206 Miscellaneous Linkage Topics 



wild-type genes -}-''^'' (red eye) and +^ (normal-shaped eye). 

 Thus the two ends could be distinguished by observation. When 

 the two strains were crossed, the Fi females were heterozygous 

 for both pairs of genes and were heteromorphic for the X chro- 

 mosome. The Fi was testcrossed to a cai^ +^ male; it was the 

 double recessive and had a normal X chromosome. This test- 

 cross produced flies that were phenotypically of the parental 

 types and had parental-type chromosomes. It also yielded, how- 

 ever, a small percentage of flies which were phenotypically re- 

 combinations and had chromosomes showing that a break and 

 realignment had occurred between the two morphologically dif- 

 ferent ends. As in maize, cytological crossing over was accom- 

 panied by genetical crossing over, and the theory of crossing 

 over was further substantiated. 



Four-Strand Crossing Over 



It has been pointed out in previous chapters that crossing 

 over occurs between only one chromatid of each homologous 

 chromosome at any one place and that crossing over occurs be- 

 tween chromatids and not between whole chromosomes. Since 

 it occurs when there are four chromatids, it is said to occur in 

 the four-strand stage or double-strand stage. In Janssens's 

 earlier work on chiasmata, however, two types of crossing over 

 were described. One type, often called total chiasmatyyy , occurs 

 when the two chromatids of one homologue break and exchange 

 with the two chromatids of the other homologue at the same 

 place. This is the type that Janssens believed to be most com- 

 mon. As a result of this method, all four gametes (or spores) 

 from one mother cell would be recombinations and there would 

 be no gametes of the parental type (Fig. 63) . He also recognized 

 that partial chiasmatypy might exist by which only one chro- 

 matid of each chromosome could exchange and four types of 

 gametes would be produced. Today it is believed that total 

 chiasmatypy never occurs, and that crossing over occurs only in 

 the four-strand stage between two chromatids, one from each 

 chromosome. 



It was pointed out in Chapter 10 that the percentage of 

 chiasmata (chiasma frequency) observed in a given region of a 

 chromosome in the prophase of the first meiotic division is twice 

 as great as the percentage of recombinations between two genes 



