236 



HEREDITY AND ENVIRONMENT 



germ cells, one bearing the dominant and the 

 other the recessive factor, and the chance com- 

 bination of these two classes of male and fe- 

 male gametes will yield on the average one 

 union of dominant with dominant, two unions 

 of dominant with recessive and one union of 

 recessive with recessive, thus producing the 

 typical Mendelian ratio, IDD : 2D(R) : 

 I RR, as shown in the accompanying diagram, 

 (Fig. 51 a, b). 



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3 



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FIG. 51. Diagram of Mendelian inheritance, in which the 

 individual is represented by the large circle, the germ cells 

 by the small ones, dominants being shaded and recessives 

 white, a, Pure dominant X pure recessive = all dominant- 

 recessives, b, Dominant-recessive X dominant-recessive = 1 

 pure dominant : 2 dominant-recessives : 1 pure recessive, 

 c, Dominant-recessive X pure dominant = 2 pure dominant : 

 2 dominant = recessive, d, Dominant-recessive X pure reces- 

 sive 2 dominant-recessive : 2 pure recessive. 



