Chapter 13 



SEX DETERMINATION (I) 



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UPPOSE reproduction had always 

 I occurred by asexual means. 

 Even so the earth would now be 

 populated by genetically different kinds of or- 

 ganisms, each variant having arisen by mu- 

 tation in a pre-existing individual that was in 

 turn produced from an unbroken line of de- 

 scent. However, this method of direct descent 

 is inefficient, at least in the respect that new 

 mutations which confer a slight biological 

 advantage would be wasted whenever they 

 occurred in a genetically ill-adapted individ- 

 ual. For such an individual or its descend- 

 ants would eventually become extinct, at 

 which time the beneficial as well as the harm- 

 ful genes would be lost simultaneously. Ac- 

 cordingly, biologically fit individuals could 

 become more fit only by waiting until the 

 rare event of mutation occurred in them. 



The biological innovation of sexuality pro- 

 vides a tremendous genetic advantage over 

 asexuality. Sexuality provides for genetic 

 recombination (cf. p< 13), and this speeds up 

 the process of the evolution of more adaptive 

 organisms. For a more adaptive genotype 

 may be produced by the combination in one 

 individual of mutant and nonmutant genes 

 originally located in different individuals who 

 may have been less or even poorly adapted. 

 Since genetic recombination normally occurs 

 each generation for each gene pair, adaptive 

 combinations of genes can originate much 

 more rapidly than could be produced by 

 mutation, which occurs much less frequently 

 per gene than once a generation. In other 

 words, sexuality provides numerous tests of 

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the adaptability of various combinations of 

 genes, whereas equivalent tests in asexually 

 reproducing lines would require an inordi- 

 nately greater length of time, since these 

 combinations would have to wait for succes- 

 sive mutations for their production. It 

 should be clear, therefore, that sexuality, 

 which produces more different adaptive geno- 

 types in a given period of time than does 

 asexuality, is primarily responsible for the 

 great variety of adapted kinds of individuals 

 that have appeared on earth in recent times. 



You will agree that it is of interest to learn 

 the basis of sex in view of its important role 

 in biological evolution. The fundamental 

 and unique feature of sex is the produc- 

 tion of gametes. So our problem may be 

 stated: What are the factors responsible for 

 the production of gametes? Gametes are 

 part of the phenotype just as are all other 

 traits of the individual. Accordingly, since 

 all traits of an organism have some genetic 

 basis (in the genes that help create the organ- 

 ism, as an organism, and in whose absence 

 there would be no organism and hence none 

 of its traits), and since all genotypes must 

 interact with the environment in order to 

 produce phenotypes, it is clear that sex is 

 also the consequence of the interaction be- 

 tween genotype and environment. We may 

 be more specific, therefore, and seek to de- 

 termine the relative importance of these two 

 factors in the production of gametes. Let us 

 evaluate these two factors insofar as they 

 affect gamete formation in different kinds of 

 organisms. 



In certain organisms, male and female 

 gametes are produced in the same individual. 

 In animals, individuals of this type are called 

 hermaphrodites (after Hermes and Aphrodite), 

 while such plants are said to be monoecious 

 ("one house"). In the case of the snail. 

 Helix, there is only one type of sex organ, or 

 gonad, which produces both eggs and sperm 

 from cells which may lie very close together. 

 In the earthworm, eggs and sperm are pro- 



