382 



MODERN THEORIES OF HEREDITY 



is that the germ-cells (or "gametes") which combine in repro- 

 duction can carry only one of any two mutully exclusive 

 characters, but that the zygote produced by their union may bear 

 Ijoth. One character, however, is usually (but by no means 

 always) dominant over the other. When the gametes of the 

 hybrids are formed each can bear only o>ic of the two zygote 

 characters, and on the ordinary laws of chance the number 

 bearing one will be approximately equal to the number bearing 

 the other. Since this occurs in both pollen and embryo-sac, we 

 have, on self-pollination, equal numbers of gametes bearing the 

 two characters mating at random, and again the result can be 

 predicted by the laws of chance. If the characters are called 

 A and a, we shall have the following matings occurring in equal 

 numbers : — 



A. female (9). x A. male (<?). 



A. J . X a. J . 



a. 9. X A. (? . 



and a. ? . x a. J' . 



But as there is no difiference between the hybrids Aa and aA we 

 may write the result 25 % aa, 50 % aA, 25 % AA, the last 

 two appearing alike owing to the dominance of A. We may 

 illustrate the theory by placing TOO red and 100 white counters 

 in a hat, and withdrawing them in pairs. We shall get approxi- 

 mately 25 all white pairs, 25 all red pairs, and 50 white and red. 

 The phenomenon of dominance may then be illustrated by placing 

 the red counters above the white in all the mixed pairs. Pairs 

 of characters such as those we have been considering are known 

 as allelomorphic pairs, or pairs of allelomorphs. Zygotes formed 

 by the union of like gametes are called homosygotes. those 

 formed from unlike gametes hcterozygotes. The table below 

 shows at a glance the theoretical constitution of gametes and 

 zygotes on Mendel's hypothesis. The supposed separating out 

 of the characters borne by the gametes at the formation of the 

 latter is called the segregation of the allelomorphic pairs. 



A ? 

 a o 



The above table shows the constitution of gametes and zygotes 

 for one pair of characters. The next table indicates the same 

 points for two pairs of allelomorphs, A,a. and B.b 



