Gene Segregatic 



13 



expression of genes in diploid condition and 

 has no relation to their mechanism of trans- 

 mission. 



It turned out, for the six other traits 

 used to test the general applicability of our 

 gene concept, that in each case one allele was 

 dominant to the other alternative one in the 

 hybrid. From this, one might be led to con- 

 clude that dominance is a universal phenome- 

 non since it was found in each of seven differ- 

 ent studies on the occurrence and transmis- 

 sion of traits based on genes in the garden 

 pea. However, before making this decision 

 examine the results of breeding certain 

 chickens. Here black X white produces blue- 

 gray Fi. Mating two blue-gray Fi produces 

 in Fo /4 black, ]n blue-gray, and K white. You 

 see in this case that dominance does not occur 

 at all, so that dominance is not a rule for the 

 phenotypic expression of alleles in hetero- 

 zygotes. Note that when dominance is ab- 

 sent, genotypes can be written with certainty 

 from a knowledge of phenotypes. 



It should be realized that it was cross- 

 fertilization that made it possible to show 

 that genes occur as pairs, which after segre- 

 gation become unpaired, then recombine to 

 form pairs in the offspring. In other words, 

 the view that the genetic material contains 

 separable paired units is based upon the re- 

 combination which these units undergo in 

 cross-fertihzation. At this point we ought to 

 consider the meaning of the term genetic 

 recombination. You will agree that the 

 genetic units themselves are not required to 

 undergo novel changes (mutations) when 

 undergoing recombination. That is, the types 

 of genes present in a genetically recombinant 

 individual had already existed before re- 

 combination. Given an individual whose 

 gene pair is AA\ segregation followed by self- 

 fertilization may produce AA' again. But, 

 this genotype is not considered a genetic 

 recombination, but rather a reconstitution 

 of the original arrangement of the units. 

 However, the self-fertilization under discus- 



sion may also produce AA or A' A' . These 

 represent two new genetic combinations, and 

 are considered to be genetic recombinations. 

 Accordingly, when events lead to the pro- 

 duction of "old" combinations and "new" 

 combinations of genes, it is only the latter 

 type of grouping which is called genetic re- 

 combination. This usage is reasonable in 

 view of the importance that new combinations 

 have in our understanding of genetic material 

 (we were able to derive the principle of segre- 

 gation only because new combinations of 

 genes were produced by sexuality). Accord- 

 ingly, genetic recombination should be identi- 

 fied with the reassortment or regrouping of 

 genes as a consequence of which new arrange- 

 ments of them are produced. Any process 

 that has the potential of producing new 

 arrangements of genetic units is, therefore, a 

 mechanism for genetic recombination. 



The phenotypic results of the experiments 

 discussed in this and the preceding Chapter 

 have led us to hypothesize the existence of 

 genetic material which is self-replicating, 

 mutable, and transmissible. The pea plant 

 experiments reveal that the genetic material 

 can be partitioned into a pair of units by 

 means of the operation or technique of genetic 

 recombination. There may be techniques or 

 operations other than recombination which 

 may be employed to study the nature of the 

 genetic material. Should these different 

 operations reveal that the total genetic ma- 

 terial is divisible into smaller units, this would 

 not necessarily imply an equivalence among 

 the units. Thus, to use a nongenetic analogy, 

 a book (equivalent to the total genetic ma- 

 terial) can be partitioned operationally in 

 terms of chapters, pages, paragraphs, words, 

 letters, illustrations, and so forth. Each 

 operation reveals something about the book, 

 but the different units by which it is described 

 are necessarily neither identical nor mutually 

 inclusive. 



The present Chapter has revealed that the 

 genetic material contains a pair of genes. 



