NATURE OF THE GENETIC tlFFECTS 373 



an acentric chromosome that failed to reach the daughter nuclei (see 

 Fig. 7-4b). However, the middle piece might in this case be able to 

 survive for a time, provided its two broken ends happened to become 

 bent around so as to touch and unite with each other, forming a ring or 

 "closed" chromosome. At least, it could survive if in this process the 

 chromonema (chromosome thread) had preserved its axial orientation, 

 but, if one end had become twisted by one or more complete turns, rela- 

 tive to the other end, then when the ring chromosome later reproduced to 

 form two ring chromatids these would find themselves interlocked and 

 hence incapable of being transported to the daughter nuclei (unless they 

 broke again — an event which might lead to further complications). The 

 rings formed without torsion would not be subject to this difficulty, but 

 any descendant cells or individuals that inherited such a ring would, of 

 course, be deficient for both end pieces of the chromosome. Whether 

 they could survive for a time despite their abnormality would then 

 depend on the size and importance of the resulting gene iml)alance. 



Since the regions of chromosomes in the neighborhood of their ends, in 

 Drosophila at least, are composed of heterochromatin (see Sect. 3), 

 which is more or less dispensable, a few cases of rings with only very tiny 

 end deficiencies are known, which result in apparently normal individuals 

 even when both the homologues of the given chromosome possessed by 

 the individuals are of this ring type. Nevertheless, these as well as all 

 other ring chromosomes tend eventually to die out in the course of 

 breeding of a population. This is because a ring chromatid, when it 

 undergoes single crossing over with its partner at meiosis, necessarily 

 gives rise (no matter whether the partner chromatid is itself a ring or of 

 normal structure) to a dicentric chromatid that fails to be transported 

 properly to the daughter nuclei. As a result, fewer germ cells capable of 

 developing into normal offspring are formed by individuals with rings 

 than by those with only non-ring chromosomes, and this reproductive 

 disadvantage leads all lines of descendants with rings eventually to 

 become extinct. 



The reproductive disadvantage occasioned by the formation of dicentric 

 crossover chromatids is not so great as might be thought. As Sturtevant 

 and Beadle (1936) have shown, this is because, when crossing over occurs 

 in the meiosis of the oocyte between two chromatids of a tetrad and not 

 the other two, any dicentric chromatid resulting, being pulled toward 

 both poles at once, tends to become stalled near the middle of the spindle 

 of the first meiotic division, leaving the two noncrossover chromatids to 

 be pulled to opposite poles, one entering the inward-lying nucleus that is 

 destined to form the egg. Then, at the second meiotic division, this 

 inner noncrossover chromatid, which is still in partial conjugation with 

 the dicentric one but more centrally placed than the latter, becomes 

 seoarated from it in such a way as to be pulled still further in, into the 



