4SO EVOLUTION, GENETICS, AND EUGENICS 



Alkaptonuria Urine dark after oxidation 



Otosclerosis Thickening of ear-drum 



Lef t-handedness Probably recessive 



Tendency to twinning Probably recessive 



Susceptibility to tuberculosis Probably recessive 



Thomsen's disease Lack of muscular tone 



Menier's disease Dizziness and roaring in ears 



Deaf -mutism Congenital deafness 



Friederick's disease Degeneration of the upper part of 



the spinal cord 



Multiple schlerosis Diffuse degeneration of nervous 



tissue 



Chorea Ordinary St. Vitus' dance 



Hereditary feeble-mindedness Probably recessive 



Hereditary epilepsy Probably recessive 



Manic depressive insanity Probably recessive 



Dementia praecox Probably recessive 



C. CHARACTERS DUE TO CUMULATIVE FACTORS 



It is suspected that a good many of the characters listed as incompletely domi- 

 nant may be due to cumulative factors producing the condition usually called 

 "blending hereditary." Those that almost certainly belong to this category are: 

 stature, body weight (except certain types of obesity), skin color, shape of head, and 

 proportions of features. 



D. SEX-LINKED RECESSIVES 



Color-blindness Lack of discrimination between red 



and green 



Night-blindness One form of this defect, which in- 

 volves inability to see in dim 

 light 



Haemophilia Free bleeding 



Gower's disease Muscular atrophy 



Neuritis optica Progressive atrophy of the optic 



nerve 



• 



Some pedigrees of dominant characters. — Perhaps the best ex- 

 ample of a dominant trait that has been thoroughly worked out is 

 brack yd actyly, a type of hand in which one joint is lacking in each digit. 

 In the normal hand the thumb has one less joint than the fingers. 

 Hence in a brachydactylous person all the fingers are thumbs and the 

 thumb is a subthumb. Such a type of hand is broad, thick, and clum- 

 sy. It is not at all a well-adapted character, but it is not so bad but 

 that it can survive. Drinkwater has studied a number of brachydac- 

 tylous pedigrees one of which is shown in Figure 81. Males in this 

 chart are shown by cT and females by 9 , solid circles indicate affected 



