THE MUTATED GENE 147 



D. Differences of Dosage Not Confined to Individual 



Genes 



Differences of dosage may also be produced by chromosomal 

 rearrangements, which add (or subtract) parts of a chromosome 

 or whole chromosomes to the chromosome set, as already stated. 

 In an ascending order, such dosage alterations would be: 



1. Loss of a whole chromosome if viable. Example: haplo-IV 

 in Drosophila. 



2. Loss of a considerable section of a chromosome. Example : 

 large deletions in Drosophila. 



3. Loss of a small section of a chromosome. Example: the 

 typical deficiencies. 



4. Addition of a small duplicated fragment. Example: the 

 Bar case. 



5. Addition of a larger duplicated fragment. Example : many 

 duplications in Drosophila. 



6. Addition of an extra chromosome. Example: all the 

 manifold trisomies in plants. 



7. Additions of more than one extra chromosome up to a 

 whole set, i.e., subtriploids to triploids. For the dosage of one 

 gene the groups 4 to 7 mean always triplicates; groups 1 to 3, 

 simplex condition. 



8. Addition of more than one single whole chromosome, 

 tetrasomics, etc. 



9. Addition of more than one whole chromosome set, tetra- 

 ploidy, etc. 



10. All combinations of these types are imaginable. 



It is clear that in all these cases no direct conclusions upon the 

 action of the gene may be drawn, because differences in the dosage 

 of many genes are always involved. Nevertheless, in a general 

 way, we are dealing with the same phenomenon of dosage increase 

 that furnishes information upon the action of groups of genes in 

 different dosage. From the standpoint of developmental 

 physiology there is only a quantitative difference between these 

 types of a dosage effect. There can be no doubt, in the light of 

 the cases reported above, that any deviation from the typical 

 dosage of a gene, resulting in different reaction velocities, disturbs 

 the normal setup of the interplay of the different simultaneous 

 and consecutive determining reactions. The result is an abnor- 



