CHAPTER XIV 

 CHROMOSOME NUMBERS AND THEIR ALTERATION 



In earlier chapters it has been shown that in the hfe cycle of sexually 

 reproducing organisms there is an alternation of two chromosome num- 

 bers, one of them being double the other. In typical cases the numeri- 

 cally smaller chromosome group consists of one genome, while the larger 

 consists of two. It has also been mentioned that higher numbers of 

 genomes are sometimes present. For reasons not well understood this 

 condition is very rare among animals, but it is of frequent occurrence 

 in plants, especially among angiosperms. In many genera of this group 

 it characterizes certain species of a genus or even all of them. Sometimes 

 a single altered individual appears in a population of diploid plants in 

 the field or breeding plot. An individual may show the increase in 

 number only in a portion of the body, this portion constituting a sector 

 or sometimes a layer of tissue overlying the normal portion. 



Plants with increased numbers of certain chromosomes or of whole 

 genomes are valuable in many ways. They furnish material for the study 

 of the action of individual chromosomes, since they may have the various 

 members of a genome present in different numerical relations. Again, 

 differences in chromosome number and the characters sometimes cor- 

 related with them are often useful in classifying related species and in 

 determining their probable origin (Chap. XVII). Finally, the change 

 in chromosome number is sometimes accompanied by morphological or 

 physiological alterations that render the j^lant more valuable com- 

 mercially. This point has increased in interest since the discovery that 

 such chromosomal changes can be induced by experimental means. 



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, 

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

 exact multiple of the monoploid, the nucleus (or tissue, etc.) is ewploid. 

 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 eleven-ploid, twelve- 

 ploid, etc. Euploid types are often said to be polyploid. In such 

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



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