HUMAN GENETICS 



327 



normal, A may fail to occur, or occur in a modified form. We shall 

 discuss later in a particular case how far Nature (the genotype) or 

 Nurture (the environment) can be said to be responsible for the mani- 

 festation of A. In the present context, the point is that in order to 

 identify human genes, genetics must learn to classify together all the 



40-50 60-70 80-00 I00-IK3 120-130 140-150 160-170 180-190 200-210 220-230 210-250- 



Fig. 134. Blood Sugar in Relatives of Diabetics. — The frequency diagram of 

 blood sugar concentration in normal individuals is shown by the full circles, 

 that of relatives of diabetics by hollow circles. Note how some of the latter, 

 without developing defmite diabetic symptoms, have an abnormally high blood- 

 sugar content. There is some doubt whether diabetes mellitus is due to a recessive 

 or a dominant gene, but whichever it is, the gene is incompletely penetrant, 

 and we are here discovering cases in which it is very weakly manifested. 



(From Pincus and White.) 



different manifestations of a single genetic 

 still largely unachieved. 



'diathesis." This task is 



2. Autosomal Dominants^ 



Probably the easiest genes to identify are rare autosomal dominants 

 with good penetrance. In a randomly mating population, the frequencies 

 of the different genotypes for a rare dominant gene A are p^ aa : 2pq 

 Aa : q^ A A, where /> + ^ = i and p is much larger than q (cf. p. 289). 



^ For mathematical methods of the foilowing three seaions, see Hogben 

 i93i> 1933. 



