408 ANNUAL REPOET SMITHSONIAN INSTITUTION, 1913. 



Mendel noticed the fact that when two varieties of plants differing 

 in one characteristic only are crossed, the red or the white color of 

 the flower, for example, all the seeds obtained therefrom produce 

 in the first generation plants having red flowers only; if these 

 hybrid red plants are crossed with them, 75 to 100 per cent of red- 

 flowered plants and 25 to 100 per cent of plants with white flowers 

 wall be obtained. If the stalks of the white flowers are afterwards 

 united, the red color would never appear; the stalks of the white 

 flowers reappeared in definite proportions in the descent from the 

 stems of the red flower united between them. 



Mendel thought that the two characteristics, red coloring and 

 white coloring, both exist in the hybrids of the first generation, 

 but the red characteristic dominates the white and appears only 

 Avhen they exist side by side. If we call " R " the red characteristic 

 and " B " the white characteristic, the formula for these hybrids is 

 R(B), the parenthesis denoting that (B) is latent, because it is 

 dominated. If a plant R(B) should be crossed with another plant 

 11(B), the two characteristics would disassociate themselves in the 

 pollen grains and in the ovules; half of the pollen grains contain 

 only li and a half contain only B ; the same with the ovules ; the 50 

 per cent of pollen gi-ains R now unite half with ovules R, half with 

 ovules B ; the fertilized ovules which result from the union have 

 now as a formula half RR, or 25 per cent and half R(B), or again 

 25 per cent. In the same way the 50 per cent of pollen grains B 

 unite half with ovules R and half with ovules B, and the fertilized 

 ovules which result from this have for a formula the half R(B) and 

 half BB, or 25 per cent R(B) and 25 per cent BB. The total is 

 25 RR, 50 R(B), and 25 BB. But the R(B) are red like the 

 RR and can not be distinguished from them. There are now 75 

 per cent of red. Unite them and the reds still give a certain pro- 

 portion of whites, which can be calculated for each generation under 

 the formulas of the law of probabilities. Without entering into the 

 detail of these formulas, understand that at the end of n generations 

 of unions between reds resulting from the first crossing, the white 

 being at each generation separated from the reproduction, we obtain 

 n~ — 1 stalks of red flowers for one stalk of w^hite flowers. On the con- 

 trary, it is well understood that the stalks of white flowers BB united 

 with each other never produce anything but the stalks of white 

 flowers ; they do not contain, either obviously or in any unseen way, 

 any red element. 



Such is Mendel's law'. It is applicable not alone to the colors of 

 plants, but to all other living beings, animals, and vegetables, to all 

 simple characters, capable of producing two and two in varieties 

 differing only by single characters. This law is found in the union 

 of gray mice (dominant) with white mice (dominated), of normal 



