A>'/'^^v 



.o^ 



'^^^l^ 



Chapter 26 

 HAPLOIDS AND AUTOPOLYPLOIDS 



Chromosome aberrations involving segments of a chromosome 

 or individual whole chromosomes are by no means the only aber- 

 rations, for many plants and some animals differ from the 

 regular diploid type by possessing only one genome or chromo- 

 somal set, or by consisting of three or more. Those forms with 

 only one chromosome set are haploids (or monojploids) whereas 

 those that have three or more complete genomes are euploids or, 

 more frequently, polyploids. Organisms with three complete sets 

 of chromosomes are triploids; those with four, five, six, and eight 

 are knov»'n, respectively, as tetraploids, pentaploids, hexaploids, 

 and octoploids. If an organism consists of more than two ge- 

 nomes, its genomes may be alike or dissimilar. If they are all 

 alike, the polyploid is an autopolyploid, but if two or more dif- 

 ferent genomes are present, the organism is an allopolyploid. 

 Thus we have the terms autotriploid, autotetraploid, allotet- 

 raploid, and similar designations. When dealing with many other 

 biological phenomena, we often find it easier to designate cate- 

 gories than to place plants and animals into them as simply as 

 we should prefer to do. The situation is the same for poly- 

 ploids, for some organisms are not easily classified as either 

 autopolyploids or allopolyploids, and some may actually be 

 autopolyploid for some chromosomes and allopolyploids for 

 others. In general, however, the designations are useful and 

 widely used. 



Haploids 



Haploid organisms contain only one genome or set of chro- 

 mosomes. In some organisms certain phases of the life cycle 

 are regularly haploid. These phases are adjusted to the haploid 

 condition, and there is nothing abnormal about their behavior, 

 for the normal condition is to behave the way they do. It is 

 only when tissue that is normally diploid develops with only 



433 



