THE CHROMOSOME THEORY OF HEREDITY 290 



by the type of male gamete with which an ovum unites. Inherit- 

 ance of sex in these species is graphically indicated in Figure 172. 



Some heterogametic males have an even diploid number of 

 chromosomes, but the synaptic mate of the x chromosome, called 

 the y chromosome, is at least different in function. The chromo- 

 some complex of Drosophila is of this type (Fig. 168). 



Other animals, including the birds and Lepidoptera, are known 

 to have heterogametic females. The behaviour of the chromo- 

 somes is similar but two kinds of ova arc produced and only one 

 kind of spermatozoa. The sex chromosomes arc called z chromo- 

 somes in such cases. 



Sex chromosomes of all kinds arc sometimes called allosomes, 

 and the remaining chromosomes autosomes. 



Abnormal Behaviour of Chromosomes. The phenomena of 

 chromosome behaviour just described are the normal. In such a 

 complex series of changes it would he surprising to find that 

 abnormalities never occur. Several modifications have, in fact, 

 been demonstrated, including (1) interchange of parts between 

 synaptic mates and (2) changes in the number of chromosomes. 

 The latter may result from multiple fertilization or from the 

 failure of chromosomes to separate during the reduction division 

 so that the chromosome complex of some of the daughter cells 

 is increased and of others decreased. 



Interchange of parts between synaptic mates cannot actually 

 be seen, but during certain stages of synapsis the thread-like 

 chromosomes have been found twisted together, and some of the 

 phenomena of heredity indicate that the separation of these mates 

 may be attended by rupture at one or more points of crossing. 

 This would result in the formation of new chromosomes, each 

 formed of parts of both of its predecessors (see Chapter XVII). 



The multiplication of chromosome numbers does not change the 

 nature of the chromosomes involved, and so may not change the 

 nature of the individual. Haploid, triploid and tetraploid com- 

 plexes are on record as well as the usual diploid complex and in 

 some cases they have been produced experimentally. It is worthy 

 of note that gigantism is a common result of the tetraploid con- 

 dition as recorded by DeVries in the evening primrose, Marchals 

 in mosses and Winkler in tomato and nightshade, yet in cases 

 of abnormal numbers of sex chromosomes no abnormality is 

 apparent except the modification of sex ratios. 



