278 THE CONTINUITY OF THE RACE 



(undesirable) qualities that had regularly or frequently appeared in the 

 original ancestral stock. One has only to consider some of the many 

 existing breeds of dogs, cattle, poultry, corn, or tobacco to realize how 

 effective such a practical manipulation of variation and inheritance has 

 been. 



Until the present century, however, the work of the practical breeder, 

 although extremely productive, had been almost wholly empirical and 

 by "rule of thumb," and the plight of the biologist was little if any better, 

 in spite of the accumulation of much careful data from observation and 

 experiment. Then, in 1900, with the rediscovery of Mendel's long-neg- 

 lected pioneering work, the biologist was given a sound foundation and a 

 powerful research method for the investigation and understanding of 

 inheritance and variation. 



Gregor Johann Mendel was an Augustinian monk and later the prelate 

 or abbot of a monastery in Brunn (now Brno), Austria. In 1866, after 

 8 years of thorough study, he announced the results of his work on the 

 inheritance of certain qualities in the garden pea. Many other workers 

 before his time had made somewhat similar investigations, but Mendel's 

 experiments were so carefully thought out and so painstakingly made 

 and recorded that he was able to discover underlying principles of in- 

 heritance that could not have been disclosed by less precise methods. His 

 procedure (which was to become one of the fundamental tools for modern 

 genetic studies) is so important that it is necessary to describe it in some 

 detail. 



The selection of experimental material. At the beginning of his 

 studies Mendel obtained as many varieties of peas as he could find. He 

 tested each variety by growing it for several generations and then selected 

 several that differed from each other in one or more distinct character- 

 istics and that proved to breed true. For example, he obtained one variety 

 in which all the individuals were tall (5 to 6 feet) and another in which all 

 the individuals were dwarf (1^ to 2 feet); a variety with smooth seeds 

 and another with wrinkled seeds; a variety in which the seeds were always 

 green and another in which the seeds were always yellow; etc. 



THE MONOHYBRID CROSS 



Mendel's simplest experiments were concerned with crosses between 

 varieties that differed from one another in a single definite quality. One 

 of these was the cross between the tall and the dwarf varieties. The 

 garden pea is normally a self-fertilizing species, so that Mendel's first 

 step was to select one plant — a tall one, for example — and remove all 

 its anthers before the pollen was ripe. The plant was thus completely 

 emasculated and incapable of fertilizing itself. When the stigmas of this 

 plant were ready for pollination, he introduced pollen from the anthers 



