388 PLANT AND ANIMAL BREEDING 



now Czechoslovakia. About 1865, Mendel bred peas in his 

 monastery garden and found that certain characters, such as 

 color of seeds, color of flowers, smooth and wrinkled coats, and 

 other characters, are inheritable. Then he began a long series 

 of experiments in which he crossed or hybridized peas having some 

 of these different characters. For example, he crossed tall plants 

 with short ones, or smooth peas with wrinkled ones. The results 

 of these crossings showed that these characters are always trans- 

 mitted to the next generation as units, not as blendings of the two 

 opposing characters. This was his first great discovery, the inherit- 

 ance of unit characters. 



The Law of Dominance. — But Mendel found, in crossing peas, 

 that the first generation of hybrids always showed a curious result. 

 One character would appear, while its opposite would seemingly 

 be lost. If, for example, smooth and wrinkled peas were crossed, 

 the hybrids were all smooth. If tall and short pea plants were 

 bred,- the hybrids were all tall, and similar results were obtained 

 with the other pairs of characters with which he experimented. 

 This gave rise to the statement that certain unit characters are 

 dominant over others, which are called recessive characters. 



The Law of Segregation. — But these recessive characters were 

 not really lost. If some of the hybrid smooth-coated peas are 

 fertilized by others of the same kind and their seeds planted, the 

 next generation (known to breeders as the F2 generation) will 

 include some pea plants bearing smooth peas and some bearing 

 wrinkled peas, in the ratio of 75 : 25. One quarter of all the peas 

 show the recessive character. If these peas having the recessive 

 character are crossed with each other in another generation, they 

 will produce all wrinkled peas, the recessive character ; and such 

 peas, when crossed again and again with peas of the same kind, 

 will continue to produce wrinkled peas. The recessive character 

 has been segregated out and is now known as an extracted recessive. 

 This shows Mendel's Law of Segregation. Its importance in plant 

 and animal breeding can readily be seen. 



The 75% of F2 peas which are seemingly all smooth-coated are 

 in reality 25% smooth and 50% mixed, that is, having both char- 

 actj5rs, the recessive hidden by the dominant. If we can separate 

 the pure dominants^ they will produce only dominants, while the 



