92 



CHAPTER 12 



SUMMARY AND CONCLUSIONS 



In previous Chapters the transmission of autosomally located genes was studied; their 

 pattern of transmission is such that reciprocal crosses between different pure lines produce 

 Fi which are genotypically and phenotypically uniform; that is, there is no dependency be- 

 tween the traits which appear and the sex of the offspring. 



In Drosophila, sex is determined by a single pair of genes located in the sex chromosomes. 

 The facts stated in the preceding paragraph are to be expected, therefore, since whenever 

 autosomal genes segregate they will do so independently of the sex genes. 



For a number of other traits in Drosophila, however, crosses between different pure lines 

 yield results which differ in reciprocal matings, the difference appearing in the phenotype 

 shown by one of the sexes. Genes behaving in such a sex-lintced way are not located auto- 

 somally. The results obtained for such genes can be explained on the basis that the Y sex 

 chromosome carries no allele of these genes, while the X does. 



In human beings and Drosophila, XY is male and XX female, while in birds and moths, 

 it is the female which is heteromorphic, and, therefore, heterogametic with reference to sex 

 chromosomes. 



Occasionally, as a consequence of nondisjunction of sex chromosomes at meiosis, chro- 

 mosome segregation fails, and gametes containing two or, complementarily, no sex chromo- 

 somes are formed. When this nondisjunction occurs in a Drosophila female homozygous 

 for an X-linked recessive, and such a female is mated to a male carrying the dominant allele, 

 some offspring appear that are simultaneously exceptions to sex-linkage and to sex chromo- 

 some content, the exceptional feature of the one accurately predicting the exceptional 

 feature of the other, and vice versa. 



Linkage and nondisjunction offer additional tests of the hypothesis that the material basis 

 of the gene is in the chromosome, a view which is supported by so many and diverse lines 

 of evidence, and contradicted by none, that it must be accepted as theory. 



REFERENCES 



Bridges, C. B., "Non-Disjunction as 

 Proof of the Chromosome Theory of 

 Heredity," Genetics, 1:1-52, 107-163, 

 1916. 



Morgan, T. H., "Sex Limited Inheri- 

 tance in Drosophila," Science, 32:120- 

 1 22, 1910. Reprinted in Classic Papers 

 in Genetics, Peters, J. A. (Ed.), 

 Englewood Cliffs, N.J., Prentice-Hall, 

 1959, pp. 63-66. 



Calvin Blackman Bridges {1889- 



1938). {By permission of Genetics, 



Inc., vol. 25, p. 1, 1940.) 



