Rules and Exceptions The rediscovery of Mendel's studies and the 

 simultaneous discovery of his principles by several independent investigators 

 aroused widespread interest. Hundreds of students immediately set to work 

 to check on the amazing new "laws" of heredity. Supporting facts were found 

 through experiments on maize, mice, hens, rabbits, silkworms, wheat, various 

 flowering plants, and many other species of animals and plants. 



Earlier experience, as well as many experiments since Mendel's time, 

 show that with some pairs of characters there is not complete dominance. In 

 the case of the blue Andalusian fowl, for example, or of the four-o'clock flower 

 there appears to be what Galton called "blended" inheritance. But from 

 further experiments we now understand that these seemingly blended hybrids 

 behave exactly as do Mendel's hybrid dominants, except that the dominant 

 factor does not completely hide the recessive one. 



William T. Bateson (1861-1926), a British surgeon and investigator, 

 had stressed the desirability of studying heredity by experimenting with 

 distinct traits that did not merge or blend gradually into others. He quickly 

 recognized the importance of the Mendelian principles and urged further 

 research. He carried on experiments himself, and on the whole his results 

 agreed with Mendel's findings. But Bateson (who, by the way, invented the 

 name genetics for "the science of heredity and variation") discovered some 

 curious exceptions to the principle of independent transmission of traits. For 

 example, purple sweet peas having long pollen grains were crossed with red- 

 flowered varieties having round pollen grains. In the second hybrid generation 

 the segregation did not yield the four possible combinations in the propor- 

 tion 9:3:3:1 (see illustration, p. 478). Instead the long-pollen and purple 

 came out together, and the round-pollen and red came out together. In other 

 experiments the large petal, or "standard", of the pea-flower appeared to 

 remain associated with color; it always droops in white flowers and is erect 

 in purple ones. That is, there is some connection, or "coupling", between 

 these two characteristics: they are not transmitted independently. 



Other exceptions appeared in the offspring of two different strains of white- 

 flowered sweet peas. The hybrids have colored flowers, and their progeny in 

 turn segregate into six different color combinations, in addition to some pure 

 whites. Here, again, the proportions did not fit the expectation according to 

 the Mendelian formula. Many scientists began to feel that they had to take a 

 stand for MendeHsm or else against Mendelism. 



Multiple Factors Although Mendel's work remained so long forgotten, 

 his selection of material was very fortunate since it enabled him to develop 

 his three "laws" in about eight years, with the least amount of confusion. 

 With other material he might have been completely baffled. The mating of 

 red wheat with white wheat, for example, yields a grain of an intermediate 

 color. In the following generation there is a breaking up into a long series of 



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