Part IV 



GENETICS AND EVOLUTION 



CHAPTER 32 



Principles of Heredity 



266. History of Genetics 



It must have been thousands of years ago when man first made one 

 of the fundamental observations of heredity— that "like tends to beget 

 like." But his curiosity as to why this is true and how it is brought about 

 remained unsatisfied until the beginning of the present century. A num- 

 ber of breeders, such as Kolreuter who worked with tobacco plants about 

 1770, crossed different varieties of plants and produced hybrids. Kol- 

 reuter recognized that parental characters were transmitted by both the 

 pollen and the ovule. Mendel's careful work with peas revealed the 

 fundamental principles of heredity, but the report of his work, published 

 in 1866, was far ahead of his time. It is clear that his work was known 

 to a number of the leading biologists of the time, such as the botanist, 

 Nageli, but in the absence of our present knowledge of chromosomes 

 and their behavior, its significance was unappreciated. 



The chromosomal details of mitosis were described by Eduard Stras- 

 burger in 1876. Eduard van Beneden (1887) discovered the process of 

 meiosis and understood its significance. Earlier that same year Weis- 

 mann had pointed out, simply from theoretical considerations, that the 

 chromosome number in gametes must be half of that in somatic cells. It 

 is conceivable that some brilliant theoretical biologist with these facts at 

 hand might have postulated that, if hereditary factors were units located 

 in the chromosomes, the mating of different parental types would yield 

 offspring in predictable ratios. However, no such mental synthesis was 

 made, and the existence of these definite ratios of the types of offspring 



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