THE LINEAR DIFFERENTIATION OF THE CHROMOSOMES 95 



The chromosome alterations are of the following types : 



1 . Deficiencies, in which a part of the chromosome is lost. In the 

 homozygous condition they usually act as lethals and they may have a 

 dominant phenotypic effect in the heterozygote. In the heterozygote 

 recessive genes in the partner to the missing section may show pseudo- 

 dominance and exaggeration similar to that mentioned in connection 

 with Haplo-IV. No crossing-over takes place in the deficient region, 

 which shows, genetically, that this part of the cross-over map is absent, 

 not merely inactivated. 



2. Deletions. — This name is often used for large deficiencies of the 

 central parts of the chromosome, both ends of which are still present. 

 They do not differ in essentials from deficiencies, in which the ends of 

 the chromosome are probably also always retained even if this does not 

 at first appear to be the case. 



3. Translocations in which part of one chromosome is broken off 

 from its normal place and joined on somewhere else, either to the 

 same or another chromosome. They are probably always interstitial, 

 that is, never at the very end of the chromosome. Branched chromo- 

 somes from lateral translocations have been observed cytologically 

 but their permanence is doubtful and they have not been analysed 

 genetically.^ 



4. DupHcations are products of breeding from translocations. The 

 organism contains its normal set of chromosomes, attached to one of 

 which is a translocated fragment which is therefore present in excess 

 and is said to be dupHcated. The presence of a dominant gene in a 

 dupHcated fragment frequentiy overcomes the presence of two reces- 

 sive allelomorphs in the normal position, whence duplications may 

 appear as gene-suppressors.^ 



5. Inversions are cases in which a section of the chromosome becomes 

 reversed in order. They can be regarded as a particular type of trans- 

 location. They are detected chiefly by their effect on crossing-over, 

 which they suppress. 



6. Simple fragmentation probably never occurs; a breakage of a 

 chromosome seems in fact always to be followed by a rejoining of 

 broken ends. If it does occur it would not be permanent as one chromo- 

 some would lack a centromere and would therefore be lost at division. 



7. Simple fusion of whole chromosomes does seem to be a possi- 

 bility if they become united in the centromere region^ — if they are 



^ Cf. Darlington 1937, Offermann 1936. 



2 Schnltz and Bridges 1932. ' Painter and Stone 1935. 



