Physcial Science

Word Count
Pasadena City College
Jiajun Zhong
Dr. Elisabeth
DNA Fact Sheet
Down Syndrome
Down syndrome (DS) becomes a chromosomal genetic defect that results when there is an error
during cell division in extra chromosome 21 (Kazemi et al., 2016). It causes intellectual
disabilities that occur in every 1 in 400 newborns.
-It occurs when some or all of an individual's cells have an extra full or partial copy of
chromosome 21 (the protein) (Crosta, 2017). Normally, there are two copies of every
chromosome, however, in DS they are three. The overexpression of a gene(s) found on the
triplicated chromosome causes DS.
- DS occurs in this manner: More than four hundred genes are found in chromosome 21 and has
many nucleotides and portrays 1.5 percent of human genome yet it is the smallest human
autosome. This chromosome contains 40% content repeat including short interspersed repetitive
elements (SINEs), long terminal repeats (LTR) and long interspersed repetitive elements
(LINEs). The pathogenesis of this is explained by gene-dosage hypothesis which explains that all
variations are because of extra copy of the chromosome 21 (Kazemi et al., 2016). Only some
genes in the chromosome 21 participate in phenotypes of the down syndrome and the products of
genes like morphogens, transcription regulators, multi-subunit proteins etc. become more
sensitive to gene dosage imbalance compared to others. Therefore, during cell division, an error
occurs (nondisjunction) which leads to an embryo with three copies of chromosome 21.
- Physical symptoms include; small stature and a short neck, flat nasal conduit, eyes that slant
upward, white spots on the iris, a protruding tongue. On average, a child with Down syndrome