10 MENDELIAN SEGREGATION 
If the factors for vestigial wings are carried by a 
pair of chromosomes (the chromosomes carrying v 
in Fig. 3) then at the ripening of the germ cells (eggs 
and sperm) such a pair of chromosomes will come 
together and at reduction separate; so that each 
germ cell will have one such chromosome and not 
the other. (See Fig. 1, e-h.) 
If such a germ cell fertilizes an egg of the wild fly 
that contains a similar group of chromosomes, ex- 
Fic. 4.—(A.) Fertilization of egg by sperm. (B.) Zygote formed by 
union of egg and sperm. (C.) Diploid nucleus. 
cept that the corresponding chromosome carries the 
factor for long wings (Fig. 4, A), the result will be to 
produce a fertilized egg (Fig. 4, C) in which one mem- 
ber of the pair of chromosomes in question comes 
from the mother and carries the factor for long, and 
the other comes from the father and carries the 
factor for vestigial wing. Since this egg with both 
factors present produces a fly with long wings, the 
vestigial character is said to be recessive to the long; 
or conversely the long is said to be dominant to the 
vestigial character. 
When the eggs and the sperm of hybrid flies of this 
origin come to ys the homologous chromo- 
