Chapter *18 



CHANGES INVOLVING 

 WHOLE GENOMES AND 

 SINGLE WHOLE CHROMOSOMES 



I: 



N THE preceding Chapters marked 

 by asterisks we used recombina- 

 tion to study the genetic materiak 

 This operation permitted us to divide the 

 genetic material into genes whose properties 

 are expressed in terms of their recombina- 

 tional behavior. The present Chapter and 

 others to follow (especially Chapters 19, 23, 

 25) deal with the study of the genetic material 

 by means of the operation of mutation. We 

 shall be especially interested to learn to what 

 extent the genetic material can be partitioned 

 into mutational units; that is, we will be 

 concerned with the possibility of describing 

 a gene in terms of its mutational behavior. 

 It has been possible for us to learn the 

 recombinational properties of genes only be- 

 cause the genetic material exists in more than 

 one alternative state and is apparently capable 

 of replicating itself and certain of its modifi- 

 cations. You can readily see that a gene, 

 which is present in homozygous condition in 

 all organisms, is not detectable, since all 

 individuals would have the same genotype, 

 and, therefore, the same range of phenotypic 

 expression. A gene can be detected only if 

 it occurs either in different numbers in differ- 

 ent individuals, or if it has an alternate 

 allele, or both, provided such a genetic differ- 

 ence produces a detectable phenotypic change. 

 A great deal of genetic variation exists 

 among living organisms (Chapter 1). We 

 have seen that some of the phenotypic varia- 

 137 



tion attributable to genes arises via sexuality, 

 by means of which new combinations of 

 already present genes may be produced by 

 segregation, independent segregation, cross- 

 ing over, and fertilization. These mecha- 

 nisms of recombination shuffle the genes, just 

 as shuffling a deck of playing cards produces 

 the great variety of different combinations 

 obtained with the same cards. However, the 

 genetic differences found in a population to- 

 day were not always present in it. 



What we are concerned with now is how 

 the genetic differences arise whose shuffling 

 produces phenotypic variation by recombina- 

 tion. Before we can study this, however, we 

 must have some way to distinguish the origin 

 of mutants, really new genetic forms, pro- 

 duced by the process of mutation, from the 

 recombination of old, pre-existent genes. 

 Consider a case, involving Drosophila, that 

 illustrates how this distinction may be made. 

 None of the flies in laboratory strains of 

 Drosophila, regardless of origin and cross- 

 breeding, have an appendage on the anterior- 

 dorsal part of the thorax. Suppose a single 

 fly occurs with an appendage in this region 

 (Figure 18-1), and this trait appears in ap- 

 proximately one half of this fly's progeny. 

 How is the new phenotypic variation, called 

 Hexaptera, to be explained? It cannot be 

 due to environmental factors alone. Hex- 

 aptera cannot be due to the interaction of 

 particular members of a pair of genes, already 

 present in the population, which happened 

 to become combined in the same zygote at 

 fertilization. For if the occurrence of Hex- 

 aptera depended upon such a combination, 

 this would have to be so rare that, following 

 segregation, this phenotype would not be 

 expected to appear in any appreciable num- 

 ber of the progeny. Hexaptera cannot be due 

 to the rare combination of two previously 

 existing unlinked nonalleles, since at most 

 only one quarter of the progeny would have 

 the novel phenotype. So, neither segregation 

 nor independent segregation is responsible 



