DETERMINATION AND INHERITANCE OF SEX 543 



carry only the yellow color and 50 per cent, the green. At fertilization 

 eggs and pollen grains meet fortuitously and according to the laws of 

 chance there are twice as many possibilities that an egg with a green 

 color-determinant (G) shall meet a pollen grain with a yellow color- 

 determinant (Y), and vice versa, as that an egg with a green deter- 

 minant shall meet a pollen grain with a green determinant, or that an 

 egg with a yellow determinant shall meet a pollen grain with a yellow 

 determinant. Hence the combinations produced may be expressed 

 thus: lYY: 2YG: IGG. Among the seed 50 per cent, are hybrids in 

 respect to color, but since the yellow color dominates over the green all 

 appear yellow, giving a total of 75 per cent, yellow peas and 25 per cent, 

 green peas. 



When pure strains of sweet peas with white flowers are crossed with 

 pure-bred peas with red flowers similar results follow. All the plants 

 of the first generation bear red flowers, showing the dominance of red 

 color over white color. When these are interbred the plants of the 

 second generation split up in the proportion of 75 per cent, red-flower- 

 bearing to 25 per cent, white-flower-bearing; or 25 per cent, will be red 

 pure dominants, 50 per cent, red hybrids and 25 per cent, white pure 

 recessives. Also when white mice (albinos) are crossed with gray mice 

 (or pigmented mice) the flrst generation are all gray, when the latter 

 are bred among themselves the second generation includes albinos and 

 gray individuals in the proportion of 1 pure gray: 2 gray hybrids: 

 1 pure white. 



In applying Mendelian principles to sex, maleness and femaleness 

 are regarded as unit characters and during the maturation of the germ- 

 cells the carriers (chromosomal elements) of the male and female quali- 

 ties are believed to be segregated in different cells, both ova and sperma- 

 tozoa, so that one half of the ova contain the male-determining factor 

 and the other half the female; and likewise the spermatozoa. The 

 result of a fortuitous intermingling of ova and sperm, according to 

 strict Mendelian laws, should produce male and female individuals in 

 the proportion of 3 :1 or the reverse. Suppose femaleness dominated, 

 then there would be 75 females in every 100 of population or of any 

 particular species. No such disproportion of sexes obtains. In fact, 

 barring a few exceptions which may be explained as adaptations or as 

 the result of a selective mortality, all species are approximately equally 

 divided into two classes with respect to sex. Further assumptions must 

 be made and pure Mendelism modified. The obvious and necessary 

 assumptions are that there are no individuals pure in sex; all are 

 hybrids ; and the sex that the organism attains is the result of a struggle 

 between the mingled male and female tendencies and the dominance, 

 now of one tendency, now of the other. The dominance may be due, 

 as Dr. Thomson, of the University of Aberdeen, suggests, to slight 



