CHROMOSOMAL MUTATIONS 



3 = 







e 



FiGLTRE 78. Heterozygous Deficiency in the Salivary Gland Chromosomes, a-d, 

 diagrammatic; e, actual specimen. (From Altenburg, "Genetics," 2nd Ed., Henry Holt 

 & Co., Inc., 1957.) 



Once a standard architecture for any chromosome is estabHshed, it ought 

 to be possible to rearrange it in various ways. Four such types of rear- 

 rangement are known. A deletion or deficiency constitutes the loss of a 

 segment of a chromosome ( Figure 78 ) . Such losses can be produced ex- 

 perimentally by high-energy radiation, and they may occur in this way 

 naturally also. At synapsis, in an organism which is heterozygous for a 

 deficiency, the unpaired portion of the normal chromosome projects to 

 one side as a loop, while those points which are present in both chromo- 

 somes of the pair are synapsed normally. 



Just the opposite of a deletion is a duplication, in which a segment of 

 a chromosome is repeated. In a heterozygote, synapsis looks very much 

 like it does in a deficiency heterozygote, this time because a region which 

 is present only once in the normal chromosome is present twice ( or more ) 

 in the modified chromosome. In the salivary gland chromosomes, the dif- 

 ference between a deficiency and a duplication can be easily detected. 

 It is believed that the production of duplications is based upon unequal 

 crossing over, that is, crossing over in which the breaks in the homologous 

 chromosomes occur at somewhat different points. This unequal crossing 

 over would produce simultaneously two gametes with a duplication and 

 two gametes with a deficiency. 



A third type of chromosomal rearrangement is an inversion, and is sim- 

 ply a reversal of a segment of a chromosome. Thus, a chromosome in 

 which the order of parts runs ABCDEFGHIJK might undergo an inver- 

 sion of the segment D to H, with the result that the order of parts would 

 run ABCHGFEDIJK. In synapsis in the heterozygote, homologous point 

 still synapses with homologous point, with the result that one chromosome 

 of the pair must form a twisted loop, while the other continues around it 

 without twisting (Figure 79). An inversion can be produced only if a 

 chromosome is broken at two points, and if it reheals in the reversed 

 relationship. They are produced frequently in radiation experiments, and 



217 



