314 Cell as Ba^is of Organic Activity 



It may be recalled that at the time of synapsis during meiosis, the 

 pairs of chromosomes may actually twist about one another as they come 

 to live close together in the formation of the primary oocyte or sperma- 

 tocyte. At this time, the portions of chromosomes may interchange. 

 The percentage of crossover is, of course, small. 



First cross: Gl Gl X gL gL 



Gametes : Gl gL 



Progeny : GlgL 



Gray-bodied, long-winged 



Second cross_: ^1 gL X gl gl 



Progeny: Gl gl Gray-bodied, vestigial-winged, 41.5% 



gL gl Black-bodied, long-winged, 41.5% 



gl gl Black-bodied, vestigial-winged, 8.5% 



GL gl Gray-bodied, long-winged, 8.5% 



Chromosomal Maps. — The farther apart genes are located on a 

 chromosome, the more likely they are to be affected by a crossover 

 Conversely if they are very close together, crossing over would seldom 

 occur. The accurate analysis of the percentage of crossover between 

 genes located on the same chromosome has made possible the construc- 

 tion of accurate chromosomal maps in Drosophila. In these maps, the 

 distance between various genes is represented as a function of the 

 amount of crossover between them. It takes very many studies to 

 give the necessary information to make these maps. Only in animals or 

 plants having a comparatively small number of chromosomes is it even 

 possible to make accurate chromosomal maps. 



Fig. 103. — Chromosomes of Drosophila. A, Male; B, female; C, enlarged portion 



of chromosome. 



Linkage represents important confirmation of the theory that genes 

 have a linear arrangement on the chromosome. In the salivary glands 

 of the Drosophila larvae, giant chromosomes are found. These chromo- 

 somes are so large that it is possible to make out much of the structure 

 that is normally invisible. On these chromosomes, alternating bands 



