372 RADIATION BIOLOGY 



transported in normal fashion at mitosis, while the excised middle piece 

 itself, being acentric, will fail to reach either of the daughter nuclei. 

 Thus the descendant cells will be aneuploid, in that they are deficient for 

 this section (see Fig. 7-4a). 



The degree and type of damage to the descendant cells caused by the 

 deficiency will, just as in the above-considered cases of deficiencies occa- 

 sioned by translocation between homologous or sister chromosomes, 

 depend upon the size of the deficiency, the importance of the missing 



genes, and the extent to which those 

 present in the undamaged homologous 

 chromosome are able in "single dose" 

 to perform the biochemical functions 

 which are ordinarily carried out by 

 the two doses present in normal cells. 



^ , , . , An offspring derived from such a de- 



FiG. 7-4. Two breaks in one chromo- ,, , i • i i -.i 



some, with all broken ends uniting so ^^^ent cell, even though provided with 

 as to give rise to a deleted and a ring a normal homologous chromosome 

 chromosome. In (a) (on left) the from its other parent, usually dies of 

 deleted chromosome, being centric, its genie imbalance (abnormal gene 

 can continue, while in (h) (on right) ^.^^-^^^ ^^^^^^^ ^^^ deficiency is a very 

 the ring is centric and can continue , 



provided it is not twisted on its axis, ^mall one. However, an extremely 



small deficiency of only one member 

 of a pair of homologous chromosomes may result in no perceptible ab- 

 normality. The deficiency will in that case behave in the same way as a 

 "recessive lethal " (see pp. 394 and 403) . It may be inherited by a succes- 

 sion of descendants. Having a slightly weakening action on them, it will 

 tend to cause the line of individuals containing it to die out. If, however, 

 it happens to persist long enough, a mating will finally occur, in some 

 subseciuent generation, between two apparently normal parents both of 

 which have one deficient chromosome of the given kind. Then an indi- 

 vidual can be produced which receives the same deficient chromosome 

 from both parents, and this individual will be killed outright by its genetic 

 abnormality. Thus in any event the deficient chromosome finally dies 

 out. 



Exceptions to the rule of the lethality of any deficiency inherited from 

 both parents are provided only by those extremely rare cases in which the 

 deficiency comprises only one or a very few genes, none of which happens 

 to be indispensable for survival. An individual with such a deficiency in 

 both homologous chromosomes might live, but would probably evince 

 its condition by one or more biochemical or morphological inadequacies. 

 These, though not directly lethal, would hamper it, so that in time — 

 though not so quickly as with a lethal — it too would undergo extinction. 



If the chromosome broken into three parts had had its centromere in 

 the middle section, then the two end pieces on uniting would have formed 



