple aneuploidy several very uncommon, except for Polyploidy (triploidy tetraploidy) whole sets chromosomes. Table 11.1. Principal Meiotic Events Outcomes Their Failures Stage Meiotic events Results unsuccessful completion events Leptotene Chromosomes become visible; lateral elements begin form Germ cell degeneration; sometimes nondisjunction Zygotene Chromosomes bouquet; each chromosome pairs with its element; unite into complex, completes pairing Germ nondisjunction Pachytene, early Recombination nodules attach central elements No over; univalent Pachytene, late During over, recombination change bars Because lack chiasmata, bivalents univalents Diplotene Homologues repel other until held together only chiasmata More visible earlier stages Metaphase I, Anaphase Metaphase II, II Orderly segregation prerequisite regular gametogenesis Univalents undergo nondisjunction, loss, misdivision, spindle abnormalities interfere