FOUNDATIONS FOR SEX 



45 



genera are 12, 14, and 18, with no sex 

 chromosomal differentiation. Types having 

 haploid, diploid, triploid, tetraploid, penta- 

 ploid, hexaploid, heptaploid, and various 

 aneuploid chromosome numbers have been 

 observed under experimental conditions 

 (Humphrey and Fankhauser, 1956; Fank- 

 hauser and Humphrey, 1959). Humphrey 

 ( 1945) induced sex reversal by grafting tes- 

 ticular jirimordia on to embryonic genetic 

 female gonads. He showed that males hav- 

 ing the genotype WW were viable, as were 

 females having the same constitution. The 

 somatic cells could be modified to either sex 

 phenotype whereas the germ cells retained 

 their genotypic constitutions as observed 

 later under natural and hormone control as 

 in fish. 



The improvement of estrogenic hormones 

 facilitated further studies on this problem. 

 Under natural conditions the sex genes are 

 effective sex determiners as normally they 

 guide the developing organism into one or 

 the other of the definite sex phenotypes. 

 When the embryonic forms of newts or 

 toads were exposed to relatively high con- 

 centrations of sex hormone-like substances 

 of the other sex, growth and development 

 were guided toward that sex rather than 

 toward that expected from the chromosomal 

 constitution of the somatic cells (Gallien, 

 1954, 1955, 1956; Chang and Witschi, 1955, 

 1956). When the male genotype was con- 

 verted to the female phenotype through this 

 treatment and was later bred to a normal 

 male, the progenies as expected on the basis 

 of the chromosomal constitution were all 

 normal males. When the female WZ was 

 sex-reversed to the male phenotype and then 

 bred to a normal female the progenies were 

 of three types, from a chromosome stand- 

 point ZZ, WZ, and WW, the ratio being 

 1 male to 3 females. The derived WW type 

 was then of female constitution showing 

 that this chromosome carries genes which 

 normally guide the organism to the female 

 phenotyjie. WW individuals had an ade- 

 quate gene content for normal development 

 of either sex in this class of organisms. The 

 observations on sex reversal in amphibia 

 were reminiscent of the experimental anal- 

 yses of sex differentiation by Baltzer on 

 Bonellia (1935) in which he quoted Har- 

 rison (1933) as saying, "A score of different 



factors may be involved and their effects 

 most intricably interwoven. In order to 

 resolve this tangle we have to inquire un- 

 der as great a variety of experimental con- 

 ditions as is possible to impose. Success will 

 be assured by the implicity, precision, and 

 completeness of our descriptions rather than 

 by a specious facility in ascribing causes to 

 particular events." Bonellia showed the 

 way for synthetic hormone in that contact 

 of the embryonic form with the female was 

 sufficient to direct development into the 

 male type. 



Sex determination in fish as in Amphibia 

 seems to be of such a nature that the results 

 of hormone treatments are revealed more 

 clearly than in Drosophila or mammals. 

 Winge (1922) found 46 chromosomes in 

 both male and female guppies. Lebistes 

 reticulatus has 22 pairs of autosomes and 2 

 X chromosomes in the female and 22 pairs 

 of autosomes and an XY chromosome set in 

 the male. Normally the Y chromosome is 

 found only in the male and is transmitted to 

 only male progeny. The genetic factors con- 

 tained in the differential segment of this 

 chromosome are not found in the females, 

 but are transmitted to all the young of the 

 male sex. The pairing segment on the other 

 hand crosses over with the X. Whether the 



Y chromosome itself also is sex deciding is 

 uncertain. As Winge said in 1922, "experi- 

 ences gained from the Drosophilia re- 

 searches have proved that one must indeed 

 take care not to state anything certain on 

 this subject." 



A fairly large number of genes had been 

 demonstrated in the X, Y, and autosomes of 

 this species by 1934. Genes which showed 

 X linkage showed crossing over to the Y 

 chromosome and vice versa (Winge, 1934; 

 Winge and Ditlevsen, 1948) . The amount of 

 crossing over showed some variation de- 

 pending on what genes were present in the 

 two chromosomes. A locus was found in the 



Y chromosome containing a set of allelo- 

 morphic genes which could not cross over 

 to the X chromosome. These alleles were 

 completely linked with a male-determining 

 clement found in this Y chromosome and 

 located near its end. The pattern of inheri- 

 tance for chromosome sex determination 



