46 



BIOLOGIC BASIS OF SEX 



was apparently XX for the female and 

 XY for the male. Events observed in selec- 

 tion experiments for sex completely altered 

 this behavior. A pair of autosomes took over 

 the role of the sex chromosomes in the ex- 

 perimentally produced race. The males as 

 well as the females were found to have the 

 XX chromosome arrangement. In effect the 

 X chromosomes became autosomes and the 

 X-linked genes were transmitted auto- 

 somally. Pure males and pure females were 

 generally obtained in the progeny for this 

 race, but sex differentiation was so weak 

 that the sex percentage was subject to wide 

 variations betweeen broods and the ex- 

 pected 50 per cent males to females was ob- 

 served only during the spring months. 

 Winge interpreted these results as indicating 

 that there were both masculine and feminine 

 elements present in the autosomes as well 

 as in the XY chromosomes which con- 

 tributed to the determination of sex. Selec- 

 tion sorts out different proportions of these 

 elements and a change occurs in the mecha- 

 nism forming the sex phenotype. XY fe- 

 males were produced which when crossed 

 to XY male segregates gave as expected 3 

 males to 1 female. On this basis the YY 

 individuals were found. The YY males on 

 crossing with normal females produced only 

 male offspring just as previously the XX 

 males when crossed to normal females gave 

 only female offspring. 



Changes of a similar type have been 

 found in nature. Gordon (1946, 1947) ob- 

 served wild stocks of the platyfish, Platy- 

 poecilus maculatus, from Mexico which 

 were XX for the female and XY for the 

 male. Platyfish from rivers in British Hon- 

 duras on the other hand were WZ for the fe- 

 male and ZZ for the male. The W and Y 

 chromosomes have many common char- 

 acteristics. Breeding data on domesticated 

 platyfish uncovered similar exceptional 

 chromosomal types with corresponding dif- 

 ferences in the sex transmission. Brcidci" 

 (1942) observed an exceptional WZ male 

 which when mated to a normal female WZ 

 had 51 daughters and 13 sons in the prog- 

 eny. The ratio is such as to indicate that the 

 females were a mixture of WW and WZ 

 genotypes. That this was probable was in- 

 dicated by the work of Bellamy and Qucal 

 (1951). Again an exceptional WZ male in 



niatings with WZ females had female prog- 

 enies of two types, WW and WZ, the males 

 being ZZ. The WW females were proved by 

 mating to normal males, ZZ, and obtaining 

 progenies which were entirely females. The 

 results indicated that the WW females were 

 less fertile and so would soon be replaced in. 

 nature by the WZ type. Aida (1921, 1936) 

 located genes in the X and the Y chromo- 

 somes of another genus of fishes, the med- 

 aka, Oryzias latipes, and studied the effects 

 of these chromosomes on sex determination 

 and sex reversal. The chromosome number 

 for the diploid was apparently 48 with no 

 prominent morphologic difference between 

 the X and Y chromosomes. The males were 

 XY and the females XX. By selection for 

 high male ratios, lines were established in 

 which the male offspring far exceeded those 

 which were female. Females having the 

 genotype XY were isolated which on cross- 

 ing to normal males gave the ratio of 3' 

 males to 1 female. One-third of the male off- 

 spring were of the YY constitution so that 

 as with the platyfish this type was viable. 

 In interpreting these results the primary 

 sexual characteristics are held to be de- 

 termined by respective genes distributed 

 throughout the autosomes and set into ac- 

 tivity by stimulating genes. The female 

 genes were held to require greater stimu- 

 lation than the male genes to be active and 

 produce their jihcnotypes. Sex was viewed 

 as determined by the differences in quan- 

 tity of the stimulating genes. Between 

 these two quantities a threshold was postu- 

 lated above which the female and below 

 which the male genes were stimulated. Sex 

 I'eversal and differences in sex ratios among 

 the offspring of these fish from sex reversed 

 males were explained as due to fluctuations 

 of the stimulating jjower or potency of the 

 X chromosome. 



Yamamoto (1953, 1959a, b) showed tlie 

 l)ossibility of reversing the phenotypic sex 

 by incorporating sex hormone-like sub- 

 stances in the diets of the developing young. 

 Functional sex reversal of the male geno- 

 types XY to those having female pheno- 

 tyi)es was accomplished by introducing 

 estrone or stilbestrol into the diet for ft 

 to 10 weeks to the extent of 50 /i.g. per gm. 

 diet from 1-day-old fry to those 11 to 16 

 mm. in length. Mating these estrone sex: 



