29 

 HEREDITY AND CELL DIVISION 



Mendel's experiments 



The first man to carry out experiments on heredity which gave 

 clear results was Gregor Johann Mendel, a teacher of physics 

 in the school of the Konigskloster at Briinn in Austria (now Brno 

 in Czechoslovakia), an Augustinian house of which he was after- 

 wards Pralat or Head. His chief work extended from 1857 to 

 1865, and was published in 1866 ; it dealt with a plant, the garden 

 pea Pisum sativum, but must be described even in a zoology book 

 because of its fundamental importance. 



Mendel introduced into his work three new principles, which 

 were the foundation of his success, (i) He worked with pure 

 lines, which had been inbred for long enough for it to be clear 

 that, if they were not crossed, they did not vary significantly in 

 the characters under investigation. His choice of the garden pea 

 was determined by the fact that, since it is normally self-fertihsing, 

 it is protected from accidental crossing. (2) He chose characters 

 (or character differences) which were clear-cut and distinct, not 

 those, such as height or intellect in man where there is a con- 

 tinuous gradation, or coat colour in dogs where the variety is 

 bewildering. He used twenty-two varieties of pea obtained from 

 seedsmen, but these could be classified as differing only in one 

 or more of seven pairs of characters, and it was to these character 

 pairs, not the whole appearance of the plant, that his attention 

 was given. (3) He kept careful statistical records of the progeny 

 of each individual cross in each generation. 



His results may be illustrated by the crosses between tall 



and dwarf plants. The former are 6 to 7 feet high, the latter 



9 to 18 inches, so that although there is some variation in height 



within each pure line there is no overlap between the two. The 



anthers of the flowers of one plant were carefully removed from 



the bud, and pollen from the other placed on the stigma ; crosses 



were carried out in both directions, so that the tall and dwarf 



parents provided pollen in turn. In thirty-seven experiments on 



ten plants he found that the hybrid seeds developed to give tall 



plants, and nothing but tall plants, whichever way the cross was 



made. The character tallness he therefore called dominant, 



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