Sex Determination 



103 



since synapsis does not occur between non- 

 corresponding regions of homologous chro- 

 mosomes, and, in the absence of pairing, 

 exchanges leading to chiasmata cannot 

 occur. 



To further establish the cytogenetic basis 

 for sex, we shall consider the results of 

 crosses between certain laboratory strains of 

 D. melanogaster. 1 One strain produces 

 about 75% males and 25% females (Figure 

 8-1 A), instead of the normal sex ratio of 

 approximately 50% males and 50% females. 

 Since just as many eggs become adult in 

 this unusual strain as in a normal one, the 

 abnormal result cannot be due to a gene 

 that affects the viability of one sex. 



In this exceptional case it can be hy- 

 pothesized that an autosomal gene is affect- 

 ing the determination of sex. This gene is 

 called transformer and is postulated to have 

 two alleles, tra + , and tra. Homozygotes for 

 tra are presumed always to form males re- 

 gardless of the X genes present ( tra tra is 

 epistatic and the X genes hypostatic), - 

 whereas heterozygotes or homozygotes for 

 tra + have their sex determined by the pres- 

 ence of the sex gene on the X (in this case 

 the X sex gene is epistatic). Accordingly, 

 XX individuals that are also tra tra will ap- 



1 Based upon work of A. H. Sturtevant. 



-These terms are defined in Chapter 4. pp. 51-52. 



figure 8-2. Some abnormal sex types in 

 Drosophila: A = superfemale; B = supermale; 

 C = intersex. {Drawn by E. M. Wallace.) 

 Compare with normal male and female in Fig. 

 2-6. p. 23. 



pear as males {transformed females), ex- 

 plaining the excess number of males in the 

 progeny. Thus, a cross of XY tra tra (male ) 

 by XXtra+ tra (female) (Figure 8-1B) 

 produces one-fourth each XY tra tra (males), 

 XY tra+ tra (males), XX tra tra (males, 

 transformed females), XXtra+ tra (fe- 

 males) — accounting for the numerical re- 

 sults. All these assumptions have been 

 tested in additional crosses and are con- 

 firmed, proving that autosomal genes are 

 also concerned with sex determination. 

 Note, however, that the tra allele is very 



