generation interval. Furthermore, except for 
poultry, each commercial producer must in 
varying degrees conduct his own breeding 
program. With these factors in mind, we will 
consider what possibilities are open to the 
animal breeder for disease, abnormalities, 
and pest control. 
QUALITATIVE TRAITS 
A wide variety of congenital abnormalities 
that interfere with body function have been 
reported for all classes of livestock. Stormont 
(1961) described over 90 abnormalities for 
cattle, sheep, swine, horses, and poultry. Those 
of genetic origin are amenable to control if 
the "carriers'' can be detected. Detection of 
carriers is straightforward in the case of the 
dominant anomalies, such as achondroplasia 
in cattle and lethal gray in sheep. Most con- 
genital disorders, for which some proof of 
inheritance is established are, however, due 
to a single autosomal recessive gene with no 
visible effects on the heterozygous individual. 
Some of the more common lethals or semi- 
lethals in the homozygous state are muscle 
contracture, semihairlessness, and legless- 
ness in swine, mulefoot, dwarfism, and short- 
spine cattle. 
What percent of our breeding program should 
be directed toward the detection and elimina- 
tion of these recessive genes? Consider the 
case of dwarfism in beef cattle. Limited evi- 
dence indicates that the heterozygote has a 
slight selective advantage because of its com- 
pact conformation, but since the homozygous 
recessive iscompletely rejected, the frequency 
of the gene is held in check. 
If all the herd sires carried the dwarf gene 
and nothing was done except to eliminate the 
easily detected dwarf, the incidence would 
stabilize at about 14.6 percent of all calves 
born. If only 80 percent of the sires used 
were carriers, the incidence would be 10 per- 
cent; 50 percent carrier sires, 4.4 percent; 
40 percent carrier sires, 3 percent; and 20 
percent carrier sires, less than 1 percent 
dwarfs (Emmerson and Hazel 1956, Smith 
1959), If control efforts are confined to pedigree 
selection on immediate ancestors, the inci- 
dence will remain rather low. Losses from 
dwarfism and other autosomal recessives 
149 
cause only a small portion of losses as com- 
pared to other diseases, and since they cannot 
be eliminated completely, efforts of balance 
must be such that the return is greater than 
the expense of the control program. 
INHERITED SUSCEPTIBILITY 
In addition to the clear-cut anomalies, sus- 
ceptibility to such diseases as cancer eye, 
bloat, and milk fever appear to be genetically 
affected. Hereford cattle are most prone to 
afflictions of the eye, but the condition is 
usually not a problem except in the latitudes 
of high solar radiation intensity. French (1959) 
observed that eye cancer is unusualin animals 
less than 4 years of age, and susceptibility 
reaches a peak at about 7 to 8 years. Bonsma 
(1949) reported that the presence of a ring of 
pigmentation around the eye gave almost 
complete protection. Since lid pigmentation is 
rather heritable (0.6-0.8), those breeders 
plagued by this condition may be advised to 
pay some attention to eye pigmentation in 
their breeding program, 
Although genetic susceptibility to such dis- 
eases as bloat and milk fever has been 
demonstrated, more information is required 
before breeding recommendations can be made. 
QUANTITATIVE TRAITS 
Diseases caused by pathogens result in the 
greatest economic losses, but the genetic 
picture is complicated by genetic variation 
in the pathogens, the genetic environmental 
interaction of the animal, possible acquired 
immunity, and perhaps yet unrecognized 
factors. In spite of these obstacles, there is 
some evidence to support selective breeding 
for reduction or control of pathogenic dis- 
eases. The example presented here are some 
of those for which present evidence holds 
limited promise. 
Mastitis 
Mastitis is the most complex and costly 
disease afflicting dairy cattle (about $400 mil- 
lion annually) in the United States. In the past 
30 years, several reports have been published 
