Gene Segregation 



detected, difference. Second, to avoid un- 

 necessary complexity in following the results 

 of the matings, we should use only strains 

 having a single major difference. 



Third, we should use only lines which can 

 be successfully cross-fertilized in both di- 

 rections; that is, where matings can be made 

 reciprocally — the line furnishing the male 

 gamete in some crosses with a second line 

 also provides the female gametes in other 

 crosses with this second line. This is a de- 

 sirable step whose purpose is to determine 

 whether it makes any difference upon which 

 parental line the offspring start their develop- 

 ment (as pea seeds formed on the maternal 

 parent). 



Fourth, all crosses should be fully fertile; 

 that is, the parental lines should be hardy 

 plants growing vigorously and producing 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 it is possible that insuflficient 

 numbers of offspring will be obtained or, 

 more important, that the offspring observed 

 will be an incomplete sample of those whose 

 development started. Deaths that occur be- 

 tween the time of fertilization and the time 

 that we make our observations regarding the 

 phenotype of the offspring may lead to serious 

 bias. Differential viability for different geno- 

 types could cause us to miss, or underestimate 

 the frequency of, certain phenotypes; this 

 would give us misleading results with regard 

 to genotypes, especially on the view that the 

 genetic material is transmitted at the time the 

 new organism starts its existence, i.e., at the 

 time of fertilization. 



Two strains of garden pea, one producing 

 colored flowers and the other colorless flow- 

 ers, satisfy the prerequisites discussed. The 

 breeding procedure followed and the ob- 

 servations made are now described, accurate 

 records of parental and offspring phenotypes 

 having been kept, of course. 



Cross-fertilizations were made reciprocally 



between pure line colored flowers and pure 

 line colorless flowers, these individuals serv- 

 ing as the parents of the first generation (Pi). 

 The offspring seeds were planted and the 

 color of their flowers scored. All these off- 

 spring, which comprise what we may call the 

 first filial generation (Fi), were phenotypically 

 uniform, having colored flowers just like one 

 of the Pi. The Fi results were the same for 

 the reciprocal matings. In the discussion 

 which follows in this Chapter and subsequent 

 ones, it will be correct to assume that all 

 crosses were made reciprocally and produced 

 identical results, unless a statement to the con- 

 trary is made. 



What can we conclude about the genetic 

 material from these results? Let us use sym- 

 bols as a shorthand method of representing 

 the genetic material — C for the genetic ma- 

 terial whose effect produces colored flowers, 

 present in all members of the colored flowered 

 pure line, and c for the genetic material 

 producing colorless flowers, present in all the 

 colorless flowered pure line individuals. All 

 Fi individuals must contain C since they pro- 

 duce colored flowers. What has happened 

 to c? Has it failed to be transmitted? 



We may learn more by permitting the Fi 

 colored to serve as P2 (second parental gen- 

 eration) and reproduce by self-fertilization 

 to yield F2 progeny. When this is done, and 

 large enough numbers of F2 are obtained 

 from each P2 plant, it is found that among 

 the offspring of every P2 some are colored 

 and some are white. In terms of genetic 

 material, then, these F2 must carry, respec- 

 tively, C or c. It is no surprise that some F2 

 contain C, but where did the c come from 

 which is necessary for colorless F2? One 

 could at first suppose that in these cases c 

 either arose spontaneously from some non- 

 genetic origin or that C mutated to c. We 

 can bypass the first possible explanation by 

 assuming that genetic material can arise only 

 from pre-existing genetic material and that this 

 material is self-reproducing {self-replicating). 



