INDUCED CHROMOSOMAL ABERRATIONS IN ANIMALS 1169 



2. Monosomies are individuals having one of the chromosomes repre- 

 sented only once (or fewer times than the rest of the chromosomes if the 

 individual is a polyploid). The origin of polysomics and monosomies 

 is due to irregularities in the functioning of the mitotic mechanism. 

 The daughter halves of the chromosomes in somatic divisions, and the 

 homologues in meiotic divisions, may undergo nondisjunction (Bridges, 

 12) and pass to the same pole of the spindle instead of to the opposite 

 poles. IVIonosomics may be produced also by elimination of chromosomes 

 in any divisions. 



3. Compounding is a union of separate chromosomes into a single body. 

 Temporary compounding of several chromosomes into single "Sam- 

 melchromosomen" is regularly observed in Ascaris and in certain insects. 

 Whether compounding ever occurs as a permanent heritable aberration 

 remains to be studied (see below). 



4. Fragmentation is a separation of one chromosome into two or more 

 independent ones. 



ABERRATIONS INVOLVING SECTIONS OF CHROMOSOMES 



This class of chromosomal aberrations may have the general name 

 of rearrangements of chromosomal materials or simply chromosomal 

 rearrangements. 



1. Translocation (Bridges, 16) is a transfer of a section of a chromo- 

 some from its normal location to a new one. In intrachromosomal 

 translocations the transfer is accomplished within a single chromosome. 

 Such translocations seem to be rare, possibly because the technique 

 used for finding translocations (see below) fails to detect them. Dubinin 

 (38, 39) gives a preliminary account of a translocation in Drosophila 

 W£lanogaster involving a transfer of a section of the left end of the 

 Z-chromosome to its right end. In inter chromosomal translocations a 

 section of one chromosome is transferred to a nonhomologous chromo- 

 some. Two main types of interchromosomal translocations may be 

 distinguished : 



a. Simple translocation (Dobzhansky, 28) is a transfer of a section 

 of one chromosome (the donor) to another chromosome (the recipient). 

 Simple translocation involves breakage of the donor chromosome only, 

 the recipient chromosome remaining intact, or at least no section of the 

 recipient being transferred on the donor. 



h. Reciprocal or mutual translocation (Muller, 76; Dobzhansky, 28), 

 known also as segmental interchange, ^ is an exchange of sections between 



2 The expression "segmental interchange" is used by several authors, especially 

 by those working on plants. This term is objectionable because "segmental inter- 

 change" was originally used to designate cytological crossing over and is still frequently 

 used in this sense. "Segmental interchange between nonhomologous chromosomes" 

 is certainly too long a term to be convenient. Furthermore, Belling and Blakeslee 

 (10) and Belling (9), who first used "segmental interchange" for reciprocal trans- 



