222 THE THEORY OF THE GENE 



(because it is confined to the male line), except, as just 

 pointed out, that it carries genes that are like those in the 

 corresponding part of the compound X-chromosomes. 

 Cases of inheritance have, in fact, been recently recorded 

 where certain genes appear to be carried by the Y-chro- 

 mosome, and such cases have been interpreted to mean 

 that the Y-chromosome itself may sometimes carry genes. 

 There is no objection to such a statement if interpreted 

 as above, but there is an obvious objection to this state- 

 ment if it is intended to mean more than this; for the 

 chromosomal sex-mechanism would break down if the 

 X and the Y of the male interchanged throughout. If this 

 happened, the two chromosomes would after a time be- 

 come identical, and the difference in balance that gives 

 males and females would be lost. 



The Y -Chromosome. 



There are two groups in which the genetic evidence has 

 been interpreted to mean that Mendelian factors may be 

 carried in the Y-chromosome. In fish, belonging to two 

 different families, it has been shown by Schmidt, Aida, 

 and Winge that the Y carries genes. In the gypsy moth 

 Goldschmidt has interpreted the result of species-crosses 

 in the same way (here the W-chromosome). The latter 

 results will be considered in the chapter on sex inter- 

 grades ; the former may now be taken up. 



In the small aquarium fish, Lebistes reticulatus, a na- 

 tive of the West Indies and northern South America, the 

 males are highly colored and strikingly different from the 

 females (Fig. 128). The females in different races are 

 closely similar to each other, while the males show char- 

 acteristic differences in color. Schmidt has found that 

 when a male of one race is bred to a female of another 

 race, the sons are like the father. If these hybrids (F x ) 

 are inbred, their sons (F 2 ) are again all, like the father, 



