sparrow: cytogenetic effects of ionizing radiations 81 



9. Gaps or achromatic lesions 



Unstained or poorly stained short regions often seen in chroma- 

 tids after irradiation. They may represent incipient breakage. 



10. Misdi vision of the centromere 



A break in the centromere at right angles to the longitudinal 

 axis of the chromosome. Such a division is in contrast to the usual 

 lengthwise splitting. Misdivision of the centromere may result in the 

 formation of isochromosomes, i.e., two chromosomes in each of which 

 both arms are identical genetically and cytologically. 



11. Transposition 



The movement of a piece of chromosome or chromatid to a new 

 position. 



12. Exchange 



Exchange occurs when broken ends produced by two separate 

 breaks rejoin in a new combination. When two separate chromo- 

 somes are involved it is called interchange, and when the two breaks 

 involved are in the same chromosome an intrachange. A ring-shaped 

 chromosome is usually such an intrachange. Isolocus chromatid 

 exchanges usually result in a dicentric and a fragment. 



13. Simple translocation 



This results from the movement of a piece of one chromosome 

 to another followed by union of the broken ends. 



14. Reciprocal translocation 



The structure arising when portions of two nonhomologous 

 chromosomes are exchanged. These are much more common than 

 simple translocations and are one type of exchange. 



15. Inversion 



An inversion occurs when a piece of a chromosome is inverted 

 180° and inserted either in its original position or in a new position 

 in the same chromosome. Inversions are of two types, depending upon 

 the position of the centromere relative to the two breaks. Paracentric 

 inversions are those confined to a single arm of the chromosome. If 

 the centromere is included, it is a pericentric inversion. 



