Oct. 27, 1923 
Otocephaly in Guinea Pigs 
179 
important genetic basis. Further, the case illustrates how a fundamental 
change in the developmental pattern at first sight perhaps suggesting the 
loss of determinants for parts of the head, may be explained much more 
satisfactorily as due to genetic factors with a very simple physiological 
effect, acting in conjunction with environmental conditions. 
SUMMARY 
Among about 40,000 guinea pigs recorded in genetical experiments of 
the Bureau of Animal Industry 82 monsters of the otocephalic type, or 
about 0.2 per cent, have appeared. These are classified in a practically 
linear series of 12 grades of defectiveness. The defects in the lower grades 
(1 to 4) center about arrest of Meckel's cartilage. Arrest of the fronto¬ 
nasal process seems responsible for grade 5. From grade 6 to grade 12 
the primary feature is the progressive arrest of the brain. Grades 7 to 9 
are cyclopeans. Grades 11 and 12 are almost headless. 
The majority (50) have appeared in one inbred family, No. 13, in which 
the frequency is 1.5 per cent. There are marked differences in the per¬ 
centages in different sublines of this family, there being indications of 
segregations of different tendencies in the early generations. 
At two or three points in the pedigree there has been a sudden jump 
in the tendency. A line producing about 1 per cent jumped to 4 per cent 
in one of its sublines in the thirteenth generation. This gave rise to a 
branch in the nineteenth generation, which is producing over 20 per cent. 
These observations demonstrate the importance of genetic differences 
between inbred lines. Analysis indicates, however, that there can be few 
or no genetic differences within such lines between the normals and the 
monsters. Their occurrence can not be explained as due to Mendelian 
segregation or to mutation, except where one whole subline becomes 
differentiated from its parent line in frequency of production. 
Inbreeding merely brings to light genetic differences. Family 13 pro¬ 
duced most of its otocephali in the later generations. All other inbred 
families, several with histories closely parallel to Family 13, produced the 
greater number in the early generations. 
The genetic basis determines an individual, not a maternal, character. 
Females of Family 13 do not produce otocephali in outcrosses to the extent 
that they do in matings with brothers. Otocephali are more likely to 
appear in crosses in which both sire and dam have blood of Family 13 than 
in other crosses. Females are twice as likely to suffer the defect as males. 
There is no appreciable difference in frequency in first and later litters 
or in last litters. 
The seasonal distribution shows a slight predominance in the winter and 
early spring, when conditions are apt to be poor, agreeing with the fluc¬ 
tuations in size of litter and mortality of the normal young before weaning, 
with a not unexpected anticipation by half a month in the former case 
and a lag of a month in the latter. 
Otocephali are born in slightly smaller litters than normals from the 
same matings. A slightly larger percentage* of their litter mates die at 
birth and between birth and weaning than normals in other litters from 
these matings. There is a corresponding but statistically insignificant 
difference in birth weight between litter mates and nonlitter mates. 
Otocephali themselves are only slightly under weight and typically have 
a well-nourished, healthy appearance. 
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