It is one of the most fre quently occurring chromosomal aberrations with minimal abnormalities visible at birth, which become more apparent at the age of three years. The phenotype is marked mainly by holoprosencephaly, brachycephaly, broad facies, blepharoptosis, downturned corners of the mouth, tooth abnormalities, broad neck with low posterior hairline, funnel chest, enlarged labia majora, hand abnormalities, mental retardation ranging from mild to severe, and other malformations. Blepharoptosis, glaucoma, coloboma, and occasional microphthalmia, hypertelorism, downslanting or upslanting palpebral fissures, strabismus, nystagmus, microphthalmia, cataracts, and cyclopia. Broad nose, epicanthal folds, and choanal stenosis. Funnel chest and rib anomalies.
Simian creases occur are the occasional dermatoglyphic abnormalities.
Coxa vara and hip dislocation and occasional leg asymmetry. Scoliosis, kyphosis, and vertebral anomalies. Male genital hypoplasia, hypospadias, and enlarged labia majora. Autoimmune disorders and immunoglobulin A deficiency. Most cases are de novo deletions and many other are due to de novo translocation between the long arms of an acrocentric and long arm of chromosome 18.