Down Syndrome (Trisomy-21) (2n = 47, +21)
occurs as a results of non-disjunction of chromosomes during
meiosis, such that the gamete contributed by one
(unaffected) parent has two copies of Chromosome 21. When
fertilized by a gamete from the other parent, with a single
Chromosome 21, the result is a trisomy (three copies: see G-banding
karyotype). Trisomy 21 is the most common trisomy in
newborns, and occurs in about 1 / 1000 live births. The
incidence is up to 50-fold higher in older mothers. The syndrome is
characterized by a distinctive facial phenotype including
epicanthic eye folds, a broad flat face, handprint, and
moderate to severe mental retardation, as well as other
medical conditions shown above. The stereotypic
phenotype of persons with Down Syndrome is often a
white male with dark hair (see second photo), however
Trisomy 21 occurs in both sexes and in all ethnicities. Many
persons with Down Syndrome function well in the community. See the website of the Down Syndrome Research
for more information.
Most or all persons with Down Syndrome are
functionally sterile, and do not pass on gametes with the
duplicated chromosome to the next generation. Thus, although
the condition is "genetic,"
it is not "inherited"
in the same sense as are single-locus traits, and its
occurence in the population is termed "sporadic". In very rare
cases, Down Syndrome may result from a balanced translocation
that attaches a substantial portion of Chromosome 21 to
another chromosome (usually Chromosome 14) [this is
indicated as 2n=46,t(q14q21)].
This form of Down Syndrome may be passed genetically (by
asymptomatic, non-Down Syndrome parents) to the next
generation. For this reason, it is considered advisable to
check the karyotype of all persons with Down Syndrome, to
provide appropriate genetic counselling.
suggests that the characteristic features of the Down
Syndrome phenotype are due to the triplication of a small
set of genes on Chromosome 21, rather than the trisomy per se. Selective
re-regulation of the expression of one or a few key "Down Syndrome Critical Region"
gene loci in utero
might therefore provide an effective means of treatment. Study of the
homologous genes in Drosophila
may provide means to this end. A former US politician's
trivialization of government-funded Drosophila research as
irrelevant to human concerns is thus misinformed.
diagnosis of Down Syndrome has used amniocentesis
to recover fetal cells from the maternal amniotic fluid,
which can then be cultured to check their karyotype. Test
results may not be possible until late in preganancy.
Several new DNA-based tests
have been developed that take advantage of Next-Generation sequencing
methods to diagnose Down Syndrome pregnancies much earlier
Carole, age 40; Children with Trisomy 21 competing at the
Special Olympics; Catherine;
Abigail, from the exhibition "More Than Meets the Eye," St
John's Arts & Culture Centre, 2009
Figures ©2002 by Griffiths et al. & ©2011 by Russell; all text
material ©2013 by Steven M.