Changes Involving Unbroken Chromosomes 



161 



these are the only ways entire chromosomes 

 can be lost. 



Mosaic Heteroploidy and Aneusomy 



Mutations leading to heteroploidy need not 

 involve germ cells or the entire organism, 

 as mentioned with respect to asexually re- 

 producing species (p. 152). Sexually repro- 

 ducing species of plants and animals may 

 also show mosaicism for ploidy involving 

 reproductive or nonreproductive tissues, or 

 both. In man, for example, a baby boy 

 has been studied who is diploid in some 

 tissues and triploid in other, normally dip- 

 loid, tissues. About 3% of cells in certain 

 human tissue cultures show such changes in 

 ploidy. 



Aneusomy can also originate at any 

 mitotic, as well as meiotic, nuclear division. 

 Thus, nondisjunction at the first nuclear di- 

 vision of a normal human zygote might pro- 

 duce one nucleus that is monosomic and one 

 trisomic for chromosome 21. In this case 

 the former nucleus is expected to die, and 

 the latter nucleus, to produce a completely 

 mongoloid individual. 



Some of the aneusomics born of older 



mothers may have originated in such a post- 

 zygotic nondisjunction, as is the case in 

 mice. If nondisjunction occurs later in 

 development, it produces complementary 

 monosomic and trisomic mutant patches in 

 a diploid background, which — in the case of 

 autosomes in man and mouse — are usually 

 expected to be lethal to the individual. That 

 such nondisjunctions or chromosome losses 

 do occur with appreciable frequency is sug- 

 gested by the frequent occurrence in human 

 adults of a few cells per hundred which are 

 scored as having one or two chromosomes 

 too few or too much. It is extremely un- 

 likely that all, or even most, of these abnor- 

 mal counts are due to experimental errors 

 in preparing or in scoring the cells. Under 

 normal circumstances one would expect the 

 aneusomic cells produced after birth to be 

 functionally inferior to their neighboring 

 euploid cells and, therefore, at a selective 

 disadvantage. 



Because of the large genetic unbalance it 

 produces, addition and subtraction of whole 

 chromosomes is a class of mutation which 

 involves a phenotypic change too drastic to 

 play a very significant role in evolution. 



SUMMARY AND CONCLUSIONS 



The mutational events involving the largest recombinational unit of genetic material 

 are euploid changes in the number of whole sets of chromosomes — heteroploidy. Ploidy 

 can increase by allopolyploidy, autopolyploidy, and polynemy. The modes of origin 

 and the breeding behavior of autopolyploids, and the origin and structure of the giant 

 polynemic chromosomes in the salivary gland of Drosophila larvae are considered in 

 detail. 



Loss or gain of part of a genome — aneuploidy — can result from nondisjunction and 

 the segregation of chromosomes in polyploids, especially those possessing an odd num- 

 ber of genomes. Not only do such mutations occur in the germ and somatic lines 

 spontaneously, but they may be initiated or have their frequency enhanced by physical 

 and chemical factors. 



The addition or subtraction of single chromosomes results in aneusomy. The ab- 

 sence of a chromosome is more detrimental to survival than an excess. Aneusomy 

 produces too drastic a phenotypic change to be as inportant in evolution as heteroploidy. 



