Deficiency 391 



tions but they have been found in some organisms, and several 

 have been reported in maize. There has been some dispute as 

 to the presence of true terminal deficiencies in Drosophila. The 

 ends of chromosomes have sometimes been thought to have pecu- 

 liar properties because they do not usually become attached to 

 one another or to broken chromosomal segments, and the name 

 telomere has been given to them. It has been suggested also that 

 true terminal deficiencies do not occur but that apparent terminal 

 deficiencies are actually intercalary deletions near one end of 

 the chromosome in such a position that only the telomere fails 

 to be eliminated. Sutton, however, has found several deficiencies 

 in Drosophila which appear to be truly terminal deficiencies. 

 She considers that the broken ends heal and thereafter become 

 functionally normal and that no previously existing telomere is 

 present at the end of the chromosome after the break has oc- 

 curred. 



As we have pointed out in Chapter 2, deficiencies may be 

 heterozygous or homozygous. If an animal has a homozygous 

 deficiency, it usually fails to survive to an adult stage, because 

 it does not have one complete set of genes. If a deficiency oc- 

 curs in the X chromosome, the effect is usually the same as if 

 there were a homozygous deficiency in an autosome, for the miss- 

 ing piece is usually not ''covered up" by a corresponding piece 

 of a homologous chromosome. Such a deficiency is usually lethal, 

 although there are a few deficiencies in the X chromosome of 

 Drosophila, such as that involving the yellow locus, which are 

 not lethal. Such nonlethal deficiencies are always very small 

 and include only one or two loci. The Y chromosome is dif- 

 ferent, for large pieces of this chromosome may be deficient 

 without producing any lethal effect on the fly. It is really not 

 so strange, however, w^hen it is remembered that much of the 

 Y chromosome does not contain any genes and that large pieces 

 could be removed without the loss of a single gene. Heterozy- 

 gous deficiencies are much more viable than homozygous defi- 

 ciencies or deficiencies in the X chromosome, but in some animals 

 even these result in death unless they are relatively short. 



In plants a deficiency frequently fails to survive in the gameto- 

 phyte generation. Since the gametophyte is haploid, as we 

 pointed out in Chapter 4, any deficiency will result in the failure 

 of this generation to have a complete set of genes. There is fre- 



