242 REPRODUCTION 



Mendel's Laws. Applying this conception of unit char- 

 acters to the results of Mendel's work we may make the following 

 summary: (a) In a hybrid where the two parents have contrib- 

 uted contrasting unit characters, one of these (the so-called 

 dominant) expresses itself, while the other (the recessive) is 

 suppressed. This is the dominance of one character over another, 

 (b) The homologous chromosomes of the parents of a hybrid, 

 namely, the chromosomes that bear corresponding groups of 

 unit characters, w r hich became associated in the fertilized egg, 

 and so continued in every cell of the body sprung from the 

 fertilized egg, become again dissociated, after a possible inter- 

 change of unit characters, during the formation of microspores 

 and megaspores, so that half of these as to any character are 

 maternal and half are paternal. This is the segregation of unit 

 characters, (c) Paternal and maternal chromosomes, modified 

 to a certain extent by the previous exchange of pangenes, again 

 become associated by the fertilization of the egg, the different 

 varieties of modified homologous chromosomes coming together 

 according to chance, and so giving rise in the long run to all 

 possible combinations of paternal and maternal unit characters. 

 This we call the recombination of unit characters. 



In the F t generation (see Fig. 132) dominance alone in opera- 

 tive; in the division of the grandmother and mother cells of the 

 spores of the F 1 generation segregation occurs; when sperm cells 

 and egg cells of the F t generation unite in fertilization, giving 

 rise to embryos of the F 2 generation, recombination of unit 

 characters takes place, as shown in Fig. 134. 



Practical Applications. Knowledge of the nature and 

 behavior of hybrids has a very important bearing on the prob- 

 lems of plant breeding. In the first place it shows us that we 

 should not expect to realize the possibilities of hybridization in 

 the F, generation. Many a culture has been thrown away at 

 this stage because the hoped-for combinations did not appear, 

 when, if it had been carried to the F 2 generation the ideal might 

 have been realized, together with many other unlooked-for and 

 valuable combinations. 



