402 Intrachromosomal Aberrations 



perfectly normal. If, however, the strain with the homozygous 

 inversion is crossed with the original strain, the offspring will 

 be heterozygous for the inversion and all will have loops at zygo- 

 tene. Especially if the inversion is large, crossing over within 

 the inversion will produce a fairly high degree of sterility among 

 the heterozygotes. This inversion may act as a barrier to hy- 

 bridization in nature and in that way may have some effect on 

 evolution within the species. 



One other property of inversions might be mentioned here. In 

 inversion heterozygotes, the inversion may act as a complete or 

 partial crossover suppressor in the pair of chromosomes in which 

 it is found. Because of this property, they are used as a tool in 

 the methods for detecting lethal mutations, as w^e have men- 

 tioned in Chapter 16. 



Plants and animals may be heterozygous for two or more in- 

 versions at the same time. If an organism has two inversions, 

 they may be in two different pairs of chromosomes or both may 

 be in the same chromosome. If both are in the same chromosome, 

 they may be independent, included, or overlapping as Dobzhan- 

 sky and Sturtevant have described. If the original chromosome 

 is ab c d ef g hi j, one that differs from it by two independent 

 inversions would he ab e d c f g ih j. A chromosome differing 

 from the original by two included inversions would he ab h g ef 

 dcij, and one that had two overlapping inversions might be 

 ab f ehg c dij. This last type could originate if the first in- 

 version resulted in the chromosome abfedcghij. If the sec- 

 ond inversion included the segments from d through h, inclusive, 

 the overlapping chromosome would be obtained. Such overlap- 

 ping inversions have occurred spontaneously a number of times 

 in Drosophila pseudoobscura. An appreciation of them is neces- 

 sary for an understanding of the historical development of the 

 various strains of this species and for a comparison of different 

 races and species. 



Translocations 



A translocation is a chromosomal aberration by which a por- 

 tion of one chromosome is transferred to another position on the 

 same chromosome or to another chromosome by some process 

 that is not normal crossing over. If nothing else occurs except 

 the transfer of one chromosomal segment to another, the aberra- 



