VARIATION AS THE BASIS FOR HEREDITARY DISTINCTIONS 



313 



somes, with the result that the original linkage relationships and the 

 meiotic behavior of the chromosomes are modified. 



3. Losses or gains of whole chromosomes ("heteroploidy"). 



4. Gains of whole sets of chromosomes, or the loss of a single set, so 

 that haploid, triploid, tetraploid, or higher polyploid individuals are 

 produced by normal diploid parents. 



Fig. 21.1. Photomicrograph of giant chromosomes from the salivary gland cells of Droso- 

 phila larvae (stained preparation). In the salivary glands of most flies the chromosomes 

 increase more than a hundredfold in length and considerably in diameter as compared 

 with their size in the cells of other tissues. They appear as cross-striped cylinders or ribbons, 

 the striations forming a constant pattern that permits identification of the chromosomes 

 and of their parts. The stainable disks that form the bands may or may not correspond to 

 single genes, but their longitudinal arrangement corresponds accurately to the gene 

 arrangement in the chromosome. In addition, the homologous chromosomes in the salivary 

 gland nuclei undergo a very intimate pairing, disk by disk, enabling identification of homol- 

 ogous disks and chromosome segments in the two chromosomes present in an individual. 

 Losses, duplications, inversions, and the like (chromosome mutations) may be detected 

 by abnormalities in the pairing of the chromosomes, which form loops or folds so as to 

 bring the corresponding disks together as much as possible. (Courtesy General Biological 

 Supply House, Inc.) 



Autogenous Variations Due to Changes in Single Genes. We have seen 

 that Mendelian inheritance is concerned with the transmission from 

 generation to generation of differences in the genotypic constitutions 

 of the original parent stocks. We have also seen something of the evidence 

 that the genotypic constitution is made up of separable and combinable 

 units — genes that occupy definite points within particular chromosomes. 

 So far we have utilized gene differences that were found "ready made" 

 in various Mendelian stocks, without question as to their origin. It is now 



