106 CELL HEREDITY 



point are tetraploid, and further back they become octoploid. Perhaps 

 the most celebrated example is that of the polytene chromosomes of some 

 insect larvae, in which each chromatid is present as many as 1024 times. 

 This type of polyploidy, in which the strands multiply but do not sep- 

 arate, is called polyteny. The term endomitosis has been applied to 

 the situation in which duplication is followed by separation of the 

 strands within a single nucleus. 



Aneuploidy 



Aneuploid cells contain odd numbers of chromosomes, other than in- 

 tegrals of the haploid number. Trisomies, for instance, contain 2u -f 1 

 chromosomes, monosomies, 2n— 1. In general, aneuploid organisms are 

 rare, although trisomic plants are occasionally found in nature. Within 

 organisms, the occurrence of aneuploid cells in normal tissues is also 

 rare, as judged from chromosome counts and cytochemical measure- 

 ments. 



Aneuploidy seems to be an important characteristic of malignant 

 tumors, and, consequently, chromosome constitution has become of great 

 interest in the study of cancer. Some investigators have proposed that 

 the development of malignancy involves increasing genotypic variability 

 of individual cells so that selection can operate to favor those capable 

 of multiplying rapidly within the host organism. It is further proposed 

 that one method of increasing variability is by altering the chromosome 

 complement. Most malignant cell lines which have been examined are 

 hypotetraploid; thus, it is the presence of additional chromosomes above 

 the diploid number which is significant. 



Recent chromosome studies of normal mouse cells immediately after 

 transfer to tissue culture have shown that explantation alone often leads 

 rapidly to the development of aneuploidy. First the tetraploid number 

 becomes predominant, and then fast growing sublines take over the cul- 

 ture; they are hypotetraploids of various sorts. These findings are 

 important not only in the investigation of cancer but also for the light 

 they cast upon experimental difficulties involved in the study of tissue- 

 cultured cells. 



CROSSING OVER 



The term crossing over refers to reciprocal genetic exchange between 

 homologous chromosomes. 



Each chromosome of meiotic prophase consists of four chromatids, two 



