About 1/1000 apparently normal females has a 47,XXX (trisomy X) karyotype. Physical anomalies are rare. Menstrual irregularity and infertility sometimes occur. Affected girls may have mildly impaired intellect and may have more school problems than siblings. Advanced maternal age increases risk of trisomy X, and the extra X chromosome is usually maternally derived.
Although rare, 48,XXXX and 49,XXXXX females exist. There is no consistent phenotype. The risk of intellectual disability Intellectual Disability Intellectual disability is characterized by significantly subaverage intellectual functioning (often expressed as an intelligence quotient < 70 to 75) combined with limitations of adaptive... read more and congenital anomalies increases markedly when there are > 3 X chromosomes. The genetic imbalance in early embryonic life may cause anomalous development.
(See also Overview of Chromosomal Anomalies Overview of Chromosomal Anomalies Chromosomal anomalies cause various disorders. Anomalies that affect autosomes (the 22 paired chromosomes that are alike in males and females) are more common than those that affect sex chromosomes... read more and see Overview of Sex Chromosome Anomalies Overview of Sex Chromosome Anomalies Sex chromosome anomalies may involve aneuploidy, partial deletions or duplications of sex chromosomes, or mosaicism. (See also Overview of Chromosomal Anomalies.) Sex chromosome anomalies are... read more .)
