Page:Popular Science Monthly Volume 85.djvu/434

430 if it is a factor which differs in the ease of the two sexes it is a "sex-determining factor." There are many parts of a germ cell, all of which may be concerned in heredity and development, but the chromosomes appear to be the seat of the differential factors for Mendelian characters.

 (abcd) their distribution in cleavage, their union into 4 pairs (Aa, Bb, Cc, Dd), in synapsis and the separation of the pairs in the reduction division. Only 2 of the 16 possible types of germ cells are shown. (From Wilson.)

2. Modifications of the Principle of Dominance.—A great number of animal and plant hybrids show one contrasting character completely dominant over the other one, as Mendel observed in the case of his peas. But in a considerable number of cases this dominance is incomplete or imperfect. When white-flowered strains of four-o'clocks are crossed with red-flowered ones the F1 plants bear neither white nor red flowers, but pink ones, and the F2 plants bear white, red and pink flowers. The whites and reds are always homozygous, the pinks heterozygous; pure white and pure red are produced only when their factors are duplex (WW), (RR); when they are simplex (WR) pink is produced. In this case red is not completely dominant over white, but the hybrid is more or less intermediate between the two parents (Fig. 56).

It has long been known that the breed of fowls called Blue Andalusian does not breed true, but in each generation produces a certain number of blacks and whites as well as blues. Bateson found that the blues are really hybrids between blacks and whites in which neither of