264 AN INTRODUCTION TO MODERN GENETICS 



with the behaviour of the centromeres.^ Observations of division show 

 that a chromosome fragment which does not possess a centromere does 

 not move to the poles in anaphase, while one with two centromeres, 

 such as might arise from fusion, often gets pulled apart by the move- 

 ment of the centromeres to opposite poles. It seems necessary to 

 assume that all viable chromosomes possess one centromere. The new 

 "fusion" and "fragmentation" chromosomes are therefore probably 

 always formed in the first place by translocation, and the appearance of 

 forms with an extra or a lacking chromosome is a secondary effect of 

 segregation or recombination (p. 95). 



In some genera of plants, chromosome numbers vary continuously 

 over a wide range. Thus in Carex"^ there are species with n = 28, 30, 

 33^ 34? 35? 37? 39? 4*^? 4^- Little is known about the exact nature of the 

 differences involved in such cases, but probably polyploidy, secondary 

 polyploidy, "fusion," "fragmentation" and even the occurrence of inert 

 chromosomes are all involved. 



6. Translocation^ Inversion, etc., in Species Formation 



Many nearly related species or local races differ in the shape and size 

 of their chromosomes in such a way as to suggest that deficiencies and 

 translocations have occurred during their evolution. Hybrids between 

 such forms will show the pairing of unequal chromosomes and the 

 formation of unequal bivalents at meiotic metaphase. The Brachystola 

 (Orthopteran) chromosomes described by Carothers (p. 46) are an 

 example, and more complex examples are common in Crepis. Similar 

 changes may often be inferred from a mere comparison of chromosome 

 shape between species which will not cross. The species of Drosophila 

 fall, in their chromosome morphology,^ into a series of types which can 

 be quite easily derived from each other by a few simple changes in- 

 volving translocations and "fragmentations" and "fusions" which we 

 have seen to be results of translocation. Careful analysis does not 

 always support the simplest hypotheses which suggest themselves in 

 such cases. Thus in D. Willistoni it might be assumed that one of the 

 rod-shaped chromosomes corresponds to the rod-shaped X of melano- 

 gaster, but actually non-disjunction has shown that the sex chromo- 

 some is really one of the V-shaped bodies, so that an unexpected 

 translocation must be involved, by which the arm of some other chro- 

 mosome became joined on to the originally rod-like X.^ 



^ Cf. Darlington 1937. ^ Heilborn 1924. 



^ Metz and Moses 1923. * Cf. Morgan, Bridges, and Sturtevant 1925. 



