56 GENETICS 



/. Two Groups of Organisms, Having Different 

 Relations of Chromosomes to Sex 



In examining the question proposed In the preceding 

 paragraph, we come upon the striking fact that in different 

 groups of organisms there are very different relations of 

 chromosomes to sex. There are two main groups, showing 

 contrasted relations In this matter. One group, which in- 

 cludes the larger number of animals and plants, shows the 

 relations that were described in Chapter I. In these, the 

 males have an unequal pair of chromosomes, X and Y (or 

 they may lack completely the smaller chromosome Y) , while 

 In the females the corresponding pair consists of two equal 

 chromosomes XX. After this group had become known, 

 the remarkable fact was discovered that there Is another 

 group of organisms, including the birds and some moths, in 

 which this situation Is reversed. In this second group It Is 

 the females that have the unequal pair of chromosomes, 

 while the males have the corresponding equal pair. This 

 Is obviously a fact requiring consideration in all attempts 

 to understand how the chromosomes operate In producing 

 diversity of sex. The two groups may be characterized as 

 follows : 



Group I : The males contain one pair of dissimilar chro- 

 mosomes, X and Y; in some cases Y is lacking entirely. In 

 addition they have a number of pairs, the autosomes, in 

 which the two members of each pair are alike. The females 

 carry an equal pair XX in place of the unequal (X + Y) 

 pair of the males; they also have of course the usual auto- 

 somes. The conditions In Group I are represented In fig- 

 ure 3. 



Thus in Group I the males are heterogametic, producing 

 two kinds of sperms In equal numbers, one set of sperms 

 bearing n autosomes plus X; the other bearing n autosomes 

 without X — but in many cases with a Y. In the females all 



