THE GENERAL ASPECTS OF HEREDITY 25 



and back-crosses will further dilute the colour. It is 

 probable that in organisms at large complete domi- 

 nance is the exception rather than the rule. Why 

 dominant mutations should be so numerous in man 

 is at present quite unexplained. In Drosophila, 

 only about a dozen dominant mutations have ap- 

 peared among some 300, all the rest being recessive, 

 and they are equally uncommon in other organisms. 



The biological characters or differences observable 

 in the human race and for the most part inherited 

 include not merely the more striking racial divergen- 

 cies, but also the innumerable structural and mental 

 or temperamental differences that we see in the 

 individuals of any population, however " pure " the 

 race. Colour of hair and eyes, height and size of 

 various parts of the body (for there is some evidence 

 that these may be independently inherited for different 

 segments), conformation of the head and features, 

 size and shape of eyes, e-ars, nose, mouth, hands, and 

 feet — there is good reason to believe that the element 

 of inheritance enters largely into the perpetuation 

 of a host of such differences as well as others more 

 minute. Everyone can cite, from his own experi- 

 ence, cases of such essentially phj^siological traits as 

 longevity and early baldness* or greyness "running 

 in families." 



* In an interesting study of the inheritance of baldness, by 

 Dorothy Osborn (1916), she tabulated the results for twenty-two 

 families and reached definite conclusions. Baldness is found to 

 be a sex-limited trait, being inherited as a dominant character 

 from father to son. In the woman it acts as a recessive, and 

 may be transmitted as such, only appearing as baldness when 

 present in the duplex (homozygous) condition. This may 

 explain the greater rarity of baldness in women. Baldness is 

 frequently associated with progressive decrease in the concentra- 

 tion of thyroid in the blood (see pp. 211 ^.). This view of early 

 baldness as a sex-limited trait is borne out by data of Sedgwick 



(1863). 



It is interesting to note in this connection that Duerden (191 8, 



