| Down syndrome (also Down's syndrome) or | | | | background, environmental factors, and |
| trisomy 21 is a genetic condition | | | | random chance. |
| resulting from the presence of all or | | | | Down syndrome can occur in all human |
| part of an extra 21st chromosome. Down | | | | populations, and analogous effects have |
| syndrome is characterized by a | | | | been found in other species, such as |
| combination of major and minor | | | | chimpanzees and mice. Recently, |
| abnormalities of body structure and | | | | researchers have been able to create |
| function. Among features present in | | | | transgenic mice with most of human |
| nearly all cases are impairment of | | | | chromosome 21 (in addition to their |
| learning and physical growth, and a | | | | normal chromosomes). |
| recognizable facial appearance usually | | | | The extra chromosomal material can come |
| identified at birth. It is named after | | | | about in several distinct ways. |
| John Langdon Down, the British doctor | | | | Trisomy 21 Trisomy 21 (47,XX,+21) is |
| who first described it in 1866. | | | | caused by a meiotic nondisjunction |
| Individuals with Down syndrome have | | | | event. With nondisjunction, a gamete is |
| lower than average cognitive ability, | | | | produced with an extra copy of |
| normally ranging from mild to moderate | | | | chromosome 21 (the gamete has 24 |
| retardation. Some individuals may have | | | | chromosomes). When combined with a |
| overall average intelligence, but | | | | normal gamete from the other parent, the |
| generally will present with some amount | | | | child now has 47 chromosomes, with three |
| of developmental disability such as a | | | | copies of chromosome 21. |
| tendency toward concrete thinking or | | | | Trisomy 21 is the cause of approximately |
| naivete. There are also a small number | | | | 95% of observed Down syndromes, with 88% |
| individuals with Down syndrome with | | | | coming from nondisjunction in the |
| severe to profound mental retardation. | | | | maternal gamete and 8% coming from |
| The incidence of Down syndrome is | | | | nondisjunction in the paternal gamete. |
| estimated at 1 per 800 to 1 per 1000 | | | | Mitotic nondisjunctions after conception |
| births. | | | | would lead to mosaicism, and is |
| The common physical features of Down | | | | discussed later. |
| syndrome also appear in people with a | | | | Robertsonian translocation The extra |
| standard set of chromosomes. They | | | | chromosome 21 material that causes Down |
| include simian crease (a single crease | | | | syndrome may be due to a Robertsonian |
| across one or both palms), almond shaped | | | | translocation. The long arm of |
| eyes, shorter limbs, speech impairment, | | | | chromosome 21 is attached to another |
| and enlarged tongue. In addition, health | | | | chromosome, often chromosome 14 |
| concerns for individuals with Down | | | | (45,XX,t(14;21q)) or itself (called an |
| syndrome include a higher risk for | | | | isochromosome, 45,XX,t(21q;21q)). Normal |
| congenital heart defects, | | | | disjunction leading to gametes have a |
| gastroesophageal reflux disease, | | | | significant chance of creating a gamete |
| recurrent ear infections, obstructive | | | | with an extra chromosome 21. |
| sleep apnea, and thyroid dysfunctions. | | | | Translocation Down syndrome is often |
| Early childhood intervention, screening | | | | referred to as familial Down syndrome. |
| for common problems, such as thyroid | | | | It is the cause of 2-3% of the observed |
| functioning, medical treatment where | | | | Down syndromes. It does not show the |
| indicated, a conducive family | | | | maternal age effect, and is just as |
| environment, vocational training, etc., | | | | likely to have come from fathers as |
| can improve the overall development of | | | | mothers. |
| children with Down syndrome. Experience | | | | Mosaicism When some of the cells in the |
| with individuals with Down syndrome | | | | body are normal and some cells have |
| shows that while some genetic | | | | trisomy 21, it is called Mosaic Down |
| limitations cannot be overcome, | | | | Syndrome (46,XX/ 47,XX,+21).[6] This can |
| education and proper care can produce | | | | occur in one of two ways: A |
| progress whatever the starting point. | | | | nondisjunction event during an early |
| Characteristics Individuals with Down | | | | cell division in a normal embryo leads |
| syndrome may have some or all of the | | | | to a fraction of the cells with trisomy |
| following physical characteristics:[1] | | | | 21; or a Down syndrome embryo undergoes |
| oblique eye fissures with small skin | | | | nondisjunction and some of the cells in |
| folds on the inner corner of the eyes, | | | | the embryo revert back to the normal |
| muscle hypotonia, flat nasal bridge, | | | | chromosomal arrangement. There is |
| simian crease, large and protruding | | | | considerable variability in the fraction |
| tongue, short neck, white spots on the | | | | of trisomy 21, both as a whole and |
| iris, excessive flexibility in joints, | | | | tissue-by-tissue. This is the cause of |
| congenital heart defect, excessive space | | | | 1-2% of the observed Down syndromes.[5] |
| between large and second toe, and fifth | | | | There is evidence that mosaic Down |
| finger with one flexion furrow instead | | | | syndrome, may produce less developmental |
| of two. In additional to observable | | | | delay, on average, than full trisomy |
| physical characteristics, individuals | | | | 21.[7] It is likely that all people have |
| with Down syndrome may have some medical | | | | an extremely small fraction of their |
| concerns. | | | | cells that are trisomy 21, with no |
| Most individuals with Down syndrome have | | | | discernable effects. |
| mental retardation in the mild (IQ | | | | Duplication of a portion of chromosome |
| 50-70) to moderate range (IQ 35-50),[2] | | | | 21 Rarely, a region of chromosome 21 |
| with scores for children with Mosaic | | | | will undergo a duplication event. This |
| Down syndrome some 10-30 points higher. | | | | will lead to extra copies of some, but |
| Down syndrome is a chromosomal | | | | not 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 | | | | |