NATURE OF THE GENETIC EFFECTS 375 



pericentric inversions these aneuploid crossover chromosomes are mono- 

 centric and therefore become transported regularly to the daughter 

 nuclei. The resulting gametes then give rise to genically imbalanced 

 offspring, which are usually unable to survive or are at least grossly 

 abnormal. This puts pericentrically inverted chromosomes, like trans- 

 located and ring chromosomes, at a reproductive disadvantage and causes 

 their eventual extinction. 



In the case of crossing over within paracentric inversions, the crossover 

 chromatids likewise have a deficiency and a duplication, but in addition 

 one of the crossover chromatids is dicentric while the complementary one 

 is acentric. As we have seen happen with dicentric rings, these crossover 

 chromosomes tend to be left near the middle of the meiotic spindle so 

 that the egg becomes provided, instead, with one of the monocentric 

 noncrossover chromatids (equally often the inverted and the noninverted 

 one). Thus the genetic damage to egg nuclei tends to be circumvented, 

 unless more than two chromatids of the tetrad have all undergone crossing 

 over within the inverted region. In the male, however, where all four 

 nuclei enter into gametes, each occurrence of crossing over within the 

 inverted region must result in at least two gamete nuclei that are either 

 deficient for a whole chromosome or, what is worse, that contain a 

 dicentric chromosome, or that become aborted by bridge formation. 

 Thus the reproductive disadvantage of paracentrically inverted chromo- 

 somes, leading to the genetic extinction of these chromosomes and of the 

 lives of descendants containing them, must work especially through the 

 males, as with ring chromosomes. But again an exception must be 

 made of Drosophila and those other relatively rare species in which there 

 is no crossing over in the male. In them paracentric inversions can sur- 

 vive rather freely, and they have, in fact, become common in some popula- 

 tions of such species, apparently without detriment to the latter. In 

 fact, in such cases, it is sometimes advantageous for the species to have at 

 its disposal structurally different alternatives of a given type of chromo- 

 some, each alternative being provided with a set of genes adapted to a 

 somewhat different type of situation. This gives one basis for divergence 

 in the evolution of adaptations. 



The smaller an inversion is, the less frec}uently will crossing over occur 

 between it and a noninverted chromosome within the region of the inver- 

 sion; hence the smaller will be its reproductive disadvantage and the 

 slower its extinction. Very small inverted regions probably never have 

 an opportunity to synapse with their noninverted homologous regions, 

 since in these cases the conjugation of the rest of the chromosome on each 

 side tends to prevent the inverted regions from attaining the necessary 

 alignment of corresponding parts with one another. In consequence, 

 these small inversions do not suffer from any appreciable reproductive 

 disadvantage and they sometimes survive indefinitely. 



