Linkage and Crossing Over Between Genes 



121 



What is the strength of linkage between 

 a given gene and several nonalleles located 

 in the same chromosome? This problem 

 can be readily studied for certain X-linked 

 genes in Drosophila. In Figure 9-5, one 

 column shows genotypes of females; the 

 other column shows the frequencies of 

 crossover combinations as detected in their 

 sons. The recombination frequencies given 

 are those found between the gene for yel- 

 low body color (y) and for each of the fol- 

 lowing: white eye (w); crossveinless wings 

 (cv); cut wings (ct); miniature wings (m); 

 forked bristles (/). For example, 13 of 

 each 100 eggs produced by the female di- 

 hybrid for v and cv carry crossovers (-| — f- 

 or ycv). What does this value, and the 

 other still different linkage values, mean in 

 terms of meiosis? 



So far, no commitment has been made 

 as to where or when crossing over takes 

 place. Since we have been concerned with 

 complete and incomplete linkage as studied 

 in successive generations of individuals, let 

 us consider only crossing over that occurs 

 in the cell line that gives rise directly to the 



figure 9-5. Crossover percentages between 

 one gene and others linked to it. 



figure 9-6. The genetic consequences ex- 

 pected after a crossing over between linked 

 genes. 



gametes (the germ line), ignoring the pos- 

 sibility that crossing over occurs in somatic 

 (nongerminal) cells. Although crossing 

 over might be premeiotic, meiotic, or post- 

 meiotic in occurrence, we shall assume that 

 all crossovers are produced during meiosis. 

 The genetic consequences of an exchange 

 (which we now call crossing over) between 

 two pairs of linked genes during meiosis 

 were discussed earlier (Chapter 4, p. 48) 

 and the assumption was made (pp. 19-22) 

 that a chiasma represents physical cytolog- 

 ical evidence that a crossing over has oc- 

 curred. 



These cytogenetic events are diagramed in 

 Figure 9-6 in somewhat more detail than 

 those originally shown (Figure 4-8). In 

 stage I, one member of a pair of homolo- 

 gous chromosomes (hollow bar) is carrying 

 the recessives a and b and the other (solid 

 bar) is carrying their normal alleles. The 

 black dots represent centromeres. The 

 homologs synapse and form a tetrad (each 



