Sex Detemunation (H) 

 FIGURE 14-1. Abnormal sex ratio in Drosophila 



A. PHENOTYPIC RESULTS 



Female x Male P 



101 



B. GENOTYPIC EXPLANATION 



XX tra tra x XY tra tra 



75% Males 



Gj ViX tra, V2X tra V2X tra, VaY tra 



' ^y /^ 25% XY tra tra (j 



25% XY tra^ tra (j 



75% XX tra tra (jl transformed) 

 - 25% XX tra 



a tra Q 



be explained as being due to some gene 

 producing a decreased viability for females. 

 It may be hypothesized, in this exceptional 

 case, that a pair of genes other than the gene 

 on the X is exerting an effect on the determi- 

 nation of sex. In this case the gene is postu- 

 lated to have two alleles, a dominant one, //•«+, 

 and a recessive one, tra. Homozygotes for 

 tra are presumed always to form males re- 

 gardless of what X genes are present (tra tra is 

 epistatic and the X genes hypostatic '), while 

 heterozygotes or homozygotes for tra+ have 

 their sex determined by the presence of one 

 2 These terms are defined in Chapter 7. 



or two sex genes on X's (in which case X 

 genes are epistatic). As a consequence of 

 this hypothesis, XX individuals that are also 

 tra tra will appear as males (transformed 

 females), explaining the excess of males in the 

 progeny. To further account for the specific 

 numerical results it is necessary to postulate 

 that the transformer gene is located auto- 

 somally (Figure 14-1 B). Thus, a cross of 

 XY tra tra (male) by XX tra+ tra (female) 

 would produce ^i XY tra tra (males), K XY 

 //•a+ tra (males), % XX tra tra (males, trans- 

 formed females), % XX tra+ tra (females). 

 All these assumptions have been tested in 

 additional crosses, and are confirmed. These 

 results prove, therefore, that autosomal genes 

 are also concerned with sex determination. 

 Note, however, that the tra allele is very rare, 

 almost all Drosophila found in nature being 

 homozygous /ra+. 



