398 



SCIENCE. 



[X. S. Vol. XVIIl. Xo. 4.'>r.. 



what the hybrid character in a given cross 

 will be. It can be determined only by ex- 

 periment, but it is always the same for the 

 same cross, provided the parents are pure. 

 Often the hybrid form resembles a sup- 

 posed ancestral condition, in which case 

 it is commonly designated a reversion. Il- 

 lustrations are the gray hybrid mice, which 

 are indistinguishable in appearance from 

 the house-mouse, and slate-colored pigeons 

 resulting from crossing white with buff 

 pigeons. 



3. Purity of the Germ-cells.— The great 

 discovery of Mendel is this: The hyhrid, 

 whatever its own character, produces ripe 

 germ-cells which bear only the pure char- 

 acter of one parent or the other. Thus, 

 when one parent has the character A, and 

 the other the character B, the hybrid will 

 have the character AB, or in cases of simple 

 dominance, A(B)* or B(A). But what- 

 ever the character of the hybrid may be, 

 its germ-cells, when mature, will bear 

 either the character A or the character B, 

 hut not both; and As and Bs will be pro- 

 duced in equal numbers. This perfectly 

 simple principle is known as the law of 

 'segregation,' or the law of the 'purity of 

 the germ-cells.' It bids fair to prove as 

 fundamental to a right understanding of 

 the facts of heredity as is the law of defi- 

 nite proportions in chemistry. Prom it 

 follow many important consequences. 



A first consequence of the law of purity 

 of the germ-eells is polymorphism of the 

 second and later hybrid generations. The 

 individuals of the first hybrid generation 

 are all of one type, provided the parent 

 individuals were pure. Each has a char- 

 acter resiilting from the combination of 

 an A with a B, let us say AB. [In eases 

 of dominance it would more properly be ex- 

 pressed by A (B) or B (A).] But in the 

 next generation three sorts of combinations 



* The parenthesis is used to indicate a reces- 

 sive character not visihle in the individual. 



are possible, since each parent will furnish 

 As and Bs in equal numbers. The possible 

 combinations are AA, AB and BB. The 

 first sort will consist of pure As and will 

 breed true to that character ever after- 

 ward, unless crossed with individuals 

 having a different character. Similarly, 

 the third sort will consist of pure Bs and 

 will breed true to that character. But the 

 second sort, AB, will consist of hybrid in- 

 dividuals, like those of which the first hy- 

 brid generation was exclusively composed. 

 If, as supposed, germ-cells, A and B, are 

 produced in equal numbers by hybrids of 

 both sexes, and unite at random in fertili- 

 zation, combinations AA, AB and BB 

 should occur in the frequencies, 1:2:1. 

 For in unions between two sets of gametes, 

 each A+B, there is one chance each for 

 the combinations AA and BB, but two 

 chances for the combination AB. 



If the three forms A A (or simply A). 

 AB and B are all different in appearance, 

 it will be a very simple matter in an ex- 

 periment to count those of each class and 

 determine whether they occur in the theo- 

 retical proportions, 1:2:1. One such case 

 has been observed by Bateson ( :02, p. 183) 

 among Chinese primroses {Primula sinen- 

 sis). An unfixable hybrid variety known 

 as 'giant lavender,' bearing flowers of a 

 lavender color, was produced by crossing 



TABLE I. 



Characters, ^^ 



I'lants bearing Flowers in Color 



1901, Lot 1. 



1901, Lot 2. 



1902, Lot 1. 

 1902, Lot 2. 



Totals 



Per cent, of whole. 



a magenta red with a white flowering 

 variety tinged faintly with pink. By seed 

 the hybrid constantly produces plants 



