Down Syndrome (Trisomy-21) (2n = 47, +21)
Down Syndrome (Trisomy-21)
is due to a non-disjunction in meiosis that results in duplication of
Chromosome 21 in the gamete
contributed by one (unaffected) parent. Trisomy 21 is
the most common post-natal trisomy, and affects about 1 / 2000 live
births (the
incidence is 50-fold higher in older mothers).
The syndrome is characterized by a distinctive facial phenotype
including an epicanthic
eye fold, a broad flat face, and moderate to severe mental retardation.
The stereotypic phenotype of persons with Down Syndrome is
a white male with dark hair (see middle photo), however Trisomy 21
occurs in both sexes and in all ethnicities. Many persons with Down
Syndrome function well in the
community.
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 manner as single-locus "genetic diseases." In 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)) (MGA2 11-35). 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.
Recent evidence suggests that critical features of Down Syndrome phenotype are due to overexpression of a very small set of triplicated loci on Chromosome 21, rather than the trisomy as a whole. Selective re-regulation of the expression of one or a few key 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: the opposition of a former US Vice Presidential candidate to Drosophila research as irrelevant to human concerns is thus misguided. See [http://www.dsrf.org/home/] 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 manner as single-locus "genetic diseases." In 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)) (MGA2 11-35). 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.
Recent evidence suggests that critical features of Down Syndrome phenotype are due to overexpression of a very small set of triplicated loci on Chromosome 21, rather than the trisomy as a whole. Selective re-regulation of the expression of one or a few key 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: the opposition of a former US Vice Presidential candidate to Drosophila research as irrelevant to human concerns is thus misguided. See [http://www.dsrf.org/home/] for more information.
Carole, age 40; Children with Trisomy 21 competing at the Special Olympics; Catherine