284 
GENETICS: C. B. DAVENPORT 
remarkable result that practically all can be traced back to some half- 
dozen individuals, including three (probable) brothers who migrated to 
America during the 17th century. But, already, numerous 'bio types' 
having specific and differential hereditary behavior have appeared. 
Thus there is a biotype in which the tremors are absent but mental de- 
terioration present; a biotype in which the tremors are not accompanied 
by mental deterioration; a biotype in which the chorea does not pro- 
gress; and a biotype in which the onset of the choreic movements is in 
early Hfe. In general, the symptomatology of chronic chorea is dissimi- 
lar in different strains of famiHes. The age of onset, the degree of mus- 
cular involvement, the extent of mental deterioration all show family 
differences and enable us to recognize various species, or biotypes, of 
the disease. These biotypes are less striking than they would be were 
it not for the extensive hybridization that is taking place between bio- 
types in random human ma tings. 
The method of inheritance of some of the elements of Huntington's 
chorea has been worked out. In general, the choreic movements never 
skip a generation and in other respects show themselves clearly to be a 
dominant trait. The mental disorder is usually of the hyperkinetic or 
manic type and this also shows itself as a dominant. The age of onset 
apparently tends to diminish in successive generations — 'law of antici- 
pation' — but this is partly, if not wholly, illusory and is due to the fact 
that in comparing the age of onset in grandfathers with that in grand- 
children we are not comparing on the same basis, for the grandparents 
are a selected lot (selected on the basis of late onset — at least late enough 
for them to become parents), while grandchildren include those in whom 
the onset is so early in life that they will never marry. If instead of 
comparing the average age of onset in successive generations, one com- 
pares the age of onset in a number of choreic parents, their parents and 
their grandparents, then the evidence for anticipation vanishes. Eight 
such series give for average age of onset of the propositus 35.5 years, 
parent 38.8, grandparents 36.9. In this series we can see no evidence 
of anticipation. 
Among the 3000 odd relatives of the 962 choreics studied many nerv- 
ous traits are recorded. Thus epilepsy is recorded 39 times, infantile 
convulsions 19 times, meningial inflammations and brain fever 51 times, 
hydrocephaly 41 times, feeblemindedness 73 times, Sydenham's chorea 
1 1 times, and tics 9 times, mostly in one small family. This incidence, 
which would seem high for an unselected population, suggests that 
chorea occurs in families characterized by a general liability to nervous 
and mental troubles. 
