SOME POSSIBLE BEARINGS OF GENETICS ON PATHOLOGY 1") 



still something more to be said. As is well-known there are many 

 grades of color-blindness. We do not know whether these grades 

 are due to fluctuating (individual) variations — assuming it to be 

 due to one gene : or whether there are several genes that differ in 

 the degree to which they produce the defect. In fact we know- 

 now of a good many cases in other animals where there are sev- 

 eral mutations of the same gene. For instance, in Drosophila 

 there is a series of ten such multiple allelomorphs for eye colors 

 that range from pure white to deep wine-red. There is still an- 

 other possible interpretation of the different kinds of color-blind- 

 ness — one which a priori would seem to be the most probable — 

 namely, that the differences are due to other modifying genes that 

 affect the extent to which the character develops. 



While in the great majority of cases, the scheme of color-blind- 

 ness is that shown by the diagram, we know that occasionally the 

 machinery may be changed to give a somewhat different result. 

 It is possible, for example, that a color-blind man married to a 

 perfectly normal woman may rarely produce a color-blind son. A 

 few years ago such a result would have appeared to upset the en- 

 tire scheme of sex-linked inheritance, today we understand how- 

 such cases may arise through a process that is called non-disjunc- 

 tion, which is best illustrated by numerous cases well worked out 

 in Drosophila. 



My second illustration has a more obvious chemical basis. 

 Hemophilia is also sex-linked in inheritance. It is known to be 

 much more common in men than in women, the explanation for 

 this is the same as in the other case. In affected individuals the 

 blood fails to coagulate quickly and the difference in chemical 

 composition of the blood is, in contrast to normal, the inherited 

 character. 



One of the most remarkable cases of heredity in man is found 

 in the so-called blood groups. As first definitely shown by Von 

 Dungern and Hirschfeld in 1910, the inheritance of the four blood 

 groups conforms to Mendel's laws. So consistent is this relation 

 that, as Ottenberg pointed out in 1921, the evidence might be 

 used in certain cases to determine the parentage of the child. 



