CYTOGENETIC CORRELATIONS AND CROSSING OVER 101 



Translocations 



A translocation is a transfer of a chromosomal segment to a non- 

 homologous chromosome. Most viable translocations detected after 

 X-ray involve reciprocal exchanges between two nonhomologous chromo- 

 somes. The first identification of a translocation was by Bridges in 1919 

 in Drosophila on the basis of anomalous breeding data, indicating link- 

 age between markers which he knew to be on nonhomologous chromo- 

 somes. Subsequently, translocations have been detected frequently in 

 natural populations, particularly in plants. In the 1930's, translocations 

 came into widespread use in genetic analysis when methods were estab- 

 lished for selecting desired combinations from large populations of 

 irradiated organisms. 



Heterozygous translocations give rise to special pairing configurations 

 in meiosis, as shown in Figure 4.4. The extent of lethality among the 

 progeny depends upon whether or not the daughter cell receives a full 

 chromosome complement. In plants, lethality is usually expressed im- 

 mediately as pollen sterility, which is detected as shriveling of pollen 

 grains, and provides a simple basis for identifying translocations in 

 breeding experiments. The point of translocation can be mapped like 

 a gene, and, with suitably marked stocks, four-armed linkage maps are 

 found. 



Inversions 



An inversion is a chromosomal rearrangement in which a segment is 

 reversed relative to the standard arrangement in the wild type. If the 

 inverted segment is confined to one arm of the chromosome, it is called 

 paracentric; if it extends across the centromere, it is called pericentric. 



of the reciprocal translocation between maize chromosomes 8 and 10. Each 

 duplicated chromosome is shown as a single line (after Rhoades, 1955, The cyto- 

 genetics of maize; in: Corn and Corn Improvement, New York, Academic Press). 



(c). Schematic model of pachytene configuration of the chromosomes shown in o 

 after duplication and pairing. Clear ovals refer to centromeres which are not yet 

 divided. Crossovers occurring distal to the centromere result in terminal associa- 

 tion of chromosome ends after contraction of chromosomes and movement onto the 

 metaphase plate. 



id). Three orientations of translocation configurations on the metaphase plate, 

 showing the genetic consequences of each. Alternate disjunction results in viable 

 progeny, since both normal chromosomes go to one pole and reciprocally translocated 

 chromosomes to the other pole. Both types of adjacent disjuntion give rise entirely 

 to lethal progeny since none of the gametes receive a full complement of genetic 

 material. 



