Down syndrome (trisomy 21)
Down syndrome is the most common chromosome condition, occurring in about 1 in 700 births. Down syndrome affects physical and intellectual development and may shorten lifespan. Down syndrome is a genetic condition that happens when an extra chromosome (chromosome 21) originates in the formation of either the sperm or the egg called “trisomy”. Even though Down syndrome is “genetic”, it is not typically an inherited condition.
Trisomy 18 is associated with a high rate of miscarriage. Infants born with trisomy 18 syndrome may have various medical conditions and a shortened lifespan. It is estimated that trisomy 18 syndrome is present in approximately one out of every 5,000 newborns.1 Trisomy 18 is due to an extra copy of chromosome 18. Even though trisomy 18 is “genetic”, it is not typically an inherited condition.
Trisomy 13 is associated with a high rate of miscarriage. Infants born with trisomy 13 usually have severe congenital heart defects and other medical conditions. Survival beyond the first year is rare. It is estimated that trisomy 13 is present in approximately 1 out of every 16,000 newborns. Trisomy 13 is due to an extra chromosome 13. Even though trisomy 13 is “genetic”, it is not typically an inherited condition.
Sex Chromosome Aneuploidies
Turner syndrome, in which one X chromosome is present instead of two, affects 1 in 2,000 girls. Klinefelter syndrome, in which an extra X chromosome is present, affects 1 in 500-1000 boys. Other X and Y chromosome-related conditions affect as many as 1 in 650 newborns. The features of these conditions are variable.
1"American Pregnancy Association." American Pregnancy Association. American Pregnancy Association, n.d. Web. 12 Oct. 2012. www.americanpregnancy.org/main/statistics.html