1. Translocations predisposing to the occurrence of Down syndrome
Translocations predisposing to the occurrence of Down syndrome. G-banded chromosomes. A-C, Robertsonian translocations. A, rob(13;21). B, rob(14;21), the most common translocation predisposing to Down syndrome. C, rob(21;21). All children of a balanced carrier of the latter translocation have Down syndrome since the complementary product, monosomy 21, is not viable. D, rcp(10;21)(p11;p12). This is a reciprocal translocation in which the end of the short arm of a chromosome 21 is translocated to the short arm of a chromosome 10 near the centromere, and most of the short arm of the chromosome 10 is translocated to the short arm of the chromosome 21 near the centromere. The resulting chromosomes appear to have the short arm (p) of 21 on the long arm (q) of 10 [der(10)] and the short arm of 10 on the long arm of 21 [der(21)] (der = derivative). Down syndrome occurs by 3:1 nondisjunction in which chromosomes der(10), der(21), and 21 all migrate to the same pole at the first meiotic division. Robertsonian translocation rob(21;22) and other reciprocal translocations involving chromosome 21 can also give rise to Down syndrome. (Courtesy of Dr. Steven A. Schonberg, University of California, San Francisco.)
Source: Down Syndrome (Trisomy 21), The Online Metabolic and Molecular Bases of Inherited Disease
Citation: Valle D, Beaudet AL, Vogelstein B, Kinzler KW, Antonarakis SE, Ballabio A, Gibson K, Mitchell G. The Online Metabolic and Molecular Bases of Inherited Disease; 2014 Available at: Accessed: October 26, 2017
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