CHAPTER III
LINKAGE

If two factors lie in the same member of a chromo-
some pair we should expect them always to be found
together in successive generations of a cross unless an
interchange can take place between such a chromo-
some and the homologous chromosome derived from
the other parent.

Whenever the two factors remain together in the
same chromosome there will be formed equal numbers
of gametes containing the two factors and of gametes
containing the normal allelomorphs of the two
factors. But if pieces of homologous chromosomes
are interchanged, then some of the gametes will con-
tain one of the factors in question, and an equal
number will contain the other factor. The process
of interchange between chromosomes is called cross-
ing over; the tendency of factors to stay together is
called linkage.

An example may make clearer this process of cross-
ing over. The factor for black body color and that
for vestigial wings both lie in the second pair of chro-
mosomes. If a black vestigial fly is crossed to a
wild fly (gray, long wings) (Fig. 21) the offspring are
gray with long wings. These Fi flies have one chro-
mosome containing both the factor for black and the
factor for vestigial, and a homologous chromosome