THE MECHANISM OF 

 MENDELIAN HEREDITY 



CHAPTER I 



MENDELIAN SEGREGATION AND THE 

 CHROMOSOMES 



Mendel's law was announced in 1865. Its funda- 

 mental principle is very simple. The units con- 

 tributed by two parents -separate in the germ cells of the 

 offspring without having had any influence on each 

 other. For example, in a cross between yellow-seeded 

 and green-seeded peas, one parent contributes to 

 the offspring a unit for yellow 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 bearing and half are 

 green bearing. This separation occurs both in the 

 eggs and in the sperm. 



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 

 cells. 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 

 Mendelian requirements of pairing and separation. 



The sperm of every species of animal or plant 



i 



