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 X-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 
AIK DIK 
Fic. 27.—Diagram showing the characteristic pairing, size relations, and shapes of 
the chromosomes of Drosophila ampelophila. In the male an X- and a Y-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 Y-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, 
Digitized by Microsoft® 
