148 Linkage and Crossing Over 



Gene Symbols and Linkage 



In the symbolic method of denoting genes which geneticists 

 have adopted to simplify their writing are ways of indicating 

 linkage. Some geneticists indicate the two homologous chro- 

 mosomes by two horizontal lines, with the genes on one chromo- 

 some above one line and those on the other chromosome below 

 the other line. More recently, this symbol has been simplified 

 by merging the lines into one. It is also written all on one line 

 by using a slanting rather than horizontal line. The two pairs 

 of genes in the heterozygote, by these methods, would be indi- 

 cated as: 



CSp . . CSp 



=^= or simply or C bp / c sp 



csp csp 



The last two methods are widely used by Drosophila geneticists 

 today and the}^ are also used by students of other organisms. As 

 the Drosophila geneticist usually indicates the wild-type gene bj^ a 

 + sign, the linkage symbols can be still further simplified to read 



4- + 



; but often even the + sign is omitted, the symbol being 



c sp 



written, . Many geneticists enclose the two genes of one 



c sp 



chromosome in parentheses. By this method of notation the 



heterozygote in this case would be indicated as (CSp) (csp). A 



still further method indicates linked genes as C Sp c sp. 



Crossing Over 



It was shown in Chapter 4 that when two homologous chro- 

 mosomes have paired at pachytene each "splits" into two chro- 

 matids. The two chromatids of one chromosome are twisted 

 about the two chromatids from the homologous chromosome, 

 and at various places one chromatid from each homologue 

 breaks. The broken ends of one chromatid join up with the 

 broken ends of the other, forming new associations and pro- 

 ducing the cross-shaped chiasmata which are seen at diplotene. 



It has just been pointed out that C and Sp always remain 

 together in the formation of the Fi gametes in the above cross 

 and that c and sp do likewise unless the chromosomes or chro- 

 matids should break. Such a break would enable the two linked 



