General Medical Information

Cytogenetic Forms

Joe Leigh Simpson

45,X/46,XY individuals have both a 45,X cell line and at least one cell line containing a Y chromosome. Based on cohort studies of 45,X/46,XY cases detected without bias in utero (prenatal genetic diagnosis), well over 90% of cases are normal males. However, cases ascertained postnatally differ, manifesting a variety of phenotypes: almost normal males with cryptorchidism or penile hypospadias, and genital ambiguity females indistinguishable from those with 45,X Turner’s syndrome. Different phenotypes presumably reflect different tissue distributions of the various cell lines. A structurally abnormal Y chromosome may be present. Because many structurally abnormal chromosomes (e.g., dicentric) are unstable, it is likely that the 45,X line arises secondarily after loss of the structurally abnormal Y.