GENETICS 



249 



dominant character by the other parent. If, now, the T^'i bla,ck rpu^h 

 animals are mated togethei;, the F^ includes all the possible combinations 

 of the two characters; some animals are black rough, some black smooth, 

 some albino rough, some albino smooth, as indicated in Fig. 197. These 

 combinations are not equally numerous, however. Of every 16 7^2 

 individuals, in the long run, 9 have both dominant characters (black rough) , 

 3 have the one dominant (black smooth), 3 have the other dominant 

 (albino rough), and one has both recessives (albino smooth). The re- 

 sults may be understood in part from Fig. 198 which, if the account of 

 the inheritance of a single pair of characters has been mastered, will 

 require no further explanation.^ 



The last line in Fig. 198 may need explanation. In the segregation 

 of the genes at the time of maturation, in F\ pigs, B segregates from h 

 entirely independently of the segregation of R from r. Thus B may 

 happen to go into the saine germ cell with U, or with r. In like manner 

 h may be carried into the same germ cell with 7^, or with r. Theoretically, 



Fig. 198. — Diagram showing the formulas of the individuals and germ cells involved 

 in a cross between a blacky smooth-coated guinea-pig and an albino, rough-coated animal. 

 The Fi animals are not represented. 



in the case of wholly independent characters such as these two, the four 

 combinations BR, Br, bR, and br should occur with equal frequency. 

 Of the eggs produced by an Fi female (BbRr) in the above case, 25 per 

 cent, should possess each of the combinations named in the last line of 

 Fig. 198. In like manner, the i^i males should produce sperms of the 

 same four hereditary compositions, 25 per cent, of the sperms being of 

 each kind. 



When the four kinds of eggs are fertilized by spermatozoa, any kind 

 of egg may be fertilized by any kind of spermatozoon, Br (egg) may 

 combine with br (sperm), or with BR, or Br, or bR. Theoretically these 

 combinations should occur with equal frequency. Each of the other 

 kinds of egg may be fertilized by any one of the four kinds of spermatozoa. 

 Sixteen combinations are thus possible. For the practice of be- 

 ginners, these combinations may well be written in the form of a checker- 

 board, in the following manner. Write each egg formula four times in a 

 horizontal row, at some distance apart. Then write each sperm formula 



