PRINCIPLES OF LIVESTOCK BREEDING. 65 
SUMMARY. 
Animals and plants are composed of microscopical units — the cells. 
Each cell has a specialized central portion, the nucleus, which con- 
tains a number of threadlike or rodlike bodies called chromosomes. 
The number of chromosomes is constant in each kind of animal or 
plant. 
Every animal begins its career in the union of two cells— the egg 
cell from the dam and the sperm from the sire. 
In a sense the reproductive cells are not produced by the parents 
but are unspecialized bits of the same material from which the 
parents themselves originally developed. Heredity consists merely 
in the retention, by the reproductive cells, of the power to develop 
into a complete individual under the proper conditions. 
Characteristics acquired by the parents through training, care, or 
accident are not transmitted to their progeny. Belief in the in- 
fluences of telegony and maternal impressions has no scientific 
foundation. 
The heredity, transmitted by the reproductive cells, is composed 
of unit factors, each of which is handed on unchanged from genera- 
tion to generation. 
Hereditary differences depend on the existence of alternative forms 
of certain unit factors. Such alternative factors are called alle- 
lomorphs of each other. 
The egg and sperm each contain, typically, a full set of the unit 
factors characteristic of the kind of animal. Their union results in 
a double set in the fertilized egg. 
This double set may be composed in part of pairs of identical factors 
and in part of pairs of alternative factors. The individual is said to 
be homozygous in regard to the former, and heterozygous in regard 
to the latter. 
It often happens that one of the alternative factors in a hetero- 
zygous individual expresses itself fully in development at the expense 
of the other. Such a factor is said to be dominant. The factor 
whose influence is suppressed is said to be recessive. The normal 
type of a species is usually dominant over deviations from normal. 
Most characteristics depend on the combined influence of a number 
of pairs of factors. 
The double set of factors in the individual is sorted out into single 
sets in the formation of the reproductive cells. Individuals produce 
only one kind of reproductive cell in those respects in which they 
are homozygous and two kinds in those in which they are hetero- 
zygous. 
Parallel to the results of genetic experiments, the microscope reveals 
the chromosomes in pairs in the body cells, in place of the single set 
