N on- Disjunction in Drosophila. 35 



CHAPTER IV. 



NON- DISJUNCTION IN DROSOPHILA. 



Analogous to the irregular distribution of meiotic chromo- 

 somes, which gives rise to such mutants as CEnothera 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 mother 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 

 11 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 



