General Medical Information

Isochromosomes for Xq

Joe Leigh Simpson

Division of the centromere in the transverse rather than the longitudinal plane results in an isochromosome, a metacentric chromosome consisting of isologous arms. Both arms are structurally identical and contain the same genes. An isochromosome for the X long arm [i(Xq)] differs from a terminal deletion of Xp in that not just the terminal portion but all of the Xp is deleted. In reality, many isochromosomes for Xq are isodicentrics, the clinical significance of which is that a minute portion of Xp is duplicated and retained in addition to duplication of the entire Xq. An isochromosome for the X long arm is the most common X-structural abnormality, but coexisting 45,X cell lines (mosaicism) are typical. Nonmosaic cases are relatively uncommon.

46,X,i(Xq) individuals almost always have streak gonads and primary amenorrhea. Occasionally menstruation is observed, but surveys continue to agree with those published by Simpson over 25 years ago in showing rarity of menstruation. The near-complete lack of gonadal development in 46,X,i(Xq) contrasts to that in 46,X, del(X)(p11) individuals, about half of whom menstruate or develop breasts.

Almost all reported 46,X,i(Xq) patients are short. Their mean height seems to be less than in 45,X. The mean height of nonmosaic 46,X,i(Xq) patients is 136 cm, and many somatic features of the Turner’s stigmata are observed. Somatic anomalies occur as frequently in 46,X,i(Xq) as in 45,X, and the spectrum of anomalies associated with the two complements generally is similar.