1^ 



THE MECHANISM OF 

 MENDELIAN HEREDITY 



CHAPTER I 



MENDELIAN INHERITANCE AND THE 

 CHROMOSOMES 



Menders fundamental law of segregation was 

 announced in 1865. It is very simple. The units 

 contributed by each parent- separate in the germ cells 

 of the offspring without having had any influence on 

 each other. For example, in a cross between yeUow- 

 seeded and green-seeded peas, one parent con- 

 tributes to the offspring a unit for j^ellow and the 

 other parent contributes a unit for green. These 

 units separate in the ripening of the germ cells of 

 the offspring so that half of the germ cells are yellow 

 producing and half are green producing. This sepa- 

 ration occurs both in the eggs and in the pollen. 



Mendel did not know of any mechanism by which 

 such a process could take place. In fact, in 1865 

 very little was known about the ripening of the germ 

 Ceils. But in 1900, when Mendel's long-forgotten 

 discovery was brought to light once more, a mechan- 

 ism had been discovered that fulfils exactly the 

 Alondelian requirements of pairing and separation. 

 M The sperm of every species of animal or plant 



