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CHAPTER 13 



genotype. In these cases, the genes neverthe- 

 less played a role, one that made possible 

 different sexual responses to differences in 

 the environment. 



Consider next the basis for gamete forma- 

 tion in still other kinds of organisms. Chla- 

 mydomonas is a unicellular plant with two 

 flagella and with a chromatophore containing 

 chlorophyll. It can reproduce asexually, by 

 means of mitotic cell division, to produce cul- 

 tures containing large numbers of individuals. 

 When cultures are derived from a single 

 ancestor no sexual reproduction is observed 

 within them. If, however, the members of 

 two such cultures are mixed together, one of 

 two events is possible — either no mating 

 occurs, or the individuals of one cuhure pair 

 with members of the other culture, fuse, and 

 produce zygotes. After two divisions of the 

 zygote, four cells are produced. Each of 

 these four cells can be grown separately and 

 each will produce cultures within which there 

 is no mating, as already explained. If now a 

 sample from each of these four cultures is 

 mixed with different portions of a fifth culture 

 (also descended from a single individual), two 

 cultures of the four will show mating (and 

 since they do we can call these sexual type +) 

 and two will not (being, therefore, of — sex). 

 Moreover, when portions of the four cultures 

 under test are combined in pairs, it is found 

 that individuals of any + culture can mate 

 to individuals in any — culture, while combi- 

 nations of two + or two — cultures do not 

 result in mating. Cytological examination 

 shows no phenotypic difference between + 

 and — individuals. 



How may these results be explained? Note 

 that among the first four cells produced from 

 a zygote only two kinds of individuals are 

 produced, exactly two being of + and two 

 of — sex. This suggests that the zygote is 

 diploid, carries a pair of genes for sex, which 



we can call -\ , and that meiosis occurs in 



the next two divisions, as a consequence of 

 which a 1 : 1 ratio of + : — is found among 



the haploid products. Therefore, it is pri- 

 marily the genotype which determines sex in 

 Chlamydomonas. 



In certain kinds of grasshoppers, females 

 have 14, and males 13, chromosomes, one of 

 the seven kinds of chromosomes having no 

 partner in the male. Thus, both sexes have 

 12 autosomes (6AA), females have two X's 

 and males one X. After meiosis, all eggs 

 contain 6A + IX, while half of sperm carry 

 6A + IX and half 6A. Sex determination is 

 obvious, zygotes with one X forming males, 

 those with two X's forming females. We 

 could consider the function of the X, in sex 

 determination in these forms, to be the pro- 

 duction of the trait of femaleness, one X 

 producing a tendency in this direction which 

 appears as male, while two X's complete this 

 tendency and produce female. 



We have already discussed, in Chapter 12, 

 the fact that the ordinary Drosophila meJano- 

 gaster female is 3AA + XX and the male is 

 3AA + X + Y. From this statement, how- 

 ever, we cannot decide the chromosomal 

 basis for sex determination in any greater 

 detail, since there are two variables involved, 

 the X's and the Y, Is the male a male be- 

 cause he has a Y, or because he has only one 

 X, or because he has both one X and one Y? 

 A decision in this matter can be arrived at 

 from the fact that flies have been obtained 

 which were shown to contain, besides 3AA, 

 either XXY, or XXYY, or XO; these were, 

 respectively, female, female, and male, prov- 

 ing that the Y is not sex determining in this 

 organism. (We had already indicated the 

 existence and sex of the first and third types 

 of individual in Chapter 12, where it was also 

 mentioned that the Y is necessary for fertility, 

 XO males having nonmotile sperm.) 



On relatively rare occasions Drosophila in- 

 dividuals appear which are abnormal in being 

 part male and part female. These individuals 

 are mosaic for the sex trait, and are called 

 gynandromorphs or gynanders (Figure 13 1). 

 The male and female parts are clearly de- 



