Segregation of Alleles 



33 



single indivisible unit. In the absence of a 

 simpler explanation for the present findings 

 with peas, it seems necessary to postulate 

 that the genetic material is not always com- 

 posed of a single indivisible unit. The ap- 

 pearance of c in F 2 can be explained by 

 making the more complex assumption that 

 each P L .(Fi) individual 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 it is assumed that there 

 is a pair of genes in each P L ., then all other 

 individuals in our experiment must be as- 

 sumed to have a pair of genes, too. For, 

 in science, we adhere to the law of parsimony 

 {Occam's rule), which states that one must 

 use the minimal number of hypotheses or 

 assumptions to explain a given set of obser- 

 vations. Instead of having some individuals 

 with paired genes and others without pairs, 

 then, all are assumed 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 Ft must have been Cc. 

 Those F 2 which are colorless must be cc. 

 Attention is called to the individuals in F 2 

 that are cc. These have colorless flowers 

 phenotypically identical with those of the 

 original colorless pure line used in the P^ 

 In fact, crosses of F 2 colorless individuals 

 either with themselves or with any other 

 colorless individual (F 2 , or pure line) pro- 

 duce all colorless progeny. In other words, 

 F- cc individuals are genotypically just as 

 pure with respect to the trait under consid- 

 eration as are pure-line individuals. This 

 is true despite the fact that both c's in the 

 F 2 had been carried in F] individuals in 

 which C was the other member of the pair 

 of genes. We conclude, therefore, that when 

 c is transmitted to the F 2 , it is uncontam- 

 inated, or untainted, by having been in the 

 presence of C in the F t , even though c had 

 not been expressed in any noticeable way in 



the phenotype of the F, individuals. We 

 can generalize this conclusion and state that 

 the nature and transmission of any gene is 

 uninfluenced by whatever its partner gene 

 may be. The members of a gene pair are 

 said to be alleles (partner genes), a term 

 also applied to alternative forms of a given 

 gene. 



Since each P 2 produced colored and color- 

 less F 2 offspring, each P 2 had the genotype 

 Cc composed necessarily of C from the 

 CC P, and c from the cc P^ This specifies 

 that one and only one member of a pair of 

 genes in a parent is transmitted to each 

 offspring, 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, status of the 

 genes becomes unpaired, or haploid, during 

 transmission; but diploidy is restored in the 

 offspring because a haploid genotype is con- 

 tributed to it by each parent. 



Accepting the hypothesis that paired genes 

 are segregated by the time they are trans- 

 mitted to progeny, are the two alleles in a 

 parent equally likely to be transmitted to 

 offspring? The F 2 produced by self-fertiliza- 

 tion of Fx Cc demonstrate that both genes 

 of a given individual are transmissible. Let 

 us test the hypothesis that both members of 

 this pair of alleles are equally transmissible. 

 If so, the Fx male parent (or part) would 

 contribute C one half the time and c the 

 other half; similarly the Fi female parent 

 (or part) would contribute C half the time 

 and c the other half. Finally, assume di- 

 ploidy is restored at random; that is, the 

 haploid gene contributed to the offspring by 

 one parent is uninfluenced by the haploid 

 gene contributed by the other parent. Ac- 

 cordingly, an offspring that receives C from 

 the male (50% of offspring) will have an 

 equal chance of receiving C or c from the 

 female, so that of all offspring 25% will be 

 CC and 25%- Cc. Those offspring receiv- 

 ing c from the male (50% of offspring) will 



