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 hy 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 oíĩspring 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

1
 

I