1 54 GENETICS 



hand, two genes that are in the corresponding parts of the 

 two chromosomes of a pair, as for example e and e', cannot 

 be brought by crossing-over into the same chromosome, be- 

 cause the break is always at the same point in the two 

 chromosomes of a pair, as illustrated in figure 34. 



The facts shown in breeding experiments agree precisely 

 with these relations. Any two recessive genes that yield 

 dominants when brought together into the same cell must, 

 as we have seen, be at different regions of the two chromo- 

 somes (like a and k in figure 35), and it is true that any 

 two such recessive genes can by exchange be brought into 

 the same chromosome. Thus, in the X-chromosome of 

 Drosophila, if white eye is originally in one individual, yel- 

 low body in another, each connected with an X-chromo- 

 some, the two may be brought by crossing-over into the 

 same single X-chromosome, as illustrated in figure 32, page 

 140. Thus they can both be present in a male, which has 

 but one X-chromosome in his cells. Similarly, eosin eye and 

 vermilion eye can be brought together into the same X- 

 chromosome. In general it is true that any two recessive 

 genes that yield dominants, when brought together into the 

 same cell, can by crossing-over be brought into the same 

 chromosome (where of course they occupy different places) . 



On the other hand, it is an experimental fact that reces- 

 sive genes that do not give dominants when brought to- 

 gether into the same zygote (and therefore must be at the 

 same location in the two chromosomes), cannot be brought 

 into the same chromosome. They are thus never present in 

 the same male, since the male has but one X. Thus, eosin 

 eye and coral eye cannot both be present in the same X- 

 chromosome, though eosin eye and vermilion eye can. 

 These, it will be understood, are experimental facts; many 

 thousand experiments have given these results. Of the en- 

 tire series of multiple allelomorphs, white — ivory — buff — 

 coral — eosin — ecru — cherry — blood, etc., no two, it is 



