FRAGMENTATION AND TRANSLOCATION 325 



bered that the metaphase chromosomes studied in making these cyto- 

 logical maps have chromonemata within them, so that a map of a 

 chromonema might show a still different spacing. For chromosomes 

 appearing clearly in the prophase as long threads with characteristic 

 chromomere patterns it should be possible to build up more accurate maps 

 than can be expected for chromosomes which are "good cytological 

 material" only at the metaphase. 



Discussion. — From the facts set forth above may be drawn a number 

 of interesting inferences, some of which are the following. A chromosome 

 contains a group of elements (genes) with special individual functions, 

 and the group may often be enlarged or diminished through translocation 

 without impairing the ability of the chromosome to behave regularly as 

 a whole. Apparently one of the chief requirements is the capacity to 

 react consistently with the spindle mechanism. 



The monoploid chromosome set, or genom, is a group of chromosomes 

 differing among themselves in the number and kind of their component 

 elements. Ordinarily all or nearly all of the genetic elements (genes) 

 are probably necessary to the normal activity of the nucleus; in other 

 words, the genom is a harmonious, differentiated system of elements, the 

 majority of which are essential parts of the system. Spores or gametes 

 with extra elements (duplications) are often fully able to function, 

 although they may not compete successfully with those having normal 

 sets. Those lacking certain elements (deficiencies) usually tend to be 

 non-viable unless the deficiency is very small and involves no essential 

 genes. A spore or gamete carrying a set with a translocation, i.e., a 

 set in which all of the elements are present but arranged in a new group- 

 ing in the chromosomes, is ordinarily viable. Hence it appears that the 

 set functions best when there are present all of its naturally evolved 

 group of elements and no more, but that the arrangement of the elements 

 within the group is of somewhat secondary importance. 



In the diploid chromosome complements of somatic cells also duplica- 

 tions are usually less detrimental than deficiencies. Moreover, a trans- 

 location involving only one of the sets (the individual therefore being 

 "heterozygous for the translocation") does not interfere greatly with 

 somatic development,^^ although there are exceptions to this, notably in 

 Drosophila.^^ In diploid somatic tissues the important consideration is 

 evidently "balance," i.e., there should be present two elements of each 

 kind and no more. The complement may be balanced when a deficiency 

 in one set is offset by a duplication in the other set. The diploid com- 

 plement must meet stricter requirements at the time of meiosis when 

 homologous elements undergo synapsis. A complement in which the two 

 sets have the same elements arranged in different ways, or in which 



12 E.g., Crepis (M. Nawaschin, 1931c). 



13 MuUer and Altenburg (1930), Dobzhansky (1930a, 1931o). 



