tetrasomy 9p tetrasomy 9p tetrasomy 9p (also known tetrasomy 9p syndrome) is a rare chromosomal disorder characterized by the presence of two extra copies of the short arm of chromosome 9 (called the p arm), in addition to the usual two. symptoms of tetrasomy 9p vary widely among affected individuals but typically include varying degrees of delayed growth, abnormal facial features and intellectual disability. symptoms of the disorder are comparable to those of trisomy 9p.