40 THE THEORY OF THE GENE 



time of conjugation the members of the same pair of 

 chromosomes are those that combine. In other words, 

 conjugation is not at random, as one might possibly have 

 inferred from the earlier accounts of the process, but 

 conjugation is always between a paternally derived and 

 a maternally derived specific chromosome. 



We may now add to this information the following 

 fact, namely, that conjugation takes place because the 

 members of a pair are alike, not because they have come 

 from a male and a female respectively. This has been 

 shown in two ways. In hermaphroditic types the same 

 union occurs, although, after self-fertilization, both mem- 

 bers of each pair have come from the same individual. 

 Secondly, in exceptional cases, the two members of a 

 pair have come from the same egg, yet presumably they 

 conjugate since crossing-over takes place. 



The cytological evidence of the conjugation of like 

 chromosomes supplies the first steps for a mechanical 

 explanation as to how an interchange might take place, 

 for, obviously, if the two members of each pair come to 

 lie side by side throughout their length, gene to gene as 

 it were, the chromosomes are brought into a position 

 where equivalent blocks might be interchanged in an 

 orderly way. Of course, it does not follow that in con- 

 sequence of their side to side apposition an inter- 

 change would necessarily follow; in fact, a study of the 

 crossing-over in a linkage group, such as the sex-linked 

 group of genes of Drosophila (where a sufficient number 

 of genes is present to furnish complete evidence of what 

 takes place in the linkage series), shows that there is no 

 interchange at all in about 43.5 per cent of the eggs for 

 that pair of chromosomes. The same evidence shows that 

 one interchange takes place in about 43 per cent of the 

 eggs; that two interchanges take place in about 13 per 

 cent (double crossing-over) and three interchanges in 0.5 



