The causes of inversions are unknown, but it is generally assumed 

 that they result from fortuitous loops which break and rejoin to give 

 the new alignment. 



Since an inversion heterozygote will give rise to gametes carrying the 

 normal chromosome and to those carrying the inverted one in equal 

 numbers, it is expected that the progeny from inbreeding such an or- 

 ganism will show a ratio of 1 homozygous normal : 2 heterozygous in- 

 versions : 1 homozygous inversion. The relative survival or fertility 

 value of these three types will determine their ultimate concentration in 

 the population. 



Translocations 

 When a piece of a chromosome becomes transferred to another (usu- 

 ally nonhomologous) chromosome, the process is described as translo- 

 cation. This is generally, and probably always, a reciprocal affair caused 

 by the mutual breakage of two overlapping chromosomes followed by 

 rejoining, and may result either in a di kinetic (having two kinetochores) 

 and an akinetic chromosome, or in two complete chromosomes which 

 have exchanged parts. The latter type is the more important and in the 

 heterozygous state results in four chromosomes being partially homol- 

 ogous with one another. If, for example, in an organism, two pairs of 

 chromosomes designated A-B, A'-B' and C-D, C'-D' are involved in a 

 reciprocal translocation, the resulting designations may be A-B, A'-D 

 and C-B', C'-D'. Association at meiosis would give a ring of all four 

 chromosomes or a configuration (e.g., a cross) which would open out 

 into a ring at metaphase (Figure 8-6). Whether balanced gametes result 

 will depend on how the four members of the ring separate at first 

 anaphase. If, for instance, adjacent chromosomes go to the same pole, 

 half the gametes will have a duplication for the A end and a deletion for 

 the C end, while the other half will be the reverse. However, if alternate 

 chromosomes go to the same pole, all the gametes will be genetically 

 balanced, with half having normal chromosomes and the other half 

 having translocated ones. The result on inbreeding is an expectation of 

 1 homozygous normal : 2 heterozygous for the translocation : 1 homo- 

 zygous for the translocation, their ultimate concentration being depend- 

 ent on the relative survival value of the three types. Experience has 

 shown that where the ring is relatively simple, there is a strong tendency 

 for separation of alternate chromosomes to the same pole, and thus 

 fertility in a translocation heterozygote is usually high so far as cyto- 

 logical considerations have a bearing on the question. 



CYTOGENETICS / 185 



