Sex Determination (If) 



105 



ried IX + lA set (being haploid) have been 

 found. As expected from their sex index of 

 1.0, these individuals or parts were female. 

 Since all known facts support the exact cor- 

 respondence between chromosomal consti- 

 tution and sexual types, we can accept 

 chromosome balance as the typical basis of 

 sex determination in Drosophila. 



What is the relationship between X-auto- 

 some balance and tra, the sex transforming 

 gene? Sex is determined by X-autosome 

 balance when the individuals carry trci+, 

 which they normally do. On those rare 

 occasions when tra tra is present, the balance 

 view does not apply, 2X + 2A sets, you recall, 

 producing male. 



We have seen in Drosophila that determina- 

 tion of sex is primarily genetic and that the 

 sex type differentiated in different parts of an 

 individual depends only upon the genotype 

 carried by these parts (Chapter 13). Autono- 

 mous sexual development of each part of 

 this organism is possible because of the 

 absence of hormones affecting sexual differ- 

 entiation. What is the situation in man? 



In human beings sexual type is determined 

 at fertilization, Y-bearing zygotes becoming 

 males, Y-less zygotes females. The manner 

 in which the appropriate sex phenotype is 

 produced can be described briefly as follows, 

 liarly in its development, the gonad is neu- 

 tral, i.e., there is no macroscopic indication 

 whether it will later form testis or ovary. 

 This early gonad has two regions, an outer 

 one called the cortex and an inner one called 

 the medulla. As development proceeds, the 

 cortex degenerates in those individuals that 

 carry a Y (males) and the medulla forms 

 testis, while in those individuals genetically 

 destined to be females the medulla degener- 

 ates and the cortex forms ovary. 



Once the testis and ovary are formed, they 

 take over the regulation of further sexual 

 differentiation by means of the hormones 

 they produce, so that these hormones direct 

 the development or degeneration of various 



sexual ducts, the formation of genitalia, and 

 other sexual characteristics. Since sexual dif- 

 ferentiation is largely under the control of the 

 sex hormones it should not be surprising to 

 find genetically normal individuals who are 

 morphologically abnormal with regard to 

 sex. For any abnormal condition in the 

 environment, which can upset the production 

 of sex hormone and/or the response of tissues 

 to it, may produce effects during development 

 which result in an abnormal sex phenotype. 

 Such abnormal sex phenotypes due to en- 

 vironmental factors could be intersexual, and 

 theoretically could be produced either from 

 genotypic males who have developed partially 

 in the direction of female or from genotypic 

 females partially differentiated in the direc- 

 tion of male. Some intersexes in humans 

 prove to be due to an abnormal genotype 

 present at fertilization, as is true for all inter- 

 sexual Drosophila; other human intersexes 

 result from genotypic mosaicism, which pro- 

 duces gynandromorphs in Drosophila where 

 sex hormones are absent. Of course you 

 realize that the phenotypes normally con- 

 sidered male and female show some vari- 

 ability. So, while it is sometimes easy to 

 classify an individual as being an intersex, 

 because that person is clearly between the 

 two sex norms, other individuals at the ex- 

 tremes of normality may not readily be 

 labeled as normal, or intersex, or supersex. 

 It is debatable whether human beings that 

 are XO or XXY, but otherwise diploid, 

 should be called intersexes, or whether they 

 should be considered of normal but of under- 

 developed sex. 



Since we have just been discussing how the 

 genotype may be related to the production 

 of intersexes, consider how the genotype is 

 related to the sex ratio, that is, the relative 

 numbers of males and females born. On the 

 average, 106 boys are born for each 100 girls. 

 At first this might surprise you, since half of 

 sperm are expected to carry X, and half Y, 

 and all eggs an X, so that the sex ratio 



