58 GENETICS IN RELATION TO AGRICULTURE 



finally there is a pair of straight ones about two-thirds as long as the 

 large curved chromosomes. In the male the same relations hold except 

 that instead of the pair of straight chromosomes there is a pair consisting 

 of one straight and one somewhat larger hooked chromosome. The 

 significance of this difference in chromosome content in the sexes will 

 be pointed out in a consideration of the inheritance of sex. The pair 

 of straight chromosomes we call the sex or X-chromosomes, the unequal 

 mate of the Z-chromosome in the male of this species is called the Y- 

 chromosome. The other chromosomes are called autosomes when it is 

 desired to distinguish them as a class from the sex chromosomes. 

 Drosophila is not unique in possessing chromosomes of such characteristic 



FIG. 27. Diagram showing the characteristic pairing, size relations, and shapes of 

 the chromosomes of Drosophila ampelophila. In the male an X- and a F -chromosome 

 correspond to the X pair of the female. On the basis of X = 100 the length of each long 

 autosome is 159, of each small autosome 12, of the whole Y 112, of the long arm of the Y 71, 

 and of the short arm of the Y 41. (After Bridges.) 



shapes and sizes; but more and more as cytology advances it is 

 becoming possible to distinguish individual chromosomes, and to 

 recognize them at every cell division. 



Moreover, the characteristic paired relations which exist among the 

 chromosomes of Drosophila are of general significance. When mature 

 germ cells are formed in an individual, reduction divisions occur by means 

 of which the chromosome number is reduced in the germ cells to one-half 

 that characteristic of the body cells. Thus the germ cells of Drosophila 

 contain four chromosomes as the result of a reduction which takes place 

 in such a manner that each germ cell contains one member of each pair 

 of chromosomes. As a consequence, the germ cell of Drosophila contains 

 two large curved autosomes, representing the two pairs of these chromo- 

 somes, one small autosome, and one X- or one F-chromosome. The 

 same thing is true for other species of plants and animals in the reduc- 

 tion divisions the chromosomes are distributed in such a manner that 

 each germ cell receives one member of each pair of chromosomes. It 

 follows from this that in general a definite number of pairs of chromo- 

 somes is characteristic of the body cells of individuals of a given species, 



