296 Criticism of Hypofhesis of Linkage and Crossing Over 



crossing over, increases the percentage of C. O.'s uf) to the middle point 

 of the chromosome, and decreases it from that point to the end. 



The accurate location of the fiictors in the chromosomes by means 



c o 



of the empirical determination of the ratio xj^^^ty appears therefore 



to be impossible. At best an approximation only can be secured, as 

 the correction for multiple crossing over is, at any rate, in the present 

 state of our knowledge, unascertainable. 



The difficulties due to differential viability may also be practically 

 insurmountable. Interference, or the hindrance to fiee crossing over 

 caused by the proximity of the factors, cannot be effective in a case 

 of single crossing over. In multiple crossing over, however, such 

 hindrances may certainly occur. A theoretical estimate of them is 

 impossible until we have at our disposal additional data such as the 

 dimensions of the chromosomes and chmmomeres at the actual moment 

 of crossing over. In the figure on p. 295, there appears to be a maximum 

 of interference. Crossing over is absolute between the first and second, 

 interference is absolute between the first and third factors. 



The crossing over hypothesis, simple enough at first sight, is in 

 reality beset with extraordinary difficulties. 



1. Two cardinal points of the hypothesis are : (a) that the distance 

 apart of the factors is only directly determinable when two pairs of 

 allelomorphs are alone involved, and {b) that the complete assortment 

 of factors is impossible unless there are as many crossings-over (and 

 schemes) as the factors, less one. As the crossing over is supposed to 

 occur indiscriminately in homozygotes and heterozygotes, the actual 

 ratios found can seldom have a simple relation to the pairs of allelo- 

 morphs whose behaviour is actually recorded. The disturbing effect 

 of the remaining pairs must always be allowed for. It is perhaps 

 impossible to arrange an experiment iti which the F^ individual is 

 heterozygotic for two pairs of allelomorphs only. 



2. When the number of pairs of allelomorphs is reduced to three — 

 Aa, Bb, and Gc — and all other disturbing features of the hypothesis are 

 set aside, a condition, which in practice is perhaps unattainable, we 

 have to note that, by hypothesis, four kinds of behaviour are traceable 

 to the same chromosome : 



(a) no crossing over, when one of the homologous chromosomes 

 is the Y chromosome. Most of the experimental data are 

 based on this hypothetical behaviour. 



