Page:Popular Science Monthly Volume 74.djvu/547

Rh 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 determinant 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: 1YY: 2YG: 1GG. 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 first 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 qualities are believed to be segregated in different cells, both ova and spermatozoa, 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