THE BEHAVIOUR OF INDIVIDUAL CHROMOSOMES 



83 



showed the characters of eyeless, shaven, and bent, for which they 

 should have been heterozygous, but even showed them more strongly 

 than they normally appear in the homozygote. This "pseudo- 

 dominance" and exaggeration is characteristic of flies which contain 

 only one dose of hypomorphic genes (p. 165), and since all the factors 

 in the IVth show pseudo-dominance, all of them must be present in 

 only one dose, that is, there must be only one IVth chromosome. 

 Cytological examination confirmed this. "Diminished" is in fact 

 haplo-IV. 



The flies with three IVth chromosomes (triplo IV) which should 



XX79 



X/y d- m(3 n) 9 yXiSn) Infersex-^ 



Fig. 36. Chromosome Complements in Drosophila melanogaster. 



(From Darlington, after Bridges.) 



result from fertihzation of non-disjunctional IV.IV eggs were searched 

 for as likely to show the opposite characters to haplo-IV flies and were 

 eventually found in the progeny of triploids. Tetra-IV flies have also 

 been obtained. These hypo- and hyper-ploid variations allow one to 

 compare the eflfects of diiFerent dosages of factors in the IVth chromo- 

 some (p. 165). 



3. Non-disjunction in Other Organisms 



Organisms with one chromosome represented three times are usually 

 known as trisomies and are found in many species, sometimes as a 

 result of primary non-disjunction, sometimes of secondary non- 

 disjunction in polyploid variants. The most complete series are in 

 plants. In Datura stramonium the haploid number of chromosomes is 



