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  Section: Introduction to Botany » Mendelian Genetics
 
 
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Pre-Mendelian Theorists and Theories

 
     
 
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Mendelian Genetics
  Pre-Mendelian Theorists and Theories
  Gregor Mendel
  Mendel’s Laws
  Mendel’s Experiments
  Applying Genetics

Jean Lamarck (that is, Jean Baptiste Pierre Antoine de Monet Lamarck, 1744-1829) asserted that species were not unalterable and that complex organisms derived from simpler ones.H e proposed four laws:
  1. Life tends to increase in the volume of every body up to a limit.
  2. New organs come into existence as a result of new wants, new desires.
  3. The action of organs develops in relation to how much they are used.
  4. All that is acquired by individuals is conserved and passed on to successive generations.
Lamarck's four laws are known collectively as the inheritance of acquired characteristics, which suggests that the hands of a newborn baby being the son of a laboring man should have calluses; that because the arms of a blacksmith grow large and strong from hammering, the children born of blacksmiths should have large arms; and that the children of a prominent pianist should learn to play well with little practice. The classic example of the inheritance of acquired characteristics relates to the giraffe, the suggestion being that its long neck resulted from succeeding generations of giraffes stretching their necks to reach the leaves high in trees. This proposition had a long tenure.

August Weismann (1834-1914) challenged the whole idea of the inheritance of acquired characteristics. He is often remembered for cutting the tails off numerous generations of mice and noting that successive generations of such mice were still born with tails. Weismann was not a silly man, however, and it would be unfair to remember him for a silly experiment. Cut-off tails are not acquired characteristics, of course; they are imposed characteristics. Weismann should instead be remembered for his theory (or observation) regarding the continuity of gern plasm. The body of an individual is made up of perishable somatoplasm. Germ plasm cannot be made anew, but rather is formed from preexisting germ plasm. An organism is a means of carrying germ plasm through time. Weismann realized that when egg and sperm unite, there is an increase in hereditary substance. He declared that somewhere along the line there must be a compensatory reduction in hereditary substance.
 
     
 
 
     



     
 
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