why is down's syndrome caused?

Question

its a genetic disorder

Answer 1

Downs syndrome is known as trisomy 21.

In English, this means that instead of the normal 2 copies of chromosome 21, people with Downs have 3 copies.

It is caused in either the mother or fathers egg or sperm when the genetic information is being sorted, and it does not divide properly (i.e. instead of exactly one of each chromosome going into each sperm or egg, one has two copies of 21, and one has none).
When this egg or sperm with two copies of 21 meets another egg or sperm with the normal one, then the resulting fetus has three copies.

Answer 2

Why? Because life sucks and then you die. God hates you. Bad luck. Take your pick.

How is it caused? It's a gene dosage problem. Not only do you need functional genes, but you need to turn them on and off at the right time, in the right places and in the right amounts. In Downs syndrome, there are three copies of chromosome 21 (the smallest chromosome but still some 45 million basepairs of DNA and hundreds of genes). The extra copy causes lots of genes to be turned on in the wrong amounts. It is the only viable autosomal aneuploidy because it is the smallest autosome. Any other autosomal aneuploidy is embryonic lethal.

The defect is usually in the female and the risk increases dramatically with the age of the mother and as her eggs approach their expiration data. There is a smaller increase in risk associated with increasing age of the father.

Answer 3

down syndrome is caused by a genetic fault on chromosome 23 i hope this helps

Answer 4

down's syndrom is a genetic disorder which occurs in 45th chromosome of human beings.

Answer 5

Down syndrome or trisomy 21 (British Down's syndrome) is a genetic condition resulting from the presence of all or part of an extra 21st chromosome. Down syndrome is characterized by a combination of major and minor abnormalities of body structure and function. Among features present in nearly all cases are impairment of learning and physical growth, and a recognizable facial appearance usually identified at birth. It is named after John Langdon Down, the British doctor who first described it in 1866.

Individuals with Down syndrome have lower than average cognitive ability, normally ranging from mild to moderate retardation. Some individuals may have average intelligence overall, but will generally have some amount of developmental disability, such as a tendency toward concrete thinking or naïveté. There is also a small number of individuals with Down syndrome with severe to profound mental retardation. The incidence of Down syndrome is estimated at 1 per 800 to 1 per 1000 births.

The common physical features of Down syndrome also appear in people with a standard set of chromosomes. They include a simian crease (a single crease across one or both palms), almond shaped eyes, shorter limbs, speech impairment, and an enlarged tongue. Health concerns for individuals with Down syndrome include a higher risk for congenital heart defects, gastroesophageal reflux disease, recurrent ear infections, obstructive sleep apnea, and thyroid dysfunctions.

Early childhood intervention, screening for common problems, medical treatment where indicated, a conducive family environment, and vocational training can improve the overall development of children with Down syndrome. While some of the genetic limitations of Down Syndrome cannot be overcome, education and proper care, initiated at any time, can improve quality of life.[1]

Down syndrome is a chromosomal abnormality characterized by the presence of an extra copy of genetic material on the 21st chromosome, either in whole (trisomy 21) or part (such as due to translocations). The effects of the extra copy varies greatly from individual to individual, depending on the extent of the extra copy, genetic background, environmental factors, and random chance. Down syndrome occurs in all human populations, and analogous effects have been found in other species such as chimpanzees and mice. Recently, researchers have been able to create transgenic mice with most of human chromosome 21 (in addition to the normal mouse chromosomes). People with Down Syndrome often have a simian crease on the palmar surface of the hand.[5] The extra chromosomal material can come about in several distinct ways. A normal human karyotype is designated as 46,XX or 46,XY, indicating 46 chromosomes with an XX arrangement for females and 46 chromosomes with an XY arrangement for males.[6]

Trisomy 21
Trisomy 21 (47,XX,+21) is caused by a meiotic nondisjunction event. With nondisjunction, a gamete (i.e., a sperm or egg cell) is produced with an extra copy of chromosome 21; the gamete thus has 24 chromosomes. When combined with a normal gamete from the other parent, the embryo now has 47 chromosomes, with three copies of chromosome 21. Trisomy 21 is the cause of approximately 95% of observed Down syndromes, with 88% coming from nondisjunction in the maternal gamete and 8% coming from nondisjunction in the paternal gamete.[7]

Mosaicism
Trisomy 21 is generally caused before conception, and all cells in the body are affected. However, when some of the cells in the body are normal and other cells have trisomy 21, it is called Mosaic Down Syndrome (46,XX/47,XX,+21).[8] This can occur in one of two ways: A nondisjunction event during an early cell division in a normal embryo leads to a fraction of the cells with trisomy 21; or a Down syndrome embryo undergoes nondisjunction and some of the cells in the embryo revert back to the normal chromosomal arrangement. There is considerable variability in the fraction of trisomy 21, both as a whole and among tissues. This is the cause of 1–2% of the observed Down syndromes.[7] There is evidence that mosaic Down syndrome may produce less developmental delay, on average, than full trisomy 21.[9]

Robertsonian translocation
The extra chromosome 21 material that causes Down syndrome may be due to a Robertsonian translocation. In this case, the long arm of chromosome 21 is attached to another chromosome, often chromosome 14 (45,XX,t(14;21q)) or itself (called an isochromosome, 45,XX,t(21q;21q)). Normal disjunction leading to gametes have a significant chance of creating a gamete with an extra chromosome 21. Translocation Down syndrome is often referred to as familial Down syndrome. It is the cause of 2-3% of the observed Down syndromes.[7] It does not show the maternal age effect, and is just as likely to have come from fathers as mothers.

Duplication of a portion of chromosome 21

Rarely, a region of chromosome 21 will undergo a duplication event. This will lead to extra copies of some, but not all, of the genes on chromosome 21 (46,XX,dup(21q)).[10] If the duplicated region has genes that are responsible for Down syndrome physical and mental characteristics, such individuals will show those characteristics. This cause is very rare and no rate estimates are possible.

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