The Principles of Clinical Cytogenetics - Extra Materials - Springer

Autosomal Aneuploidy 149
21 and indicating that complete monosomy 21 is almost always incompatible with life. Most patients
died before 2 years of age, although one male child survived to 11 years. The phenotypic features
were similar to those observed in the mosaics and included intrauterine growth retardation, postnatal
growth and mental deficiencies, microcephaly, hypertelorism with downward slanting palpebral fissures,
large low-set ears, prominent nose, cleft lip/palate, and micrognathia. Abnormals muscle tone,
mostly hypertonia, was common. Cardiac anomalies were present in a few cases.
Monosomy 22
Four cases of mosaic monosomy 22 in liveborns have been reported (171–174). All four were
male. One was a 34-week premature infant with gastroschisis who died from intracranial hemorrhage
shortly after birth. No dysmorphic features were noted, and an autopsy was not performed (173). Two
patients had growth and developmental deficiencies, microcephaly, and mild facial dysmorphism. The
fourth patient was a 30-week premature infant with facial features of DiGeorge syndrome, hypertonicity,
limited extension of major joints, and flexion contractures of all fingers.
POLYPLOIDY
Fig. 9. Karyotype of a triploid fetus (69,XXX).
Polyploidies are numerical chromosome abnormalities with changes in the number of complete
sets of chromosomes. They are usually incompatible with fetal survival and are extremely rare in
liveborns.
Triploidy
The chromosome number in triploidy is 3n=69 (see Fig. 9). It is estimated to occur in approximately
1% of all human conceptions and is found in 17–18% of all chromosomally abnormal abortuses
(175,176). Only very rarely do triploid conceptuses survive to term. Two distinct phenotypes
have been recognized (177). One type presents as a relatively well-grown fetus with or without