Translocation trisomy in heritable Down Syndrome

    Children with Down Syndrome are typically born to asymptomatic parents as a result of difficulties that arise during meiosis. Persons with Down Syndrome are rarely fertile, and so do not  have the opportunity to pass the extra Chromosome 21 on to offspring. Thus, although Down Syndrome is a "genetic disease", it is not typically an "heritable disease" like Huntington Disease.

    Some forms of Down syndrome are heritable, as shown above. In the germline of an asymptomatic individual, reciprocal translocation between the long arms of Chromosomes 14 & 21 results in gametes that carry a large portion of Chromosome 21 on a Chromosome 14 centromere. In combination with a standard gamete from the other parent, this results in a 2n=46 karyotype with a segmental aneuploidy for Chromosome 21, producing a Down Syndrome infant. The karyotype is formally desribed as 2n = 46, t(14q21q).

    Unlike standard Trisomy-21,  the translocated chromosome may be passed to an asymptomatic child (the gene locus set is substantially complete) and transmitted to the next generation. In rare cases, a 2n=46,t(q21q21) translocation attaches two Chromosome 21 long arms to a single centromere. All of the children conceived by such a one will then have three copies Chromosome 21,  and therefore Down Syndrome.

Figure after ©2002 by Griffiths et al.; all text material ©2011 by Steven M. Carr