No. 573] CHROMOSOME HYPOTHESIS OE LINKAGE 533 



Repulsion ™ 13? ^ Ve™ ^2 ^ ^ — — 



The three values are— red stigma red flower 35, red 

 flower long style 11, and red stigma long style 37. Of 

 these, red stigma red flower is based upon the most data, 

 and may therefore be taken as our base line. The value 

 for red stigma long style should be 35 — 11 or 24, if the 

 order of genes is long, red stigma, red flower ; but 35 + 11 

 or 4(>, it' the order of genes is red stigma, red flower, long. 

 The value shown by the table is 37. This means that long 

 lies to the right of red at a locus 46. 



R s R L 



The apparent discrepancy between the values 46 and 

 37 is due in most part to double crossing over, the effect 

 of which is always to lower large values disproportion- 

 ately more than short. When the discrepancy is known, 

 the amount of double crossing over can be calculated 

 approximately. Here the amount of double crossing over is 



That is, 4.5 per cent, of all the gametes are the result of 

 double crossing over. A somewhat larger amount of data 

 from a back cross in which all the individuals are effective 

 would give by direct experiment a true value for the 

 amount of double crossing over. 



A chromosome diagram should be built up of values 

 independent of double crossing over. According to our 

 experience with Drosophila, if there is not more than ten 

 per cent, of crossing over between two genes, the double 

 crossing over is negligible. Thus in the first chromosome 

 in sweet peas, the values obtained from the experiments 

 are not changed by double crossing over. However, in the 



