74 The Source of Variability 



having many small chromosomes, they are viable. Dobzhansky 

 (1955) noted that in the blue grass Poa aneuploids are common 

 in natmal populations. The change in numbers of specific chromo- 

 somes will shift the gene balance and dosage levels for all the 

 characters influenced by the chromosomes involved (Fig. 27) 



How commonly aneuploids serve as the basis for changes in 

 chromosome numbers in different phylogenetic lines is not known. 

 Consulting tables of the chromosome numbers of related animals 

 (see White, 1954, pp. 186, 193-195, 198-200), it is apparent that 

 small changes in chromosome numbers are the rule in many ani- 

 mal groups, and it is quite possible that aneuploids have been 

 instrumental in initiating some of these changes. 



On occasion the entire chromosome assemblage fails to divide 

 during meiosis, so that a germ cell is formed with double the normal 

 genome complement. These, under various circumstances, give rise 

 to offspring having 3n, 4n, or higher multiples of chromosomes, 

 such individuals being termed polyploids. Polyploids have been 

 produced experimentally by use of the drug colchicine, which in- 

 hibits spindle formation in dividing cells but does not interfere with 

 the duplication and separation of the chromosomes on the equatorial 

 plate. 



Polyploidy has played an important part in plant evolution, but 

 only a minor role in animal evolution. In plants whole large groups 

 such as the ferns may be primarily polyploid (Manton, 1950). In 

 animals few cases of polyploidy have been demonstrated (White, 

 1954), and most of these are parthenogenic forms such as certain 

 weevils and earthworms. 



CHANGES IN CHROMOSOME ORGANIZATION 



During meiosis various segments of the chromosome may become 

 broken or detached and then rejoined in an order different from 

 the original. If a rejoined unit has either no centromere or two 

 centromeres, it is incapable of proper mitosis and ultimately dis- 

 appears. If the rejoined unit has a single centromere it can, other 

 factors permitting, continue to divide and persist normally. 



The rearrangement is expressed and detected as a rearranged or- 

 der of genes in the gene string. Four types are recognized (Fig. 28). 



I. A deficiency (B). Here a gene or gene sequence has dropped 

 out {efg becomes e). In the homozygous condition, deficiencies 

 are usually lethal, but even these types may persist in hetero- 

 zygotes. 



