98 



CHAPTER 7 



next; both arc capable of imitation and sub- 

 sequent replication of the new form; both 

 segregate during gametogenesis so that they 



occur unpaired in the gamete; both show 

 independent segregation for different pairs; 

 both are combined randomly at fertilization. 

 It was also hypothesized that the chromo- 

 some is larger than a rccombinational gene 

 (the smallest rccombinational unit of the 

 genetic material ). since a gene was described 

 as the largest distance along the length of a 

 chromosome within which an exchange lead- 

 ing to a chiasma cannot form. 



These parallels still might be considered 

 merely coincidental. The present chapter 

 provides the following additional tests of the 

 idea that chromosomes function as the ma- 

 terial basis for genes: 



Se.x-linka^e. detected by the nonrandom 

 association between sex and the genes for 

 certain traits, was found to be an exception 

 to the mode of transmission of the auto- 

 somal genes studied previously. This phe- 

 nomenon was explained only by the assump- 

 tion that certain genes have an allele in the 

 homologous chromosome of a pair in one 

 sex but no allele in the homologous chromo- 

 some in the other sex. Hemizygosity was a 

 necessary assumption in the case of the 

 Drosophila male and the poultry female. 

 This genetic aberration was exactly paral- 

 leled by the occurrence of a pair of hetero- 

 morphic homologs in the Drosophila male 

 and poultry female, one homologous mem- 

 ber being present as a pair in the female 

 Drosophila and in the male chicken. 



Finally, in Drosophila, an exception to the 

 exception of sex-linkage was found and ex- 

 plained genetically as resulting from the 

 failure of the members of a single pair of 

 sex-linked genes to segregate. This failure 

 produces gametes containing two or no 

 alleles of a given sex-linked gene. This 

 genetic nondisjunction was shown to result 

 from chromosomal nondisjunction; that is, 



Calvin Blackman Bridgi s (1889-1938). 

 (From Genetics, vol. 25. p. 1. 1940.) 



the members of a pair of X chromosomes 

 failed to segregate during meiosis. From 

 chromosomal nondisjunction, it was pre- 

 dicted that the different genetically excep- 

 tional individuals would have different spe- 

 cific and unique sex chromosomal composi- 

 tions, and further tests proved this was the 

 case. 



In the light of these results, the view that 

 the chromosomes are the material basis for 

 all the genes so jar studied can no longer be 

 considered merely a hypothesis based upon 

 limited — therefore possibly circumstantial — 

 evidence, but must now be accepted as a 

 theory supported both by all the typical and 

 all the atypical recombinational properties 

 of genes and of chromosomes. 



Ordinarily, no further comment will be 

 made in this book about new tests that sub- 

 stantiate the theory. Henceforth, assume 

 that all tests do so unless note is made to 

 the contrary. 



