GENETIC FACTORS IN THE ORIGIN OF DIVERSITY 397 



evolution of chromosomes — that chromosomes were originally small and 

 that in the course of evolutionary history they have increased in size by 

 the formation of repeats, accompanied by gene mutations of the con- 

 tained genes. 



Sometimes the number of genes in a chromosome is not changed but the 

 order or sequence of genes is altered. Such an inversion is shown in 



A. Normal chromosome 



B. Deletion or deficiency 



D. Inversion 



FIG. 17.11. Chromosonfial aberrations — three struc- 

 tural types. 



Fig. 17.1 ID, where the middle section of the chromosome, involving genes 

 D through J , has become reversed or inverted. Since all the genes are 

 present in normal number, the eflfects of inversions are not so drastic as are 

 the effects of deletions and duplications. Effects on the phenotype may be 

 produced, however, because of what is known as position effect. A gene 



