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Cytogenetics Lab Virtual Tour  :  Examples of Chromosome Abnormalities

Cytogenetics Lab Virtual Tour

Examples of Chromosome Abnormalities


Trisomy 21 (Down syndrome)

example example

Down syndrome is the result of Trisomy 21. The birth prevalence is approximately one in 700 and the prevalence increases with advanced maternal age. Approximately 95% of cases are the result of nondisjunction and 4% are the result of an unbalanced translocation, which can be familial or can occur de novo. Typical features associated with the syndrome include epicanthal folds, down slanting palpebral fissures, transverse palmar crease, short stature, hypotonia, mental retardation, and cardiac defects. The developmental delay and physical findings may be less severe in the rare patients with mosaic Trisomy 21. There is an increased susceptibility to infection, an increased risk for developing neoplasia, an increased frequency of autoantibodies, and early aging.

Abstracted from: Teebi A, Kennedy S, Chitayat D, Teshima I, Unger S, Babul-Hirji R, Shuman C, Weksberg R. Atlas of Pediatrics. Edited by Ronald Laxer, Ronald M. Laxer, Elizabeth Lee N. Ford-Jones, Jeremy N. Friedman, J. Ted Gerstle. ©2005 Current Medicine, Inc.

Trisomy 18 (Edwards syndrome)

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The birth prevalence of Trisomy 18 is one in 5000 to one in 7000. The male-to-female gender ratio is one to four. Most cases are due to meiotic nondisjunction. The most common findings include intrauterine growth retardation, hypertonia, microcephaly, prominent occiput, narrow palpebral fissures, small mouth, micrognathia, and low-set malformed ears. The neck is short with redundant nuchal skin; the sternum is short. Most cases have cardiac abnormalities, and inguinal and umbilical hernias are common. The limbs show overlapping fingers with clenched hands, and radial ray abnormalities. Club feet, rocker bottom feet, and prominent heels are common. Over one half of affected patients will die within the first 2 months of life and fewer than 10% survive the first year.

Abstracted from: Teebi A, Kennedy S, Chitayat D, Teshima I, Unger S, Babul-Hirji R, Shuman C, Weksberg R. Atlas of Pediatrics. Edited by Ronald Laxer, Ronald M. Laxer, Elizabeth Lee N. Ford-Jones, Jeremy N. Friedman, J. Ted Gerstle. ©2005 Current

Chronic Myelogenous Leukemia and the Philadelphia Chromosome

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The hallmark of chronic myelogenous leukemia (CML) is a specific karyotypic rearrangement known as the Philadelphia chromosome (Ph chromosome). The Ph chromosome was first described by Nowell and Hungerford in 1960 as the shortening of chromosome 22. In 1973, it was further characterized by Rowley as the reciprocal translocation between the long arms of chromosomes 9 and 22, t(9;22)(q34;q11). In 1985, the chimeric gene encoded by t(9;22), bcr/abl was defined and cloned by Heisterkamp et al..

With the use of standard banding techniques, the Ph chromosome can be identified in 85% to 90% of patients who fulfill morphologic criteria for diagnosis of CML. In approximately half of the remaining CML patients, a "masked" Ph chromosome resulting from complex chromosomal rearrangements that involve one or two chromosomes in addition to 9 and 22 can be found on more detailed examination. In many of the truly Ph-chromo-some-negative patients, the chimeric gene bcr/abl or its transcription product can be detected by fluorescence in situ hybridization or reverse-transcriptase polymerase chain reaction.

Abstracted from: Gorman R, Finiewicz K. Atlas of Cancer. Edited by Maurie Markman, Matt Kalaycio. ©2002 Current Medicine, Inc.

Turner syndrome

Turner Syndrome is the most common sex chromosome abnormality of girls, occurring in 1 per 2000 live female births. In Turner Syndrome all or part of one of the X chromosomes is missing; 60% of affected girls have a 45, X karyotype. Short stature relative to the family occurs in nearly 100% of girls with Turner Syndrome (the average adult height of North American girls with Turner Syndrome is 4 feet 8 inches). Therefore, blood chromosome analysis should be considered in any girl with unexplained short stature. Typical features in addition to short stature include a short, broad neck, stocky build, cubitus valgus, shield chest, short metacarpals, high arched palate, low hairline, scoliosis, multiple nevi, and lymphedema. Premature oocyte degeneration usually occurs by the time of birth or soon after and is associated with streak gonads and infertility. Fewer than 1% of girls with Turner Syndrome are fertile, although more than that may have some spontaneous pubertal development, which is not sustained. Girls with Turner Syndrome should be screened for cardiac and renal anomalies and autoimmune hypothyroidism, which occur with increased frequency in this disorder.

Taken from: Shulman D, Bercu B. Atlas of Clinical Endocrinology: Neuroendocrinology and Pituitary Disease. Edited by Stanley Korenman (series editor), Mark E. Molitch. ©2000 Current Medicine, Inc.