Sex Chromosomes and Sex-Linked Genes 



SUMMARY AND CONCLUSIONS 



99 



Up to now we have studied genes located in autosomes. We have found that the 

 recombination of autosomal genes is such that reciprocal crosses between different pure 

 lines produce F, which are genotypically and phenotypically uniform; that is. there 

 is no dependency between the traits which appear and the sex of the offspring because 

 autosomal genes segregate independently of the genetic material in sex chromosomes. 



In Drosophila, sex is not the only trait directly associated with genetic material 

 located in the sex chromosomes. Several other traits in Drosophila yield results which 

 differ in reciprocal matings made between lines that are pure for different alternatives: 

 the difference appearing in the phenotypes shown by one of the sexes. Genes behaving 

 in such a sex-linked way are not located autosomally. The Y sex chromosome carries 

 no allele of these genes, and the X does. 



In human beings and Drosophila, the XY sex chromosomal constitution is male and 

 the XX female; 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, 

 chromosome segregation fails, and gametes are formed containing two or, comple- 

 mentarily, no sex chromosomes. When this nondisjunction occurs in a Drosophila 

 female homozygous for an X-linked recessive gene and such a female is mated to a 

 male carrying the dominant allele, some offspring appear that are simultaneously ex- 

 ceptions to sex-linkage and to sex chromosome content; the exceptional feature of the 

 one accurately predicts the exceptional feature of the other, and vice versa. 



Sex-linkage and nondisjunction offer additional tests of the hypothesis that the ma- 

 terial basis of all the genes studied thus far is in the chromosomes. This view is sup- 

 ported 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 Inheritance in Drosophila," Science, 32:120-122, 1910. 

 Reprinted in Classic Papers in Genetics, Peters, J. A. (Ed.), Englewood' Cliffs 

 N.J.: Prentice-Hall, 1959, pp. 63-66. 



QUESTIONS FOR DISCUSSION 



7.1. Under what circumstances would sons fail to receive a Y chromosome from 

 their father? 



7.2. In the cross X""Y by X"X" what would you expect to be the genotypes of the 

 zygotes produced, after sex chromosome nondisjunction during meiosis in both 

 male and female Drosophila? What is the phenotypic outcome in each case? 



7.3. If a trait is found to be due to a gene unlinked to any autosome, does this prove 

 that the gene is linked to a sex chromosome? Explain. 



7.4. A husband and wife both have normal vision, although both their fathers are 

 red-green color-blind. What is the chance that their first child will be: 



(a) a normal son? (c) a red-green color-blind son? 



(b) a normal daughter? (d) a red-green color-blind daughter? 



