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** DSA News **

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Welcome to our News and Media section. Here you'll find our latest
news, press releases, terminology guides and tips to to help you engage
with the media.

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how is down syndrome inherited


Genetic origins of Down syndrome - Wikipedia, the free encyclopedia

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** Genetic origins of Down syndrome **

From Wikipedia, the free encyclopedia
Jump to: navigation, search
Chromosome 21 from Human Genome Program
Normal human Karyotype.

*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 can occur 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 their normal chromosomes).^[1]

A normal human karyotype is shown here. Every chromosome has two copies. In
the bottom right, there are chromosomal differences between males (XY) and
females (XX), which do not concern us. 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.^[2] For this section, we will use females for the karyotype
designation (46,XX).

The extra chromosomal material can come about in several distinct ways.
These are explained in the following sections.

*Contents*

· 1 Trisomy 21
· 2 Translocation Down Syndrome
· 3 Mosaicism
· 4 Duplication of a portion of chromosome 21
· 5 See also
· 6 References

*Trisomy 21[edit]*

Karyotype for trisomy Down syndrome. Notice the three copies of chromosome
21.

Trisomy 21 (47,XX,+21) is caused by a meiotic nondisjunction event.^[3] A
normal gamete (either egg or sperm) has one copy of


Source: en.wikipedia.org/wiki/Genetic_origins_of_Down_syndrome

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