36 



CHAPTER 3 



genetic units themselves are not required to 

 undergo novel changes (mutations) when 

 undergoing recombination. That is. the 

 types of genes present in a genetically re- 

 combinant individual existed before recom- 

 bination. Given an individual whose gene 

 pair is AA', segregation followed by self- 

 fertilization may produce A A' again. This 

 genotype is not considered to be a genetic 

 recombination, but rather a reconstitution 

 of the original arrangement of the units. 

 The self-fertilization under discussion may 

 also produce A A or A' A'. These represent 

 two new genetic combinations relative to the 

 parental combination, and are considered to 

 be genetic recombinations. Accordingly, 

 when events lead to the production of "old" 

 combinations and "new" combinations of 

 genes, only the latter type of grouping is 

 called genetic recombination. This usage is 

 reasonable in view of the importance that 

 new combinations have for our understand- 

 ing of genetic material (it was possible to 

 derive the principle of gene segregation only 

 because new combinations of genes were 

 produced via sexual reproduction). Ac- 



O NORMAL FEMALE V 



I I NORMAL MALE r>f 



\/ UNKNOWN 



9 AFFECTED W 



I AFFECTED ^ 

 f") — i— I Marriage Line 



QrO 



sn 



Offspring Line 



Offspring, in order 

 of birth | I, to r. ) 



Dizygotic Twins 

 Monozygotic Twins 



FIGURE 3-2. Symbols used in human pedigrees. 



cordingly, genetic recombination should be 

 identified with any reassortment or regroup- 

 ing of genes which results in a new arrange- 

 ment of them. Any process, like segregation 

 or fertilization, that has the potential of pro- 

 ducing new arrangements of genetic units is. 

 therefore, a mechanism for genetic recom- 

 bination. 



The phenotypic results of the experiments 

 discussed in Chapter 1 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 divided into a pair 

 of units by means of the operation or tech- 

 nique of genetic recombination. Techniques 

 or operations other than recombination can 

 be employed to study the nature of the ge- 

 netic material. Should other operations also 

 divide the total genetic material into units, 

 this would not necessarily mean that the 

 units revealed by different operations are 

 equivalent. Thus, to use a nongenetic anal- 

 ogy, a book (equivalent to the total genetic 

 material ) can be described operationally in 

 terms of words, letters, numbers, pages, il- 

 lustrations, 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. 



What bearing has the discovery of segre- 

 gating alleles upon the hypothesis that the 

 chromosomes represent genetic material? 

 Both genes and genomes are unpaired in 

 gametes and paired in zygotes. Can a 

 genome be the physical basis of a gene? 

 Though the gametes contain a single genome, 

 this is usually constituted (ignoring the ex- 

 changes leading to chiasmata, for the mo- 

 ment) of replicas of some maternal and 

 some paternal chromosomes. Since segre- 

 gated genes are uncontaminated by their 

 former alleles, being just as pure in the 

 gametes as they were when they entered the 

 organism at fertilization, one can reject the 



