Each human cell contains 23 pairs of chromosomes, a total of 46. Chromosomes carry genetic information. One half originates from the mother and the other from the father. A trisomy occurs when a chromosome exhibits three copies rather than the normal two. Advanced maternal age is a risk factor for trisomies. Children with a trisomy often exhibit mental retardation and have a reduced life expectancy.
Trisomy 21, also known as Down Syndrome, is one of the most common genetic birth defects, affecting approximately 1 in about 700 newborns. Down Syndrome is caused by an additional copy of the chromosome 21. It often involves a mild to moderate mental and physical development as well as heart defects and may lead to a reduced life expectancy.
Trisomy 18, also known as Edwards Syndrome, is ascribed to an additional copy of chromosome 18. Affected individuals often suffer from heart defects and other intellectual and physical impairments. The prevalence of trisomy 18 is approximately 1 in 5,000 births.
Trisomy 13, also called Patau Syndrome, is caused by an extra copy of chromosome 13. Babies born with trisomy 13 die often by the age of 2 due to severe congenital heart defects and other body impairments. The prevalence of trisomy 13 is approximately 1 in 16,000 births.
Sex Chromosome Disorders
Chromosomes X and Y determine the sex of a human being. Sex chromosome disorders are caused by missing, additional or incomplete copies of chromosomes X or Y. Klinefelter Syndrome (XXY), and Ullrich-Turner Syndrome (X0), also known as Monosomy X, are associated with aberrations that involve sex chromosomes. Klinefelter Syndrome is seen in approximately 1 in 10,000 female births. Since X and Y aberrations are usually much less severe in their effects, they may remain undetected for a lifetime.