Chromosome 9, trisomy 9p is a rare chromosomal syndrome in which a portion of the 9th chromosome appears three times (trisomy) rather than twice in cells of the body. Chromosomes are found in the nucleus of all body cells. They carry the genetic characteristics of each individual. Pairs of human chromosomes are numbered from 1 through 22, with an unequal 23rd pair of X and Y chromosomes for males and two X chromosomes for females. Each chromosome has a short arm designated as "p," a long arm identified by the letter "q," and a narrowed region at which the two arms are joined (centromere). Chromosomes are further subdivided into bands that are numbered outward from the centromere. For example, the short arm of chromosome 9 includes bands 9p11 to 9p24, and the long arm includes bands 9q11 to 9q34.
In trisomy 9p, the trisomy (or duplicated material) may involve a portion of the short arm (9p), the entire short arm, or the short arm and a portion of the long arm (9q) of chromosome 9. Evidence suggests that, in many cases, associated symptoms and findings may be relatively similar among affected infants despite differing lengths of the trisomic (duplicated) segment of 9p. However, in those with larger trisomies (e.g., extending to middle or end [distal] regions of 9q), additional features may also be present that appear to correlate with the extent of the duplication. Additionally, certain individuals with duplications of specific areas of chromosome 9p have developed any symptoms or only very mild or subtle symptoms.
Children with Trisomy 9p are affected by delays in reaching developmental milestones like crawling or walking (developmental delays), growth deficiency, and distinctive malformations of the skull and facial (craniofacial) region. As children grow older, intellectual disability may become apparent.
In some instances, additional physical abnormalities may also be present, such as other skeletal defects and/or structural malformations of the heart that are present at birth (congenital heart defects). In some cases, the trisomy appears to result from a balanced chromosomal rearrangement in one of the parents; in others, it is thought to arise from spontaneous (de novo) errors very early in embryonic development that occur for unknown reasons (sporadically).
The chromosomal abnormality was originally reported in the medical literature in1970. Trisomy 9p was first proposed as a distinct syndrome with characteristic symptoms and findings in 1975.