INHERITANCE OF A CHROMOSOME 3 AND 21
TRANSLOCATION IN THE FETUSES, WITH ONE ALSO
HAVING TRISOMY 21, IN THREE PREGNANCIES
IN ONE FAMILY Pazarbasi A1,*, Demirhan O1, Alptekin D1, Ozgunen FT2, Ozpak L1,
Yilmaz MB1, Nazlican E3, Tanriverdi N1, Luleyap U1, Gümürdülü D4 *Corresponding Author: Associate Professor Ayfer Pazarbasi (Ph.D.), Çukurova University, Faculty of Medicine,
Department of Medical Biology and Genetics, 01330 Adana, Turkey; Tel.: +90-322-338-6060/3498; Fax:
+90-322-338-6572; E-mail: payfer@cu.edu.tr page: 91
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INTRODUCTION
Balanced reciprocal translocations result from
exchange of fragments between two chromosomes,
without any gain or loss of genetic material, and are
a common form of chromosomal abnormalities, occurring
in about 1 in every 625 newborns [1-4]. Although,
these translocation carriers usually do not
exhibit any particular phenotypes, there is a balanced
complement of genes. These are responsible for a
high incidence of infertility, pregnancy loss, mental
retardation, behavioral abnormalities, morbidity and
mortality. Carriers of reciprocal translocations have
reduced fertility and thus form an increased risk of
having a spontaneous abortion or an unbalanced
karyotype in their offspring [1,3,5-11]. Pure trisomy
due to non disjunction of chromosome 21 is responsible
for 96.0% of Down Syndrome with a recurrence
risk of less than 1.0%. Parental karyotype is not
required in non disjunction type of trisomies [12-15]. The population risk for trisomy 21 is 1 in 700
births but some couples are at a much higher risk
owing to parental translocation or mosaicism [2].
Trisomy 21 due to reciprocal translocations are
caused by exchange of euchromatic regions of
chromosome 21 with the euchromatin regions of
different autosomes or gonosomes. In addition to
trisomic regions in various lengths and location of
chromosome 21, unbalanced forms also show partial
mono-somy for the exchanged regions of the other
translocation chromosome. As a result, the phenotype
in the carriers of an unbalanced translocation is
not consistent [6]. Since these translocations rarely
occur, there are no reliable data for their incidence,
but their frequency is assumed to be less than 1:1000
in standard trisomy cases. According to reports in
previous literature, the most common partners for a
reciprocal translocation seem to be chromosomes 18
and 22 [11].
The true mechanisms responsible for structural
rearrangement at segregation remain unknown.
There is some evidence that considers chromosomal
translocations as a risk factor for aneuploidy, therefore,
translocations have to be considered in combination
with aneuploidy analysis [10]. In one of
our recent studies we assessed a family in which the
translocation between chromosomes 12 and 16 segregates;
one of the eight progenies carried the 47,XY,
+21,t(12;16)(q24;q24) karyotype and presented with
Down Syndrome [10]. His mother was phenotypically
normal, one brother and one sister also carried
the same translocation. Apparently, this rearrangement
occurred due to the unbalanced chromosome
segregation of the mother [t(12;16)(q24;q24)mat].
Here, we present a segregation of a balanced
translocation between chromosomes 3 and 21 [t(3;21)
(q21; q22] of a phenotypically normal mother that
led to one offspring with a viable balanced translocation
and Down Syndrome, one child with a normal
karyotype and one phenotypically normal fetus with
the translocation. This rearrangement apparently originated from the mother [t(3;21)(q21;q22)mat].
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