292 CALVIN B. BRIDGES 



THE RELATION BETWEEN COINCIDENCE AND MAP DISTANCE 



The coincidence of 61 observed in this case is relatively very 

 high. A coincidence under 5 is expected for cases in the first 

 chromosome where similar map distances are involved. This 

 higher coincidence may mean that for some reason the freedom 

 of crossing over is rhuch less in this region of the second chromo- 

 some than it is in the first chromosome. The 17.5 units of map 

 distance between black and vestigial may correspond to as 

 great a length of actual chromosome as is involved in cases in 

 the first chromosome where the coincidence is the same but the 

 map distances are nearly three times as great. On the other 

 hand, instead of the higher coincidence being due to a lower 

 'coefficient of crossing over/ it may be due to a relatively short 

 'average internode.' The length of chromosome represented by 

 a given map distance may be the same in the two regions com- 

 pared, but in the second chromosome the mechanism of double 

 crossing over may not require so long a section of chromosome be- 

 tween successive crossovers. If the average length of the inter- 

 node was shorter because of this closer spacing of doubles, then 

 a greater proportion of doubles would occur in the given region 

 from black to vestigial, and coincidence would be correspondingly 

 higher. However, the interest of these problems in double 

 crossing over is out of all proportion to our progress in their 

 solutions. It seems likely that it will be possible to derive a more 

 satisfactory method of expressing these relations than is provided 

 by the present formula; the new formulation must take account 

 of separate factors analyzable in the process, and permit of their 

 adequate representation. The conclusions based on the old 

 formula must be regarded as provisional. 



THE USE OF PURPLE IN MAPPING OTHER GENES, CURVED, 



STREAK, ETC. 



The 'map' of the second chromosome began to be useful when 

 the order and spacing for the three genes black purple and 

 vestigial were roughly established by the determination of the 

 third value, that for black purple (December, '12). The three 



