Deficiency 393 



condition often found in an organism that is heterozygous for a 

 deficiency. 



Although homozygous deficiencies are rare in both plants and 

 animals and deficiencies almost never survive in the microga- 

 metophyte, McClintock has reported several deficiency mutants 

 in maize that not only are viable in the sporophyte but are also 

 transmitted through the pollen. All these are minute defi- 

 ciencies that have arisen from changes in ring-shaped chromo- 

 somes. 



Pairing at zygotene in an organism with a deficiency is very 

 easy to understand if one keeps in mind the rule that, to the 

 fullest extent possible, pairing between a chromosome and its 

 homologue is between identical parts only so that every gene lies 

 alongside the same gene or one of its alleles. The configurations 

 to be found between a deficient chromosome and its normal 

 homologue are seen in Figs. 17 and 57. 



It is not known how deficiencies arise spontaneously. It is 

 possible that terminal deficiencies arise by a simple breaking of 

 a chromosome; but although such breaking is readily under- 

 standable as the result of X-ray treatment, it is not so easily 

 understood if no external agent is applied. Intercalary deficien- 

 cies after X-raying may result from two breaks in a chromo- 

 some with a dropping out of the broken piece or by one break 

 at two places in a twisted chromosome, followed by a fusion of 

 the broken ends. It has been suggested that these deletions may 

 occur spontaneously following illegitimate crossing over, a cross- 

 ing over between noncorresponding parts of homologous chromo- 

 somes or betw^een nonhomologous chromosomes. Small deficien- 

 cies may be produced by an, X-ray hit that breaks two adjacent 

 gyres of the chromonema when it is coiled in early prophase 

 (Fig. 102), as Sax has pointed out, and may even arise sponta- 

 neously from a similar break and fusion, as Husted has found. 

 Studying a series of X-ray-induced deletions in Tradescantia, 

 Rick suggests that the large rod-shaped ones result from single 

 hits and large ring-shaped ones from two interstitial hits, whereas 

 the small deletions appear to have been caused by two breaks 

 in adjacent gyres of the relic coils following one or two hits. 



If a piece breaks out of a chromosome, its future depends upon 

 whether or not it includes the centromere. If the deleted piece 

 lacks this important chromosomal structure, it usually fails to 



