Sex Determination 



107 



present among the siblings: All gynanders, 

 originating as postulated, will be weakly 

 forked in their female parts; their male parts 

 will have either normal or strongly forked 

 bristles and hairs, depending upon whether 

 the lost X carried / or f :1 \ respectively. 

 Experimental results obtained confirm ex- 

 actly these expectations. 



Gynanders also occur in moths. Whereas 

 male moths usually have large, beautifully 

 colored wings and females, small stumps of 

 wings, gynanders have been found with 

 wings like the male on one side and those 

 like the female on the other side. The ex- 

 planation for these exceptions is similar to 

 that given for Drosophila. In the case of 

 the moth, however, the gynander usually 

 starts as a male zygote (XX). 



Although most gynanders in Drosophila 

 and other insects in which the male has the 

 heteromorphic sex chromosomes, can be ex- 

 plained in this manner, some gynanders 

 originate another way. In extremely rare 

 cases, an abnormal egg is produced after 

 meiosis which contains not one but two hap- 

 loid gametic nuclei. Because polyspermy 

 sometimes occurs in insects — that is, more 

 than the one sperm normally involved in 

 fertilization enters an egg — one of the two 

 haploid egg nuclei may be fertilized by an 

 X-carrying sperm, the other by a Y-carrying 

 one. The resultant individual is approxi- 

 mately a half-and-half gynander. This type 

 of gynander can be identified if the two 

 paternal (or the two maternal) haploid 

 gametic nuclei are marked differently for a 

 pair of autosomal genes. 



Man and Mouse 



In human beings sexual type is determined 

 at fertilization, XY zygotes becoming males; 

 XX zygotes, females. In early development, 

 all sex organs or gonads are neutral; that is, 

 they give no macroscopic indication whether 

 they will later form testes or ovaries. The 

 early gonad has two regions, an outer 



one, the cortex, and an inner one, the 

 medulla. As development proceeds, the 

 cortex degenerates in those individuals that 

 carry a Y (male), and the medulla forms 

 a testis; in individuals genetically determined 

 to be females, the medulla degenerates, and 

 the cortex forms an ovary. 



Once the testis and ovary are formed, they 

 take over the regulation of further sexual 

 differentiation by means of the hormones 

 they produce. The hormones direct the de- 

 velopment or degeneration of various sexual 

 ducts, the formation of genitalia, and other 

 secondary sexual characteristics. Since sex- 

 ual differentiation is largely controlled by 

 the sex hormones, it is not surprising that 

 genetically normal individuals are morpho- 

 logically variable with regard to sex. Any 

 change in the environment that can upset 

 the production of, or tissue response to, sex 

 hormones can produce effects which modify 

 the sex phenotype. So, the phenotypes nor- 

 mally considered male and female show some 

 variability — providing some of the spice of 

 life. Genetically normal persons exposed to 

 abnormal environmental conditions can dif- 

 ferentiate phenotypes that lie between the 

 two normal ranges of sex type, and, there- 

 fore, are intersexual in appearance. Though 

 it is sometimes easy to classify an individual 

 as being an intersex because the person is 

 clearly between the two sex norms, other 

 individuals at the extremes of normality can- 

 not readily be labeled normal, or intersex, 

 or supersex. Intersexual phenotypes due to 

 environmental factors can result either from 

 genotypic males who have developed par- 

 tially in the direction of female, or from 

 genotypic females partially differentiated in 

 the direction of male. 



Otherwise-diploid individuals are known 

 who have various numbers of sex chromo- 

 somes. Only one type has a single sex chro- 

 mosome; this is the X0 individual, who is 

 female. The typical phenotypic effect of 

 this condition is called Turner's syndrome 



