CHAPTER XX 

 HETEROPLOIDY 



The course of modern cytology was markedly influenced by two dis- 

 coveries made in the early years of the century. Firstly, it was observed 

 that when Drosera rotundifolia (20 chromosomes) was crossed with 

 D. longifolia (40 chromosomes), there resulted a hybrid with 30 chromo- 

 somes, of which 10 were contributed by rotundifolia and 20 by longifolia. 

 When synapsis occurred in the microsporocytes in this hybrid only 10 

 bivalents were formed, 10 chromosomes remaining unpaired. Irregular 

 distribution of these unpaired chromosomes in the meiotic mitoses then 

 followed (Rosenberg, 1904a, 19096). Secondly, it was found that while 

 (Enothera Lamarckiana had 14 chromosomes (diploid), its mutant gigas 

 had 28 (tetraploid) (Lutz, 1907; Gates, 1908a). Since that time the 

 number and the behavior of chromosomes in related types and their 

 hybrids have been very extensively studied, especially in plants, where 

 the phenomena are best displayed. The results of these studies will now 

 be summarized in three chapters. 



Terminology. — A nucleus with some number other than the true 

 monoploid or diploid number of chromosomes is said to be heteroploid. 

 This term and others given below are also applied to cells, tissues, indi- 

 viduals, races, or species with such nuclei. When the number is an exact 

 multiple of the monoploid, the nucleus (or tissue, etc.) is euploid. The 

 terms designating the multiples up to 10, beginning with the triple 

 number, are as follows: triploid, tetraploid, pentaploid, hexaploid, hep- 

 taploid, octoploid, enneaploid, decaploid. The higher multiples, which 

 are of rarer occurrence, are usually designated as 11-ploid, 12-ploid, 

 and so on. Euploid types are often said to be polyploid. In such species 

 the zygotic and gametic chromosome numbers are, for example, hexaploid 

 and triploid, or tetraploid and diploid, rather than diploid and mono- 

 ploid as in the types selected for discussion in foregoing chapters.^ 

 Non-heteroploid groups may be called homoploid. 



A nucleus (or tissue, etc.) with some number other than an exact 

 multiple of the monoploid number is aneuploid. When the number is a 

 little lower than some multiple it is hypoploid; when it is a little higher it is 

 hyperploid. Obviously, a number falling between the diploid and triploid 

 numbers, for example, may be called either hyperdiploid or hypotriploid. 



1 It was formerly customary to refer to the zygotic and gametic numbers as "dip- 

 loid" and "haploid," regardless of the number of chromosome sets actually present. 



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