INHERITANCE IN ORGANISMS 



559 



since the genes are similar all progeny will be alike and will resemble both 

 parents. If two parents, both homozygous but for different characters, 

 are crossed, the offspring will all be hybrid and will exhibit the character 

 which is dominant. In crossing these hybrid or heterozygous individuals 

 it is clear that different genes may be distributed to the different sex 

 cells which the individuals develop and that thus when these unite wdth 

 other cells, varying combinations of genes will occur. For instance, 

 considering only one pair of genes, if the dominant gene received from 

 one parent is indicated by A and the recessive gene received from the 

 other parent, by a, some of the sex cells will possess A and others a. In 



Parenis 

 P 



Cametes P 



Black 

 Bb 



Black 

 Bb 



Gametes Of? 



(D 





I 



Fig. 385. — Diagram showing the results of crossing two guinea pigs differing by one 

 character and each homozygous for that character, and the checkerboard showing the 

 results in the F2 generation. {From sketches by D. D. Whitney.) 



the union of sex cells two A's may be brought together, or two a's, or an 

 A may be united with an a. The individuals with two ^'s and those with 

 two a's are homozygous, while those with A and a are heterozygous or 

 hybrid for this pair of characters and will show the dominant character. 

 These symbols being placed in circles corresponding to the cells, the facts 

 may be fitted into diagrams showing oogenesis, spermatogenesis, and 

 fertilization in successive generations as indicated in the accompanying 

 schemes (Fig. 584). From these schemes it is apparent how segregation, 

 is brought about by the chance union of similar genes and thereby produc- 

 ing the homozygous types from hybrid parents. In the discussion of these 

 successive generations it is usual to refer to the generation which forms 

 the starting point as the P (parental) generation; the first generation pro- 



