Chapter 11 



CHANGES INVOLVING 

 UNBROKEN CHROMOSOMES 



E 



xcept for Chapters 6 and 8 

 the preceding chapters sought 

 to determine the characteris- 

 tics of the genetic material through the oper- 

 ation of genetic recombination. This opera- 

 tion revealed the existence of different re- 

 combinational units of the genetic material, 

 which in order of size, include the genome, 

 the chromosome, and the genes in a chro- 

 mosome — the smallest recombinational unit 

 being the recombinational gene. 



This chapter begins a study of the genetic 

 material through the operation of mutation. 

 We shall be especially interested in learn- 

 ing the extent to which the genetic material 

 can be divided into mutational units, always 

 remembering that the recombinational and 

 mutational units may or may not be mate- 

 rially identical. 



We have been able to learn the recom- 

 binational properties of the gene only be- 

 cause it exerts a detectable phenotypic effect, 

 and because it exists in an alternative state. 

 One can readily see that if a gene were pres- 

 ent in the same form in all organisms, it 

 would not be detectable, since all individuals 

 would have the same genotype and, there- 

 fore, the same range of phenotypic expres- 

 sion. In other words, the genes detected 

 thus far in this book were only those that 

 occur either in different numbers in different 

 individuals, or have an alternate allele, or 

 both, provided that such a genetic difference 

 produced a detectable phenotypic change. 

 149 



A great deal of genetic variation of this 

 kind exists among living organisms. We 

 have seen that some of the phenotypic varia- 

 tion attributed to genes is actually due to 

 sexuality which by segregation, independent 

 segregation, crossing over, and fertilization 

 produces new combinations of already-pres- 

 ent genes. These mechanisms of recombina- 

 tion shuffle the genes, just as shuffling a deck 

 of playing cards produces the great variety 

 of card combinations. 



Detecting Mutations 



We would like to learn two things concern- 

 ing genetic differences; namely, what they 

 are, and how they are produced. To do this 

 we must first have some way to distinguish 

 between a mutant — a really new genetic 

 form produced by the process of mutation — 

 and a recombinant for already-existing genes. 

 We can use an example in Drosophila to 

 illustrate how this distinction may be made. 

 Suppose (as was the case at one time) none 

 of the flies in laboratory strains has an ap- 

 pendage on the anterior-dorsal part of the 

 thorax. Then a single fly occurs with an 

 appendage in this region (Figure 11-1) and, 

 when crossed with the wild type of a dif- 

 ferent strain (outcrossed), this trait appears 

 in approximately one half of the progeny. 

 How is the new phenotypic variant (He.x- 

 aptera) to be explained? 



If the culture conditions had not changed. 

 Hexaptera could not be due to environmental 

 factors alone. Could Hexaptera result from 

 a new combination of already-existing ge- 

 netic units? It could not be due to the inter- 

 action between two particular alleles al- 

 ready present in the population which hap- 

 pened to combine in the same zygote at 

 fertilization, for such a combination would 

 have to be rare and, following segregation, 

 this phenotype would not be expected to 

 appear in any appreciable number of the 

 progeny of the outcross. Moreover, it could 



