GENERAL ZOOLOGY 



^ 



I 

 "ir 

 "III 



Two pairs of homologous chromosomes 

 showing positions of allelomorphic genes. 



y 



B(T}b 



cf\c 



d" '>D 



C 



Crossing over: The chromosomes of the 

 pair shown in A may twist about one an- 

 other as in C and break in the plane of the 

 dotted line so that comparable sections are 

 exchanged as shown in D. 



a 1' "A 

 b" "JB 



Deletion: One member of the chromosome 

 pair shown in A may twist on itself as in E 

 and break io the plane of the dotted line so 

 that an internal section containing gene c is 

 lost, or deleted, as shown in F. 



Inversion: One member of the chromo- 

 some pair shown in A may twist on itself 

 as in G and break in the plane of the dotted 

 line so that the section containing genes B 

 and Cis inverted as shown in H. 



Duplication and deficiency: If one mem- 

 ber of the chromosome pair shown in A 

 comes to lie across the other as shown in I 

 and a break occurs in the plane of the dotted 

 line, the chromosome on the left in J will 

 have a duplication and contain both gene d 

 and gene D, and the chromosome on the 

 right will have a deficiency of the section 

 containing gene D. 



A 

 B 

 C 



"3 



It 

 2 



Translocation: One member of the chro- 

 mosome pair shown in A may come to lie 

 across one member of the chromosome pair 

 shown in B, as seen in K. If a break occurs 

 in the plane of the dotted line, sections 

 of non-homologous chromosomes are ex- 

 changed, or translocated, as shown in L. 



Fig. 6.23. Crossing over and various chromosomal aberrations; diagrammatic. 



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