VI CROSSING OVER 



179 



distances from one another by the relative frequency of crossings over 

 between them. 



In Drosophila, for some unknown reason, crossing over takes place 

 in the female only, never in the male. This would be explicable if only 

 the sex-linked characters were concerned (see below), since the factors 

 for these are presumably carried in the X chromosome, of which the male 

 has only one (its mate, the Y, being apparently inert). The rule applies 

 equally, however, to characters which are not sex-linked and of which 

 the factors must therefore be carried in other chromosomes. Thus the 

 characters for body colour (grey, dominant over black) and shape of 

 wing (long, dominant over vestigial) behave in the male as an inseparable 

 couple, all the gametes of a male hybrid between a grey, long and a black, 

 vestigial, being either grey, long or black, vestigial. The correspond- 

 ing female hybrid, however, gives indeed a majority of grey, long and 

 black, vestigial gametes, but also a small percentage of grey, vestigial, 

 and black, long. Conversely, a male hybrid between a grey, vestigial, 

 and a black, long, gives only gametes grey, vestigial, and black, long, 

 while the female hybrid gives a majority of these, with, however, a small 

 percentage of grey, long and black, vestigial. It appears therefore that 

 in Drosophila exchange of substance between homologous chromosomes 

 occurs in syndesis of the female but not in that of the male ; a fact 

 which seems to indicate that the full explanation is not given by the 

 hypothesis of crossing over in its simplest form. 



Tanaka (1915) has described a case among silkworms where the 

 reverse condition obtains, crossing over occurring in the gametogenesis 

 of the male hybrid but not in that of the female. 



The above brief summary of the points of contact between the 

 observations of cytologists on one hand, and the results of the Mendelian 

 method of studying heredity on the other, must suffice, for to follow it 

 up by multiplication of detail and instances would lead us beyond the 

 subject-matter of this book. Moreover, it is not necessary, as the subject 

 has quite recently been treated in special pubHcations — e.g. Doncaster, 

 The Determination of Sex, 1914 a ; and Morgan, Sturtevant, MuUer and 

 Bridges, The Mechanism of Mendelian Heredity, 19 15 — to which the 

 reader desirous of further information is referred. These works also 

 deal with the problem of sex-linked (or sex-limited) inheritance (the 

 connection between the sex chromosomes and the determination of 

 sex has already been dealt with in Chapter IV.). This term covers those 

 cases in which certain characters are transmitted only by gametes bearing 

 the sex chromosome ; e.g. in Drosophila certain characters such as red 

 eye can be transmitted by any egg, but only by the female-producing 

 spermatozoa. Similar cases are known in the cat and in man {e.g. colour 



