10 



CHAPTER 2 



The second explanation can be eliminated by 

 the fact that in the pure line containing C, 

 mutations to c are found to be thousands of 

 times more rare than the occurrence of c 

 among the Fo. So, if the P2 (Fi) were geno- 

 typically like pure line C individuals, as we 

 have assumed, mutation could not be the 

 explanation for the difference in breeding 

 behavior between Pi C and P2 C. 



In the absence of a simpler explanation, 

 we are faced with the necessity of postulating 

 that the genetic material is not always com- 

 posed of a single indivisible unit. The appear- 

 ance of c in F2 can be explained by making 

 the more complex assumption that each 

 P2 (Fi) contains not only C but c as well; 

 in other words, that in some individuals the 

 genetic material contains two units. Let us 

 use the word gene to refer to a unit or restricted 

 portion of the genetic material. But, if we 

 assume that there is a pair of genes in the P2, 

 we shall have to apply this rule to all other 

 individuals in our experiment. For, in sci- 

 ence, we obey the law of parsimony {Occam's 

 rule) which states that we must not multiply 

 hypotheses or assumptions needlessly. So, 

 instead of having some individuals with 

 paired genes and others with these singly, 

 we shall require all to have a pair of genes 

 in their genetic material. Accordingly, the 

 two pure lines and the Pi must have been CC 

 and cc, and all Fi must have been Cc. Those 

 F2 which are colorless must be cc. 



Now your attention is called to the indi- 

 viduals in F2 that are cc. These have color- 

 less flowers that are phenotypically identical 

 with those of the original pure line of color- 

 less used in the Pi. And, in fact, crosses of 

 F2 colorless individuals either with themselves 

 or with any other colorless individual (F2, 

 or pure line) produce all colorless progeny. 

 In other words, F2 cc individuals are geno- 

 typically just as pure with respect to the trait 

 under consideration as are pure line indi- 

 viduals. This is true despite the fact that both 

 c's in the F2 had been carried in Fi individuals 



where C was the other member of the pair of 

 genes. We conclude, therefore, that when c 

 is transmitted to the F2 it is uncontaminated, 

 or untainted, by having been in the presence 

 of C in the Fi even though it had not been 

 expressed in any noticeable way in the pheno- 

 type of those individuals. We can generalize 

 this conclusion and state that the nature and 

 transmission of any gene is uninfluenced by 

 whatever its partner gene (allele) may be. 



Since each P2 produced colored and white 

 F2 offspring, each P2 had the genotype Cc 

 composed necessarily of C from the CC Pi 

 and c from the cc Pi. This specifies that one 

 and only one member of a pair of genes in a 

 parent is transmitted to each individual off- 

 spring, so that in the transmission process the 

 members of a parental pair of genes must 

 become separated, or segregated, from each 

 other. The paired, or diploid, condition of 

 the genes, then, becomes an unpaired, single, 

 or haploid (monoploid) one during transmis- 

 sion, but diploidy is restored in the offspring 

 because a haploid genotype is contributed to 

 it by each parent. 



Accepting the hypothesis that paired genes 

 are segregated at the time they are transmitted 

 to progeny, are the two alleles in a parent 

 equally likely to be transmitted to offspring? 

 We already know, from the F2 produced by 

 self-fertilization of Fi Cc, that both genes of 

 a given individual are transmissible. Let us 

 test the hypothesis that both members of a 

 pair of alleles are equally transmissible. If 

 so, then, the male parent (or part) would con- 

 tribute C half the time and c half the time; 

 similarly 50% of the time C and 50% of the 

 time c would be contributed by the female 

 parent (or part). Finally, assume diploidy 

 is restored at random; that is, the haploid 

 gene from one parent enters an offspring 

 without regard to the haploid gene contrib- 

 uted by the other parent. Accordingly, an 

 offspring which receives C from the female 

 (50% of offspring) will have an equal chance 

 of receiving C or c from the male, so that of 



