Chromosomal Aberrations

Three well-known autosomal chromosome disorders associated with trisomies of entire autosomes are sometimes found in live births. These are trisomy 21 (Down syndrome), trisomy 13, and trisomy 18. Growth retardation, mental retardation, and multiple congenital anomalies are associated with all three trisomies. However, each has distinctive morphological characteristics, which are presumably determined by the extra dosage of the specific genes on the additional chromosome.

Down syndrome (chromosome 21) is the most frequent trisomy found in humans, and one of the most common conditions encountered in genetic counseling. General characteristics are mental retardation, distinctive palm prints, and a common facial appearance. The average life expectancy is now much greater thanks to improvements in medical care. Generally, individuals with Down syndrome have affable personalities and are able to be partially independent. The incidence of Down syndrome is about 1 in 800 children and is often associated with later maternal age (as may also be the case with other aneuploids).

Down syndrome appears to be related to the difference in gamete formation (gametogenesis) between males and females. In females, oocytes are formed before birth and held in a static state until ovulation. In the case of older mothers, an oocyte may be in this stage for more than forty years, during which time environmental factors may affect the genetic material. In trisomy 13 and trisomy 18 patients, congenital abnormalities are much more severe. These individuals generally do not live much beyond birth. Both trisomy 13 and trisomy 18 result in syndromes characterized by specific dysmorphic features and severe organ malformations.

In addition to trisomies involving the autosomal chromosomes, aneuploidy may also involve the sex chromosomes. Two examples are Turner's syndrome and Klinefelter's syndrome. As mentioned previously, Turner's syndrome is a monosomy involving the X chromosomes. Turner's syndrome females possess forty-five chromosomes (45, X) as compared to clinically normal forty-six (46, XX). They are usually sterile and short in stature with some neck webbing. Klinefelter's syndrome patients have a trisomy involving the This karyotype of Turner's syndrome is characterized by a missing X chromosome. sex chromosomes and thus have forty-seven chromosomes (47, XXY). Klinefelter's syndrome individuals are sterile males possessing some female characteristics. These chromosome abnormalities are of interest especially for their implications in infertility and abnormal development.