FUTUEE OF THE SCIENCE OF BREEDING 167 



the first place, one of the groups of factors is very small, con- 

 taining only two factors, as compared with the 20-40 of 

 the other groups, while one of the pairs of chromosomes is also 

 very much smaller than the others. Secondly, there are 

 grounds for supposing that one pair of chromosomes is 

 definitely connected with sex determination, and at the same 

 time it has been demonstrated that one of the groups of factors 

 is peculiarly related to sex, whereas the other three groups are 

 independent. 



A strong 'prima facie case is thus made out for the close 

 connection between chromosomes and factors. But here a 

 difficulty has to be faced. If two factors belonging to a given 

 group, let us say those for grey colour as opposed to black 

 and for long wing as opposed to short, both enter by the 

 same gamete — ^it matters not whether by the maternal or the 

 paternal one — then we should expect these factors to keep 

 together when a series of gametes arises. If gametogenesis 

 implies merely the resolution of the dual structure of the cells 

 of the individual, merely the separation of the descendants of 

 the maternal and paternal chromosomes which went into the 

 cross between long-winged grey and short- winged black, then 

 the only two kinds of gamete should be grey long and black 

 short. As a matter of fact it has been shown experimentally 

 that a certain proportion of grey short and black'long gametes 

 are also formed. 



The American school gets over this difficulty in a most 

 ingenious way. Some years ago it was shown by the cytologist 

 Jannsens, that at certain stages in cell division the chromo- 

 somes of some animals tend to coil round one another, 

 spiralwise. It is suggested that at the points of closest 

 contact the paired chromosomes stick together, so that when 

 the subsequent separation occurs portions of the paternal and 

 maternal members of the pair become interchanged (Fig. 6). 

 In this way is formed a certain proportion of chromosomes in 

 which the factors for grey and long wing have become dis- 

 tributed by " crossing over " as it is termed. These are the 

 chromosomes which enter the gametes that give rise to the 

 short-winged greys and the long-winged blacks respectively. 



