132 
R. Ruggles Gates. 
MUTATIONS AND EVOLUTION. 
By R. Ruggles Gates. 
CHAPTER IV. 
Non-Disjunction in Drosophila. 
NALOGOUS to the irregular distribution of meiotic 
jLJl chromosomes, which gives rise to such mutants as (Enothera 
lata and (E. scintillans, is the phenomenon in Drosophila which 
Bridges (1916) has called non-disjunction. To make this matter 
clear it must be remembered that Drosophila melanogaster has four 
pairs of chromosomes of different sizes, including a pair of sex 
chromosomes XX in the female and an unequal pair XY in the 
male. These pairs separate like the others in meiosis, so that half 
the sperms have an X chromosome and half a Y, while all the eggs 
after maturation have an X. Furthermore, we need to know that 
the Y chromosome, as in other insects, is inactive, carrying no known 
factors and taking no active part in sex-determination. A large 
number of sex-linked characters are now known, including many 
eye colours, and as these characters are distributed in crosses exactly 
as the X chromosomes are distributed—each daughter receiving one 
X from her father and one from her rhother while each son receives 
his single X from his mother—we must suppose that sex-linked 
characters are determined by the presence of the X chromosome 
and that, in short, the determiners for such character-differences 
are borne by the X chromosomes. Moreover, the phenomena of 
“crossing over” or separation of two sex-linked characters which 
went into the cross from the same parent, i.e ., carried by the same 
X chromosome, determine the relative position of such determiners 
in the chromosome, on the assumption that the farther apart they 
are along its length the more frequently crossing over (from the 
wrapping of the chromosomes around each other and subsequently 
breaking apart in new positions) will take place. On the basis of 
the percentages of crossing over, chromosome maps have been 
constructed for all four chromosomes, in which are indicated the 
relative positions of the various genes or determiners along their 
length. It will be understood that crossing over occurs between 
the members of a pair of chromosomes, but not between one pair 
and another. The amount of breeding work involved in thus 
determining that all the mutations fall into four linkage groups and 
the relative positions of the members of each group with reference 
