CHROMOSOMAL MUTATIONS 



crossing over is unequal in a fly homozygous for this duphcation, then the 

 resulting chromosomes will be a normal one in which this segment is 

 present only once, and one in which the segment is present three times. 

 The normal allele of Bar now becomes simply the unmodified chromo- 

 some. 



Chromosomal Rearrangements DifFerentiating Species. Thus far, 

 two position effects have been discussed, one resulting from a transloca- 

 tion, the other from a duplication. Both types play a role in evolution. The 

 morphology and synaptic behavior of the chromosomes of many species 

 suggest that duplications may be present. Duplications have been sug- 

 gested as a possible source of "new" genes, the duplicated ones presum- 

 ably becoming completely different from the original through repeated 

 mutations. Evidence of this is found in the common occurrence of pseudo- 

 alleles. These are genes of very similar effects which are located so close 

 together in the chromosome that they are very rarely separated by cross- 

 ing over. That new genes might arise in this way is suggestive, but there 

 are some difficulties. No examples of strongly divergent pseudoalleles are 

 known, and some geneticists consider them simply as evidence that lim- 

 ited regions of the chromosomes are concerned with unified functions. 



Translocations certainly play a role in producing changes of chromo- 

 some number. For example, Makino has studied the chromosomes of the 

 Japanese loaches (teleost fishes of the family Cobitidae) Misgiirnus an- 

 guillicaudatiis and Barbatula orecis. These species have 26 and 24 pairs of 

 chromosomes respectively, and these are, for the most part, rod-shaped 

 (Figure 82). However, two pairs of the chromosomes of B. oreas are 

 V-shaped, and Makino drew the unavoidable conclusion that the reduc- 

 tion in chromosome number has been accomplished by translocations, thus 

 uniting originally distinct chromosome pairs. Similarly, in Drosophila 

 there are six basic chromosome arms which in different species are com- 

 bined in different ways to give haploid numbers varying from three to six 

 (Figure 83). Thus in D. melanogaster, the X and fourth chromosomes 

 consist of one arm each, while the second and third chromosomes consist 

 of two arms each. But in D. palustrls there are six pairs of chromosomes, 

 all of which have only a single arm. D. texana has five pairs of chromo- 

 somes, of which four have a single arm while one has two arms. Finally, 

 D. pinicola has only three pairs of chromosomes. Two of these have two 

 arms, and the third a single arm. But undoubtedly the small chromosome 



Figure 82. The Chromosomes 

 OF Misgurnus anguillicaudatus 

 AND Barbatula oreas. Note the 

 two V-shaped pairs in the latter. 

 ( From Makino, Cytologia, V. 12, 

 1941.) 



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