FRAGMENTATION AND TRANSLOCATION 317 



In certain tissue cultures it appears that the cells are especially 

 susceptible to the disturbing effect of X-rays just before the visible pro- 

 phasic changes begin. ^ The results of such derangements are sometimes 

 not evident until the succeeding anaphase, when the lack or abnormal 

 position of spindle-attachment regions, together with other agencies, 

 interferes with the regular movement of the affected chromosomes. 

 Marked chromosomal alterations are obtained in Zea by irradiating the 

 young ear during fertilization or soon afterward, when the embryos are 

 beginning their development.* Of special interest is the induction of 

 aberrations by subjecting the young ears to treatment with heat, since the 

 temperatures used need not be higher than those occasionally occurring 

 under natural conditions in the field. 



Behavior of Free Fragments. — In the behavior of fragments much 

 depends upon the spindle-attachment region. Researches on Drosoyhila 

 and Zea indicate that a fragment not including the attachment region 

 tends to be lost in mitosis unless it is translocated to another chromosome 

 with such a region. For example, when a deletion occurs in Zea as shown 

 in Fig. 179, 2, the rod-shaped fragment with the attachment region sur- 

 vives and the ring is lost; but when the ring includes the attachment 

 region, the ring remains and the rod is eliminated.^ One case is known in 

 which the break passed through the elongated attachment region, both 

 ring and rod then possessing functional portions of it. In one strain of 

 Zea a minute chromosome consisting of an attachment region and a single 

 chromomere has been observed. 



Ordinarily deletions tend to affect the viability of the cells or organ- 

 isms concerned, but this is found to vary with the amount and nature 

 of the material eliminated; it also varies at different stages of the life 

 cycle. When viability is not too seriously impaired, it can be shown by 

 genetical tests that the cells actually lack certain genes; in other words, 

 the deletion of a portion of a chromosome has created a genetic deficiency. 

 The first known case of this kind was one in which a normal gene, dominat- 

 ing one for the waltzing character in certain mice, was lost along with a 

 portion of a chromosome (Painter, 1927). Some genetic deficiencies 

 have been interpreted as being due to an inactivation of certain genes 

 rather than to an actual chromosomal deletion. 



The method of determining the location of genes in the chromosome 

 by means of induced deletions may be illustrated by a case in Zea. It 

 had been shown previously that the satellited chromosome (number VI) 

 carries a gene PI, causing a dominant purple color in the plant. Pollen 

 from homozygous recessive plants {'pl pi), and therefore all carrying pi, was 



^Strangeways and Oakley (1923), Strangeways and Hopwood (1926). 

 * Stadler (1928a, 19306, 1931a), Randolph (1932). 



^ The peculiar behavior of such ring-shaped chromosomes has been correlated with 

 a certain type of variegation (McClintock, 19326). 



