IS TWINNING HEREDITARY? 
Problem Much More Complicated Than It Appears at First Sight—Two Kinds of 
Twins—Possible Influence of the Father—The Frequency of Multiple Births 
C. H. DANFORTH 
Department of Anatomy, Washington University Medical School, St. Louis, Mo. 
P \HE layman is often impatient with 
the indefinite character of much 
of our eugenic information. It 
is not easy for him to understand 
the difficulties that attend an investiga- 
tion into the heredity of what may seem 
to be a single clear cut characteristic. 
For this reason it may be of interest to 
point out a few of the obstacles met in 
such a study. The question of a possi- 
ble hereditary tendency for twin produc- 
tion may be selected for this purpose. 
The problems encountered are not 
identical with those that would be met 
in the investigation of the heredity of 
some other characteristic but they are 
in a measure similar and will serve to 
furnish a typical example. 
The occasional occurrence of twins 
among the offspring of man and other 
animals that usually produce only one 
offspring at a time, has always been a 
matter of interest to both the popular 
and the scientific mind. This interest 
is due not so much to the fact that these 
mammals may sometimes produce two 
or more young at a birth as to the 
extreme similarity that frequently exists 
between the young thus produced. 
The causes that result in twin produc- 
tion, however, are not fully understood, 
and still less is known of the rdle 
heredity plays in this connection. 
If one attempts to learn more about 
the heredity of twinning he is forced to 
consider a number of subsidiary ques- 
tions which are in themselves of consid- 
erable interest. It is the main purpose 
of this paper to call attention to the 
bearing of some of these secondary 
considerations in relation to the question 
of an hereditary tendency toward twin 
production. Incidentally a few frag- 
mentary data from family histories 
are also presented, but the writer is 
still collecting this material! and has 
little hope that it will be in shape for 
final publication for some time. 
THE ORIGIN OF TWINS 
One of the first questions to be 
raised at the beginning of an investiga- 
tion into the existence or non-existence 
of any hereditary tenaency is: What is 
the exact nature of the characteristic 
in which this tendency is to be sought? 
It might seem that nothing could present 
less difficulty in this connection than 
twinning. But such is far from being 
the case. 
In the first place, it may be recalled, 
embryologists hold that twins arise in 
two very different ways. In one case, 
two separate egg cells, from the same 
ovary or from opposite ovaries, are 
fertilized each by a separate sperm cell. 
The resulting embryos develop inde- 
pendently like the different members of 
an ordinary litter. They need have no 
greater resemblance to each other than 
brothers and sisters born at different 
times, and the chance that they will be 
of like sex is the same as for any two 
consecutive children in the same family. 
Such twins are variously designated as 
fraternal, heterologous, biovular. ‘There 
seems to be ample evidence that many 
pairs are of this sort. 
In the case of the other class of twins, 
it is claimed that both members of the 
pair are derived from a single egg that 
has been fertilized by a single sperm. 
At some time subsequent to fertilization 
two centers of growth appear in the 
1 The writer of this paper will welcome family histories in which several pairs of twins occur, 
or other data bearing on the question of hereditary twinning. 
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