deletion of the proximal short arm of chromosome 12 is rare and occurs de novo. Microcephaly, narrow forehead, pointed nose and micrognathia are present. Mental and growth retardation are significant but inner organ malformations are generally not present. 12p trisomy nearly always results from a familial translocation. The phenotype includes turricephaly with flat apex, high bulging forehead, flat rectangular face, pronounced hypertelorism, a very short nose with a broad and poorly defined bridge, a short neck with cutaneous folds, ear abnormalities, hypotonia, severe growth and mental retardation and signs of precocious aging in adolescents. Tetrasomy 12p is consistent with pallister-killian syndrome. The critical region appears to be confined to 12p11.2.
12q2 trisomy is uncommon and results most frequently from malsegregation of a parental translocation. The patients show a relatively large head with frontal bossing, rectangular face with chubby cheeks and short limbs, especially in the proximal segment. Mental retardation is severe and 62b growth retardation variable. Among others, genes for lactate dehydrogenase B, phenylalanine hydroxylase and haemolytic anaemia due to glyceraldehyde -3-phosphate dehydrogenase deficiency are assigned to chromosome 12.