32 



(MAI' I I K 3 



ciprocal matings arc desirable in order to 

 determine whether it makes any phenotypic 

 difference to the progenj upon which paren- 

 tal line the offspring start developing (note 



that pea seeds form on the maternal parent ). 



All crosses should also be fully fertile; 

 that is. the parental lines should be hardy 

 plants that grow vigorously and produce full 

 sets of seed capable of growing to maturity, 

 not only when self-fertilized but when 

 crossed to each other reciprocally. If this 

 precaution is not taken insufficient numbers 

 of offspring may be obtained or, more im- 

 portant, the offspring observed may com- 

 prise a biased sample of those starting de- 

 velopment. Deaths that occur between the 

 time of fertilization and the time the pheno- 

 typic observations regarding the offspring 

 are made may not occur at random. Differ- 

 ential viability for different genotypes could 

 cause one to miss, or underestimate the fre- 

 quency of, certain phenotypes; this would 

 give misleading results with regard to geno- 

 types, especially on our present view that 

 the genetic material is transmitted at the 

 time the new organism starts its existence, 

 i.e., at the time of fertilization. Of course, 

 accurate records of lineage and of parental 

 and offspring phenotypes must be kept. 



Two strains of garden pea, one producing 

 colored flowers and the other colorless 

 flowers, satisfy the prerequisites discussed. 

 Cross-fertilizations are made reciprocally 

 between pure-line colored flowers and pure- 

 line colorless flowers, these individuals serv- 

 ing as the parents of the first generation 

 (P,). The offspring seeds are planted and 

 the color of the flowers produced is scored. 

 All the offspring, which comprise what may 

 be called the first filial generation (Fi), are 

 phenotypically uniform, having colored flow- 

 ers just like one of the P,. The F, results 

 are the same for the reciprocal matings. In 

 the discussion that follows in this and subse- 

 quent chapters it will be correct to assume 

 that all crosses were made reciprocally and 



produced identical results, unless a statement 

 to the contrary is made. 



What can one conclude about the genetic 

 material from these results? Let us use 

 symbols as a shorthand method of repre- 

 senting the genetic material — C for the ge- 

 netic material whose action produces colored 

 flowers, present in all members of the col- 

 ored-flowered pure line, and r for the genetic 

 material producing colorless flowers, present 

 in all the colorless-flowered pure-line indi- 

 viduals. All F 1 individuals must contain C 

 since they produce colored flowers. What 

 has happened to c? Has it failed to be trans- 

 mitted? 



More may be learned by permitting the 

 Fi plants with colored flowers to serve as 

 P L > (parents of the second generation) and 

 reproduce by self-fertilization to yield F 2 

 progeny. When this is done, and sufficient 

 numbers of F 2 are obtained from each P 2 

 plant, one finds among the offspring of every 

 P 2 that some are colored and some are white. 

 In terms of genetic material, these F L > must 

 carry, respectively, C or c. It is no surprise 

 that some F L . contain C, but where did the c 

 come from? In these cases, one could at 

 first suppose either that c arose spontaneously 

 from some non-genetic origin or that C 

 mutated to c. The first explanation can be 

 bypassed in view of the previous assump- 

 tions (p. 10) that genetic material can arise 

 only from pre-existing genetic material, and 

 that this material is self-reproducing (self- 

 replicating). The second explanation can 

 be eliminated by the observation for the 

 pure line containing C that mutations to c 

 are thousands of times rarer than the occur- 

 rence of c among the F L >. If the P-(F X ) are 

 genotypically like pure-line C individuals, as 

 assumed, mutation cannot be the explana- 

 tion for the difference in breeding behavior 

 between P,C and P L C. 



The results of the bean experiments in 

 Chapter 1 are consistent with the view that 

 the genetic material in any individual is a 



