Sex Chromosomes and Sex-Linked Genes 



97 



bers of the gene pair. Any egg produced 

 following nondisjunction will usually be fer- 

 tilized by a sperm carrying either an X or 

 a Y in addition to a haploid set of auto- 

 somes. (Nondisjunction can also occur dur- 

 ing meiosis in the male. We can ignore 

 this complication here, because it is an in- 

 frequent event and the probability that an 

 egg produced after nondisjunction would be 

 fertilized by a sperm produced after nondis- 

 junction is negligible.) 



If the hypothesis of chromosomal nondis- 

 junction is valid, it should be consistent with 

 the genetic results. After nondisjunction the 

 exceptional eggs produced by a white 

 (X"X ,r ) female would be either X W X W or 

 (zero designating the absence of the 

 homolog normally expected to be present). 

 The normal sperm produced by a dull-red 

 (X' t+ Y) male would carry either X w+ or 

 Y. The expected genotypes of Fi following 

 random fertilizations between these gametes 

 are given in Figure 7-9. 



Let us momentarily ignore the sex of these 

 exceptional offspring and classify them only 

 for eye color. Type 1 would be dull-red- 

 eyed, type 2 white-eyed, type 3 dull-red- 

 eyed, and type 4's eye color undetermined. 

 The genetic observations would be explained 

 if types 1 and 4 were lethal; type 2, female; 

 type 3, male. (On the hypothesis that XX 

 is female and XY is male, it is reasonable 

 to assume that types 1 and 4 would be 

 neither, and therefore might be lethal. ) 

 Even more specific requirements must be 

 fulfilled before accepting these hypotheses, 

 namely, that each exceptional white female 

 must prove to be XXY cytologically; that is, 

 such females must have, in addition to the 

 normal diploid chromosomes of a female, 

 an extra chromosome which is Y. More- 

 over, each exceptional male must have, in 

 addition to the normal autosomes, one X 

 but no Y. When the somatic cells of excep- 

 tional females and males are examined cyto- 

 logically, these chromosomal prescriptions 



are found to be filled completely. It is also 

 possible to show that YO zygotes are lethal, 

 and that X"*X"X" individuals— these usu- 

 ally die before adulthood — are dull-red-eyed. 

 Although XY individuals are fertile males, 

 XO flies are invariably sterile males. This, 

 therefore, implies that the Y chromosome is 

 necessary for male fertility, the trait being 

 attributable to a gene on the Y which has 

 no allele on the X. Moreover, our sex chro- 

 mosome formula for maleness must be mod- 

 ified to include XY and XO individuals and 

 similarly the femaleness formula also modi- 

 fied to include XX and XXY individuals. 



Chromosomes as Genetic Material 



We should now re-evaluate the hypothesis 

 that the chromosomes serve as the material 

 basis for genes. In preceding chapters, the 

 following parallels were found between the 

 properties and behavior of genes and of 

 chromosomes: both come only from pre- 

 existing counterparts; both are self-replicat- 

 ing; both occur as pairs in all cells of the 

 diploid stage of sexually reproducing organ- 

 isms except gametes; both are replicated in 

 each mitotic division; both maintain their 

 individuality from one mitotic division to the 



Y (4) Y 



figure 7-9. Genotypic expectation after fer- 

 tilization of nondisjunctionally produced eggs 

 by normal sperm. 



