SPECIAL PROBLEMS OF MITOSIS 46 



It has been found necessary to distinguish between 

 two kinds of polyploidy which are called auto -poly- 

 ploidy and alio -polyploidy. An auto -polyploid is an 

 organism with more than two haploid sets of chromo- 

 somes which are all alike — it has arisen by doubling 

 of the chromosomes in an individual which is not a 

 hybrid. In an alio -polyploid, on the other hand, the 

 doubling has taken place in a hybrid, so that the 

 several haploid sets are not all identical, having been 

 derived from different parent species. 



Polyploidy may occur within a single species, as in 

 the case of the Cruciferous plant Biscutella laevigata 

 where diploid, triploid, tetraploid, pentaploid and 

 hexaploid plants have been found ^^^ or as between 

 different species of the same genus. A good example 

 of the latter phenomenon is the Section Lapathum 

 of the genus Rumex (Table V). Intraspecific poly- 

 ploidy is of course always auto-polyploidy, while the 

 interspecific kind may be either auto- or alio -poly- 

 ploidy. 



A special t3rpe of polyploidy occurs where one or 

 more chromosomes, but not the whole haploid set, 

 are present more than twice in the complete chromo- 

 some set ; this is called polysomy. The most common 

 t3rpe of polysomy is trisomy where one or more chro- 

 mosomes are present three times, the others being 

 only present twice. The opposite phenomenon where 

 a polyploid lacks one or more chromosomes from one 



TABLE IV 



If n = the haploid number of one species and n that of 

 another, then — 



2n = diploid number of first species 



2n = ,, „ ,, second species 



n + n = diploid number of hybrid between them 



3n and 3n = the auto-triploids 



4n and 4m = the auto-tetraploids 



2n + 2n = the allo-tetraploid 



2n + 1 = a trisomic ) these terms are partially inter- 



4n — 1 = an aneuploidi changeable 



4n -f 2n and 3n + 3n = different kinds of allo-hexaploids 



