Sex-Linkage 



91 



tion for different pairs. It has also been 

 hypothesized that the chromosome is larger 

 than a gene, since the latter is delimited also 

 as the largest distance along the length of a 

 chromosome within which a chiasma cannot 

 form. Although these parallels still might be 

 considered merely coincidental, the present 

 Chapter provides additional tests of the idea 

 that chromosomes function as the material 

 basis for genes. 



Sex-linkage, detected by the nonrandom 

 association between gene transmission and 

 sex, was found to be an exception to the mode 

 of transmission of the autosomal genes here- 

 tofore discussed. This phenomenon could 

 only be explained by assuming that certain 

 genes did not have alleles in the homologous 

 chromosome of a pair in one sex but did have 

 alleles in the homologous chromosome of 

 the other sex. This assumption of hemizy- 

 gosity was necessary for the male of Drosoph- 

 ila and for the female of chickens. Such 

 a genie aberration was exactly paralleled by 

 the occurrence of a pair of heteromorphic 

 chromosomes in the case of the DrosophiJa 

 male and chicken female, one member of 

 which was present as a pair in the DrosophiJa 

 female and chicken male. 



Finally, an exception to the exception of 

 sex-linkage was found in Drosophila which 

 could be explained genically as the failure 

 of the members of a single pair of sex-linked 

 genes under observation to segregate. This 

 genie nondisjunction could be explained as be- 

 ing based upon chromosomal nondisjunction, 

 the failure of the members of a pair of X chro- 

 mosomes to segregate. The failure of these 

 chromosomes to segregate predicted that the 

 genetically exceptional individuals would 

 have a specific and unique sex chromosomal 

 composition, which further tests proved was 

 indeed the case. -=- 



It must be concluded that the idea of the 

 chromosome as the material basis of the gene 

 should no longer be considered merely an 

 hypothesis based upon limited, and therefore 

 possibly circumstantial, evidence, but must 

 now be accepted as a theory supported both 

 by all the typical and all the atypical features 

 of the transmission of genes and of chromo- 

 somes. Usually no comment will be made in 

 subsequent Chapters when new tests of the 

 theory further substantiate it, and you may 

 assume henceforth that all tests do so unless 

 note is made to the contrary. 



EGGS 



w w 

 X X 



w w 

 X X 



SPERM 



n\ 



% 



OFFSPRING \ 



www 

 (J) XXX 



w w 

 (2) X X Y 



w 

 (3) X 



(4) Y 



FIGURE 12-10. Genotypic expectation from the 

 fertilization of nondisjunctionally produced eggs 

 by normal sperm. 



m 



