66 BULLETIN 905, U. S. DEPARTMENT OF AGRICULTURE. 
to be observed in the reproductive cells. There is convincing evi- 
dence that the chromosomes are the bearers of the hereditary factors. 
In most cases two factors which come into an individual's heredity 
from the same parent are no more likely to go into the same repro- 
ductive cell than two factors which were derived from opposite 
parents. 
In some cases, on the other hand, there is a tendency for factors 
which enter a cross together to come out together in the second 
generation. Such factors are said to be linked with each other in 
inheritance. 
The hereditary factors of a species fall into groups in such a manner 
that those in the same group are all linked with one another, while 
those in different groups show no linkage. It is believed that each 
linkage group contains the factors which are carried by a given 
chromosome. 
Sex is normally determined by a difference in the chromosomes. 
In some cases, including mammals, the male produces two kinds of 
sperms, male and female determining, respectively. In other cases, 
including birds, it is the female that is heterozygous for sex. 
A number of characteristics are known which are linked with sex 
in inheritance in such way as to indicate that they are carried by the 
chromosome that determines sex. 
No practical means of modifying the sex ratio is yet known, with 
the possible exception of inbreeding and selection. 
The colors of farm livestock depend in most cases on relatively 
few unit factors. The same is true of a few other characteristics, 
such as the polled condition of cattle and the various comb shapes 
of poultry. 
Differences in size, type, and function are believed to depend in 
most cases on many factors. Their inheritance can not, so far as 
known at present, be controlled by such direct methods as the 
simpler characteristics. 
There is equal inheritance from the sexes. 
Prepotency depends neither on breed nor on sex, nor does the 
prepotency of an individual in one respect indicate his prepotency 
in others. In part, prepotency depends on the nature of the heredity 
back of particular characteristics, especially on the dominance of 
the factors which are involved. In part, it depends on the fixation 
of heredity, which means the making of all pairs of factors homozy- 
gous. 
Variation is composed of four elements : that due to different com- 
binations of hereditary factors, that due to the appearance of new 
hereditary factors (or mutations), that due to outside conditions 
which can be controlled, and that due to uncontrollable conditions, 
such as chance irregularity in development. Some characteristics 
are determined to a greater degree by heredity than others. 
