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CHAPTER 8 



tilized eggs, grown in the absence of adult 

 females develop as females; they develop 

 as males in the presence either of adult fe- 

 males or simply an extract of the female's 

 proboscis. In this case, then, differentiation 

 as a whole including sexual differentiation, 

 is regulated by the presence or absence of a 

 chemical messenger manufactured by fe- 

 males. 



Nothing has been stated about the specific 

 genetic basis for the determination or dif- 

 ferentiation of sex in any of the examples 

 given in this section because different sexes 

 or gametes are determined not by genetic 

 differences between cells, organs, or indi- 

 viduals, but by environmental differences 

 acting upon a uniform genotype. The genes, 

 nevertheless, must play a role in all these 

 cases by making possible different sexual 

 responses to variations in the environment. 



Importance of Sexuality 



Even if reproduction occurred only by asex- 

 ual means, the earth would now be popu- 

 lated by genetically different kinds of or- 

 ganisms, each variant having arisen by muta- 



tion in a pre-existing individual who was, 

 in turn, produced from an unbroken line 

 of descent. This method of direct descent 

 is inefficient, however, since biologically fit 

 individuals must wait for the rare occurrence 

 of mutation to make them more fit. 



The biological innovation of sexuality has 

 a tremendous genetic advantage over ascx- 

 uality by providing genetic recombination 

 which speeds up the process of the evolu- 

 tion of more adaptive organisms. A more 

 adaptive genotype may be produced in one 

 individual by the combination of allelic and 

 nonallelic genes originally located in two 

 parents who, individually, may have been 

 less well or even poorly adapted. Since 

 genetic recombination normally occurs each 

 generation for each gene pair, adaptive com- 

 binations of genes originate much more rap- 

 idly by recombination than by the relatively 

 rare event of mutation. It should be clear, 

 therefore, that sexuality, which produces a 

 greater variety of adaptive genotypes in a 

 given period of time than asexuality, is pri- 

 marily responsible for the great variety of 

 adapted kinds of individuals that have ap- 

 peared on the Earth in recent times. 



SUMMARY AND CONCLUSIONS 



An understanding of the basis of sex requires the answer to two questions: What is 

 responsible for the onset of meiosis? What is the basis for the formation of different 

 kinds of gametes? Only the latter question is discussed in significant detail. In some 

 cases the environment and in other cases the genotype is primarily responsible for sex 

 determination. In the latter cases, sexual differences can often be correlated with 

 cytogenetic differences. 



Genes responsible for sex determination are located not only in the sex chromosomes 

 but in the autosomes as well. Although sex type may be changed through the action 

 of a single pair of genes, a given sex is usually the result of the interaction of several, 

 and probahly many, pairs of genes. Sex is, therefore, a polygenic trait (Chapter 5). 



Chromosomal differences found among zygotes serve as visible manifestations of 

 differences in the balance of genes concerned with sex. Whenever, as in female 

 Drosophila, genie balance is unaffected by the addition or subtraction of whole sets of 

 chromosomes, sex also is unaffected. However, changes in chromosome number which 

 produce intermediate genie balances also produce intermediate sex types — intersexes; 

 those which make the balance more extreme than normal produce extreme sex types — 

 supersexes. 



