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.
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.