F组染色体异常特别是20-三体的发生率虽然不高,其临床症状尚未确立,但全部有智力低下.另可见小头,两眼间距宽、斜视,鼻根低下,小颌,耳壳异常,脐疝,腹股沟疝等形态异常外,也可合并严重的心血管异常,肺及其他内脏畸形.20-三体的嵌合体表现型可正常,也可有严重异常.Djalali等指出如培养的羊水细胞见50%以上的三体型时,60%以上表现型异常.即使在显带染色发达的今天,仅用细胞遗传学的方 F group of chromosomal abnormalities, especially the incidence of 20-trisomy, although not high, the clinical symptoms have not been established, but all have mental retardation.Other visible small head, two wide eyes, strabismus, nasal low, small jaw, concha Abnormalities, umbilical hernia, inguinal hernia and other morphological abnormalities, but also may be associated with severe cardiovascular abnormalities, lung and other visceral abnormalities .20-trisomy chimeric phenotype may be normal, there may be severe abnormalities .Djalali pointed out that such as culture Of amniotic fluid cells see more than 50% of the trisomy, more than 60% of phenotypic anomalies even in the development of banding developed today, only cytogenetics