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CHAPTER 13 



ders. This type of origin may be identified 

 if the gynander is concurrently mosaic for a 

 sex-Hnked or autosomal gene for which the 

 mother was heterozygous, or if it is con- 

 currently mosaic for an autosomal gene for 

 which the father was heterozygous. 



Gynanders occur also in other insects. In 

 moths, for example, where males may 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 explanation of these exceptions is similar 

 to that given for Drosophila. However, in 

 the case of the moth, the gynander usually 

 starts as a male zygote (XX). In insects, in 

 general, gynanders usually have a sharp 

 borderline between male and female parts 

 because hormones play a relatively small role 

 in differentiation, each body part forming 

 largely according to the genotype it contains. 



In man and the mouse, males are usually 

 XY and females XX. In these organisms, 



XO individuals are female (such human beings 

 are usually underdeveloped females, demon- 

 strating Turner's syndrome), while XXY in- 

 dividuals are underdeveloped males (such 

 human beings having Klinefelter's syndrome). 

 People who are otherwise diploid but are 

 XXXY, XXYY, or XXXXY are abnormal 

 males, while those who are XXX or XXXX 

 are abnormal females. In man and the 

 mouse, then, the presence of a Y chromosome 

 determines the male sex; note that the chro- 

 mosomal relation to sex is different from that 

 found in Drosophila. 



In the last group of organisms discussed 

 (beginning with Chlamydomonas) we have 

 seen that the genotype is primarily responsible 

 for sex determination; in some cases the 

 genetic basis for this is indicated by differences 

 in the chromosomal make-up of the two sexes. 

 In certain organisms the presence of one or 

 of two chromosomes of a kind determines 

 sex, in other cases sex is determined by the 

 presence or absence of a single chromosome. 



SUMMARY AND CONCLUSIONS 



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

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

 kinds of gametes? Only the latter question was discussed here in any detail. It was found 

 that in some cases the environment and in other cases the genotype is primarily responsible 

 for sex determination. In the latter cases sexual differences can sometimes be correlated 

 with differences in chromosome content. 



REFERENCES 



Hannah-Alava, A., "Genetic Mosaics," Scient. Amer., 202:111 



30, 1960. 



Lancet, No. 7075, Vol. 1, 1959, pp. 709-716. 



Wilson, E. B., "The Chromosomes in Relation to the Determination of Sex in Insects," 

 Science, 22:500-502, 1905. Reprinted in Great Experiments in Biology, Gabriel, 

 M. L., and S. Fogel (Eds.), Englewood Cliffs, N.J., Prentice-Hall, 1955, pp. 254-257. 



QUESTIONS FOR DISCUSSION 



13.1. If sexual reproduction is as advantageous as discussed, why do so many organisms 

 still reproduce asexually? 



13.2. Does the study of sex determination offer any test of the theory that chromosomes 

 furnish the physical basis for genes? Explain. 



