| Down syndrome or trisomy 21 (British | | | | background, environmental factors, and |
| Down's syndrome) is a genetic condition | | | | random chance. Down syndrome occurs in |
| resulting from the presence of all or | | | | all human populations, and analogous |
| part of an extra 21st chromosome. Down | | | | effects have been found in other species |
| syndrome is characterized by a | | | | such as chimpanzees and mice. Recently, |
| combination of major and minor | | | | researchers have been able to create |
| abnormalities of body structure and | | | | transgenic mice with most of human |
| function. Among features present in | | | | chromosome 21 (in addition to the normal |
| nearly all cases are impairment of | | | | mouse chromosomes). People with Down |
| learning and physical growth, and a | | | | Syndrome often have a simian crease on |
| recognizable facial appearance usually | | | | the palmar surface of the hand.[6] The |
| identified at birth. It is named after | | | | extra chromosomal material can come |
| John Langdon Down, the British doctor | | | | about in several distinct ways. A normal |
| who first described it in 1866. | | | | human karyotype is designated as 46,XX |
| Individuals with Down syndrome have | | | | or 46,XY, indicating 46 chromosomes with |
| lower than average cognitive ability, | | | | an XX arrangement for females and 46 |
| normally ranging from mild to moderate | | | | chromosomes with an XY arrangement for |
| retardation. Some individuals may have | | | | males. |
| low intelligence overall, but will | | | | Trisomy 21 |
| generally have some amount of | | | | Trisomy 21 (47,XX,+21) is caused by a |
| developmental disability, such as a | | | | meiotic nondisjunction event. With |
| tendency toward concrete thinking or | | | | nondisjunction, a gamete (i.e., a sperm |
| naiveté. There is also a small number | | | | or egg cell) is produced with an extra |
| of individuals with Down syndrome with | | | | copy of chromosome 21; the gamete thus |
| severe to profound mental retardation. | | | | has 24 chromosomes. When combined with a |
| The incidence of Down syndrome is | | | | normal gamete from the other parent, the |
| estimated at 1 per 800 to 1 per 1000 | | | | embryo now has 47 chromosomes, with |
| births. | | | | three copies of chromosome 21. Trisomy |
| The common physical features of Down | | | | 21 is the cause of approximately 95% of |
| syndrome also appear in people with a | | | | observed Down syndromes, with 88% coming |
| standard set of chromosomes. They | | | | from nondisjunction in the maternal |
| include a simian crease (a single crease | | | | gamete and 8% coming from nondisjunction |
| across one or both palms), almond shaped | | | | in the paternal gamete.[8] |
| eyes, shorter limbs, speech impairment, | | | | Mosaicism |
| and protruding tongue. Health concerns | | | | Trisomy 21 is generally caused before |
| for individuals with Down syndrome | | | | conception, and all cells in the body |
| include a higher risk for congenital | | | | are affected. However, when some of the |
| heart defects, gastroesophageal reflux | | | | cells in the body are normal and other |
| disease, recurrent ear infections, | | | | cells have trisomy 21, it is called |
| obstructive sleep apnea, and thyroid | | | | Mosaic Down Syndrome (46,XX/47,XX,+21). |
| dysfunctions. | | | | This can occur in one of two ways: A |
| Early childhood intervention, screening | | | | nondisjunction event during an early |
| for common problems, medical treatment | | | | cell division in a normal embryo leads |
| where indicated, a conducive family | | | | to a fraction of the cells with trisomy |
| environment, and vocational training can | | | | 21; or a Down syndrome embryo undergoes |
| improve the overall development of | | | | nondisjunction and some of the cells in |
| children with Down syndrome. While some | | | | the embryo revert back to the normal |
| of the genetic limitations of Down | | | | chromosomal arrangement. There is |
| Syndrome cannot be overcome, education | | | | considerable variability in the fraction |
| and proper care, initiated at any time, | | | | of trisomy 21, both as a whole and among |
| can improve quality of life | | | | tissues. This is the cause of 1–2% of |
| Characteristics | | | | the observed Down syndromes.There is |
| Example of white spots on the iris | | | | evidence that mosaic Down syndrome may |
| known as Brushfield’s | | | | produce less developmental delay, on |
| Spots.Individuals with Down syndrome may | | | | average, than full trisomy 21. |
| have some or all of the following | | | | Robertsonian translocation |
| physical characteristics: oblique eye | | | | The extra chromosome 21 material that |
| fissures with small skin folds on the | | | | causes Down syndrome may be due to a |
| inner corner of the eyes, muscle | | | | Robertsonian translocation. In this |
| hypotonia, a flat nasal bridge, a single | | | | case, the long arm of chromosome 21 is |
| palmar fold (simian crease), a | | | | attached to another chromosome, often |
| protruding tongue (due to small oral | | | | chromosome 14 (45,XX,t(14;21q)) or |
| cavity, poor muscle tone, and an | | | | itself (called an isochromosome, |
| enlarged tongue near the tonsils), a | | | | 45,XX,t(21q;21q)). Normal disjunction |
| short neck, white spots on the iris | | | | leading to gametes have a significant |
| known as Brushfield spots, excessive | | | | chance of creating a gamete with an |
| flexibility in joints, congenital heart | | | | extra chromosome 21. Translocation Down |
| defects, excessive space between large | | | | syndrome is often referred to as |
| and second toe, and a single flexion | | | | familial Down syndrome. It is the cause |
| furrow of the fifth finger. Most | | | | of 2-3% of observed cases of Down |
| individuals with Down syndrome have | | | | syndrome.[8] It does not show the |
| mental retardation in the mild (IQ | | | | maternal age effect, and is just as |
| 50–70) to moderate range (IQ 35–50), | | | | likely to have come from fathers as |
| with scores for children with Mosaic | | | | mothers. |
| Down syndrome (explained below) some | | | | Duplication of a portion of chromosome |
| 10–30 points higher. In addition, | | | | 21 |
| individuals with Down syndrome can have | | | | Rarely, a region of chromosome 21 will |
| serious abnormalities affecting any body | | | | undergo a duplication event. This will |
| system. | | | | lead to extra copies of some, but not |
| Down syndrome is a chromosomal | | | | all, of the genes on chromosome 21 |
| abnormality characterized by the | | | | (46,XX,dup(21q)). If the duplicated |
| presence of an extra copy of genetic | | | | region has genes that are responsible |
| material on the 21st chromosome, either | | | | for Down syndrome physical and mental |
| in whole (trisomy 21) or part (such as | | | | characteristics, such individuals will |
| due to translocations). The effects of | | | | show those characteristics. This cause |
| the extra copy varies greatly from | | | | is very rare and no rate estimates are |
| individual to individual, depending on | | | | possible. |
| the extent of the extra copy, genetic | | | | |