THE MECHANISM OF 
MENDELIAN HEREDITY 
CHAPTER I 
MENDELIAN INHERITANCE AND THE 
CHROMOSOMES 
Mendel’s 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 yellow- 
seeded and green-seeded peas, one parent con- 
tributes 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 
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 
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 
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