The most common polysomy involving the sex chromosomes is a disorder known as Klinefelter’s syndrome. Klinefelter’s results from the non-disjunction of one or more X chromomosomes during meiosis leading to an abnormal karyotype (chromosome number or structure). The majority of non-disjunction events are believed to occur during oocyte formation; however, polyploidy may also occur during spermatogenesis but tends to result in monosomies, especially Turner’s Syndrome (karyotype 45 XO).
Over 80% of individuals with Klinefelter’s have a chromosomal karyotype of 47 XXY, as opposed to the normal male karyotype of 46 XY. Less often, polysomies including XXXY and XXYY are observed in association with Klinefelter’s. All polysomies in which a Y chromosome is present result in the development of male reproductive tract structures exclusively. Scientists think that even a single copy of certain Y chromosome genes such as AMH (Anti-Mullerian Hormone) is sufficient to inhibit the formation of female gonads.
Most of the time, Klinefelter’s produces few distinct anatomical symptoms before puberty. By adolescence, however, males with Klinefelter’s tend to be over the 90th percentile in height and begin to develop several feminine features including a broad pelvic diameter, lack of facial hair, and gynecomastia (growth of breast tissue). In most cases, males with Klinefelter’s are infertile owing to testicular atrophy which leads to the production of few or no sperm.
In contrast to Kleinfelter’s, the karyotype 47 XYY (an extra Y chromosome) is not associated with any specific anatomic abnormalities. This polysomy arises exclusively during spermatogenesis, since oocytes carry only X chromosomes. This condition may occur in as many as one in 1,000 live male births; however, since 47 XYY males appear developmentally normal and can father children, most cases probably go undetected. The 47 XYY karyotype is associated with a higher incidence of learning disabilities and autism spectrum disorders.
The polysomy XXX results in an anatomically normal female. The 47 XXX karyotype may occur in as many as one in 1,000 live female births; however as with 47 XYY, most cases go undetected. A small percentage of women with 47 XXX experience premature ovarian failure, marked by irregular menses and early menopause (prior to age 40).
As in autosomal trisomies such as Down’s Syndrome, advanced maternal age is a risk factor for Klinefelter’s and 47 XXX. Advanced paternal age may also increase the risk of chromosomal non-disjunction; however, in the case of polysomy 47 XYY, most instances are believed to arise as a sporadic germline event independent of a man’s age.