Genetics 313 



A Cross Involving Linked Genes.— An example of this linkage is 

 to be found in Drosophila where the gene for body color is located on 

 the same chromosome as that for length of wing. In diagraming such a 

 cross, the linked condition is indicated by writing the two letters desig- 

 nating the two characters together with a bar above them. 



PARENTS 



qL GI 



ql 



Fig. 101. — Crosses involving linked genes. Genes located on the same chromosome 



are inherited together. 



If a gray-bodied vestigial-winged fly (GI GI) is crossed with a 

 black-bodied, long-winged_fly_(gL gL), the offspring will all be gray- 

 bodied and long-winged (gL GI). This will, of course, be identical with 

 the results obtained if the_t\vo characters were located on different 

 chromosomes. H th[s fly (gL GI) is crossed with a black-bodied vestig- 

 ial-winged fly (gi gl)^the offspring will be 50 per cent gray-bodied 

 vestigial-winged (GI gl) and 50 per cent black-bodied long-winged 

 (gL gi). In this latter cross, the offspring are Hke the original par- 

 ents and no new combinations are found. 



Fig. 102. — Diagrams to show how crossing over occurs at synapses. 



Crossing Over. — At times in a cross such as described above, in- 

 dividuals with the combination of gray-bodied, long-winged and black- 

 bodied, vestigial-winged appear. These can be accounted for only by 

 assuming that portions of the chromosomes must have been exchanged. 

 This crossing over or exchange of portions of chromosomes actually 

 does occur. 



