30 SOME POSSIBLE BEARINGS OF GENETICS ON PATHOLOGY 



heritance is like that of all sex-linked characters as shown in 

 Fig. 14. 



All males that get their single X with this tumor-gene will die : 

 therefore, since no adult males carry it, normal males must be 

 used for mating in each generation. They are mated to females 

 that are heterozygous for the chromosome carrying the tumor 

 genes. Such matings as I have said always give two daughters 

 to one son. But since half the daughters are normal and half 

 carrv the gene for tumor it is desirable to be able to pick out the 

 latter from the stock. Therefore we have made use of a trick we 

 call "marking the chromosome," which means that we use a male 

 whose sex chromosome carries a known gene near the tumor 

 locus. By using this type of male in successive generations we 

 get two types of daughters : one type like their surviving brothers 

 in eye color that do not carry the tumor-gene and the other 

 daughter with normal eyes that carries it. We use only the latter 

 to continue the stock, but we could eliminate the tumor from the 

 stock at once by using the other kind of daughters. 



Curiously enough the tumor no longer appears in the inbred 

 stock but reappears again on out-breeding. Nevertheless the 

 sex-ratio in the inbred stock continues as before, and since the 

 missing males are those with red eyes we know that the tumor- 

 gene is still present and doing its deadly work — only now the 

 young male larvae die even before they reach the age at which the 

 tumor is due to appear. 



So far I have spoken of heredity as though that term had be- 

 come synonymous with Mendelian heredity. Those of us who 

 are at work on Mendelian inheritance are often criticized as 

 too narrow. It is said that we do not recognize that anv other 

 kind of inheritance takes place. I do not think the criticism is 

 quite fair, because, in the first place, the very great number of 

 variations studied has been shown to conform to the Mendelian 

 principles or at least to be capable of such interpretation. There 

 are, however, a few exceptional cases. In certain albino plants 

 it has been shown that the inheritance of albinism can be traced 

 to the behavior of the chlorophyll bodies in the cytoplasm. The 



