Translocation Down syndrome

4 Apr

Translocation Down syndrome

Translocation

reciprocal or eucentric translocation (Figure A, right) is a two-break aberration that results in an exact in- terchange of chromosomal segments between two nonhomologous chromosomes and produces two monocentric translocated chromosomes. A nonrecip- rocal or aneucentric translocation (Figure A, left) is a two-break aberration that results in a dicentric and an acentric translocated chromosome. A three-break interchromosomal translocation (Figure B) can gen- erate a deficient chromosome and a recipient chro- mosome containing an intercalated segment of the other nonhomologous chromosome. This is called an insertional translocation. See Appendix C, 1923, Bridges; Burkitt lymphoma, Cattanach translocation, centric fusion, Philadelphia (Ph1) chromosome. translocation Down syndrome familial Down syndrome, caused by having three copies of chromo- some 21—two as separate chromosomes and one translocated to another chromosome, usually num- ber 14. In families where one parent is a transloca- tion heterozygote, the probability of having a child with Down syndrome is .33. See Appendix C, 1960, Polani et al. translocation heterozygote an individual or cell in which two pairs of homologous chromosomes have reciprocally exchanged nonhomologous seg- ments between one member of each pair. Each chromosome pair contains both homologous and nonhomologous segments, i.e., one normal (un-

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