[Cliap. XXXVIII HYBRID SEGREGATION 461 



growth, all types of hybrids are fertile, the plants do not have to be pre- 

 vented from cross-pollinating, the characters chosen are easily recog- 

 nizable and may be combined and separated in different plants as if each 

 depended upon a single hereditary factor. Because of this simplicity 

 some phases of the experiment were relatively easy and the results were 

 not difficult to analyze. For the same reason most students in elementar)' 

 botany learn something about Mendel and his hybrid peas. They also 

 learn something about Hugo de Vries and mutation, but very little about 

 his evening primroses, for the heredity of primroses is pecuharly complex 

 and not the sort of thing one enjoys during his first approach to the 

 study of heredity. 



Some explanations. We now know that the garden pea has seven pairs 

 of chromosomes. Consequently each sperm and egg has but seven 

 chromosomes, while each cell of the vegetative body of the plant has 

 fourteen chromosomes. The genes that specifically condition the four- 

 teen characters listed earlier occur separately in these fourteen chromo- 

 somes. Thus they appear to be unit factors, and the characters they con- 

 dition appear to be independent unit characters. In one chromosome is 

 the factor of tallness; in its homolog, or synaptic mate, is the factor of 

 dwarfness. During reduction division these two chromosomes become 

 separated. Consequently each speiTn or egg of a hybrid pea will have 

 but one of these two height factors — a fact often referred to as the purity 

 of gametes, or as Mendel's law of the segregation of hereditary factors. 

 Half of the total number of microspores and resultant sperms will have 

 one of these height factors, and half will have the other height factor. 

 A similar distribution of height factors also occurs in the megaspores and 

 in the resultant eggs and fusion nuclei in the embryo sac. 



Similarly in another pair of homologous chromosomes are the factors 

 that condition flower color; in another pair are the factors that condition 

 seed form, and so on for each of the seven pairs of homologous chromo- 

 somes and seven pairs of contrasting factors. Furthermore, one factor of 

 each pair of contrasting factors completely dominates the influence of the 

 other. 



Evidentlv, if either height of plant, flower color, flower position, seed 

 form, seed color, pod form, or pod color is considered separately without 

 regard to the other characters, the results of hybridization in the garden 

 pea are as simple and as easy to decipher as were the two examples 

 described in the previous chapter. 



