Reciprocal Translocations. A double-segment or reciprocal translocation begins when breaks occur in two diferent chromosomes. The broken fragments are then exchanged, so that each afected chromosome contains a fragment of the other. If no chromosomal material is gained or lost in this process, the translocation is considered balanced. he prevalence of reciprocal translocations approximates 1 in 600 births (Nussbaum, 2007). Although the balanced translocation carrier is usually normal phenotypically, repositioning of specific genes within chromosomal segments can cause abnormalities. he risk of a major sttuctural or developmental abnormality in an apparent balanced translocation carrier is approximately 6 percent. Interestingly, using CMA technology, up to 20 percent of individuals who appear to have a balanced translocation are found instead to have missing or redundant DNA segments (Manning,i2010).
Balanced translocation carriers are at risk to produce unbalanced gametes, resulting in abnormal ofspring. As shown in
FIGURE 13-7 A carrier of a balanced translocation may produce offspring who are also carriers ofthe balanced rearrangement (8), ofspring with unbalanced translocations (C, D), or ofspring with normal chromosomal complements (A).
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Figure 13-7, if an oocyte or sperm contains a translocated chromosome, fertilization results in an unbalanced translocation-monosomy for part of one afected chromosome and trisomy for part of the other. he observed risk of a specific translocation can often be estimated by a genetic counselor. In general, translocation carriers identified after the birth of an abnormal child have a 5-to 30-percent risk of producing liveborn ofspring with an unbalanced translocation. Carriers identiied for other reasons, for example, during an infertility evaluation, have only a 5-percent risk. his is likely because the gametes are so abnormal that conceptions are nonviable.
Robertsonian Translocations. These involve only acrocentric chromosomes, which are chromosomes 13, 14, 15,i21, and 22. he acrocentric chromosomes have extremely short p arms. In a robertsonian translocation, the q arms of two acrocentric chromosomes fuse at one centromere to form a derivative chromosome. he other centromere and both sets of p arms are lost. Because the number of centromeres determines the chromosome count, a robertsonian translocation carrier has only 45 chromosomes. Fortunately, the p arms of the acrocentric chromosomes-the satellite regions-contain redundant copies of genes that code for ribosomal RNA. As these are present in multiple copies on other acrocentric chromosomes, their loss does not afect the translocation carrier, who is usually phenotypically normal. However, when the derivative chromosome is paired during fertilization with a haploid chromosome from the partner, resulting ofspring will be trisomic for that chromosome.
Robertsonian translocations are found in 1 in 1000 individuals. The incidence of abnormal ofspring approximates 15 percent if a robertsonian translocation is carried by the mother and 2 percent if carried by the father. Robertsonian translocations are not a major cause of miscarriage and are found in fewer than 5 percent of couples with recurrent pregnancy loss. When a fetus or child is found to have a translocation trisomy, both parents should be ofered karyotype analysis. If neither parent is a carrier, the recurrence risk is extremely low.