A typical human cell contains 23 pairs of chromosomes, with half inherited from each parent. Down syndrome is a genetic condition that occurs when a child has a full or partial extra copy of chromosome 21. The additional genetic material alters the course of development and causes the characteristics associated with Down syndrome. According to the Centers for Disease Control and Prevention, approximately one in every 700 babies in the United States is born with Down syndrome.
Down syndrome is a genetic condition, but only one percent of cases have a hereditary component. The chromosomal abnormalities happen by chance. Both mothers and fathers can pass on the mutation. Genetic counseling can indicate an increased likelihood of offspring developing the condition, and a blood test done during the third month of pregnancy can indicate its presence.
Down syndrome is usually identified at birth by certain physical traits including low muscle tone, a single deep crease across the palm of the hand, a slightly flattened facial profile, and an upward slant to the eyes. However, these features may be present in babies without Down syndrome; a chromosomal analysis called a karyotype is commonly done to confirm the diagnosis.
There are three types of Down syndrome:
- Trisomy 21 - Approximately 95% of cases of Down syndrome are Trisomy 21. This variant develops when there is an addition chromosome 21 in every cell in the body.
- Translocation - This occurs when a section of chromosome 21 breaks off and attaches itself to another chromosome - typically, chromosome 14. Approximately 4% of Down syndrome cases are due to translocation.
- Mosaic - This rare (less than 1% of cases) type of Down syndrome occurs when cells have a combination of numbers of chromosomes, some with the usual 46 and others with 47.
All children with Down syndrome possess some degree of cognitive delay, ranging from mild to severe. Language delay and memory impairment are the most common. Comorbidities including hearing loss, heart defects, epilepsy, and blood disorders are also common in children with Down syndrome.