"This term is applied to the fact of recurrence of traits in a number of blood relatives, due to their possession of a common germ-plasm. Hereditary traits are those whose development depends chiefly upon germinal factors, genes, or determiners."1

Heredity of a trait is direct, when the trait is found in parents and offspring; atavistic, when one or more generations are skipped; collateral, when the trait is found prevailingly in collateral relatives and not necessarily in the direct line of ancestry.

It is similar when the condition present in the patient is the same as that in an ascendant or collateral relative; in the opposite case it is dissimilar. The latter form is by no means uncommon: among the ascendants and collateral relatives of psychotic patients are to be found instances not only of similar psychoses, but also of dissimilar ones and of epilepsy, feeble-mindedness, criminality, temperamental abnormalities, sex immorality, and other neuropathic manifestations.

The fact that nervous and mental diseases are often transmitted by heredity was known to Hippocrates and has since his time been amply attested by psychiatric hospital statistics, but the exact conditions under which such transmission occurs have never been fully understood. Especially perplexing has been the seeming irregularity in the working of heredity as presented, on the one hand, in the above-mentioned facts of atavistic and collateral heredity and, on the other hand, in the frequent failure of transmission of neuropathic traits. Recent investigations have, however, revealed some data which seem to indicate that some mental disorders are transmitted from parent to offspring in the manner of a trait which is, in the Mendelian sense, recessive to the normal condition.1

1 Definition kindly furnished by Dr. C. B. Davenport, in a personal communication.

The bearing of the Mendelian theory seems to be of such importance in this connection that a brief statement of it may not be considered out of place.

The total inheritance of an individual is divisible into unit characters each of which is inherited more or less independently of all the rest and may therefore be studied without reference to other characters.

The inheritance of any such character is believed to be dependent upon the presence in the germ plasm of a unit of substance called a determiner.

With reference to any given character the condition in an individual may be dominant or recessive: the character is dominant when, depending on the presence of its determiner in the germ plasm, it is plainly manifest; and it is recessive when, owing to the lack of its determiner in the germ plasm, it is not present in the individual under consideration.

The dominant and recessive conditions of a character are often designated by the symbols D and R, respectively.

To make the matter clearer we may take as an example of a Mendelian character the case of eye color.

The brown color is the dominant condition while the blue color is the recessive condition, as has been shown by Davenport.2 It would seem that the inheritance of brown eyes is due to the presence in the germ plasm of a determiner upon which the formation of brown pigment is the anterior layers of the irides depends.

On the other hand, the inheritance of blue eyes is believed to be due to the lack of the determiner for brown eye pigment in the germ plasm; for the blue color of eyes is due merely to the absence of brown pigment, the effect of blue being produced by the choroid coat shining through the opalescent but pigment-free anterior layers of the irides in such cases.

1 H. H. Goddard. Heredity of Feeble-Mindedness. Bulletin No. 1, Eugenics Record Office, Cold Spring Harbor, N. Y. - A. J. Rosanoff and Florence I. Orr. A Study of Heredity in Insanity in the Light of the Mendelian Theory, Bulletin No. 5. - C. B. Davenport and D. F. Weeks. A First Study of Inheritance in Epilepsy. Bulletin No. 4.

2 Science, N. S., Vol. XXVI, Nov. 1, 1907, pp. 589-592.

It must be borne in mind that as regards the condition of any character every person inherits from two sources, namely, from each parent. Therefore, with reference to any character he may be pure bred or hybrid.

A case of inheritance of a character from both parents is spoken of as one of duplex inheritance and is often designated by the symbol DD.

A case of inheritance of a character from only one parent is spoken of as one of simplex inheritance and is designated by the symbol DR.

A case in which a character is not inherited from either parent, therefore exhibiting the recessive condition, is spoken of as one of nulliplex inheritance and is designated by the symbol RR.

We are now in a position to estimate the relative number of each type of offspring according to theoretical expectation in the various combinations of mates.

There are but six theoretically possible combinations of mates. Continuing to make use of the case of eye color as an instance of a Mendelian character, let us consider in turn each theoretical possibility.

1. Both parents blue-eyed (nulliplex): all children will be blue-eyed, as may be represented by the following biological formula:

RR X RR = RR.

2. One parent brown-eyed and simplex (that is to say, inheriting the determiner for brown eye pigment from one grandparent only), the other blue-eyed: half the children will be brown-eyed and simplex and the other half blue-eyed:

DRXRR = DR+RR.

3. One parent brown-eyed and duplex, the other blue-eyed: all the children will be brown-eyed and simplex:

DDXRR = DR.

4. Both parents brown-eyed and simplex: one-fourth of the children will be brown-eyed and duplex, one-half will be brown-eyed and simplex, and the remaining one-fourth will be blue-eyed (nulliplex):

DR X DR = DD+2DR+RR.

5. Both parents brown-eyed, one duplex the other simplex: all the children will be brown-eyed, half duplex and half simplex:

DDXDR = DD+DR.

6. Both parents brown-eyed and duplex: all the children will be brown-eyed and duplex:

DDXDD = DD.

It will be readily seen from these formulae that in attempting to predict the proportions of the various types of offspring that may result from a given mating it is necessary to know, not only whether the character is in each parent dominant or recessive, but in the case of the dominant condition also whether it is duplex or simplex.

Turning again to the case of eye color, an individual with blue eyes we know to be nulliplex as he has no brown pigment in his eyes and therefore could not have inherited the determiner for brown eye pigment from either parent. But now are we to judge in the case of a brown-eyed person whether he has inherited the determiner for that character from both parents or only from one? We can judge this only by a study of the ancestry and offspring of the individual.

To put the whole matter in a nutshell, the essential difference between a dominant and a recessive condition of a character is in the fact that in a case of simplex inheritance the dominant condition is plainly manifest while the recessive condition is not apparent and can be known to exist only through a study of ancestry and offspring.

This is important because it constitutes the criterion by which we are able to determine whether any given inherited peculiarity or abnormality is, as compared with the average or normal condition, dominant or recessive.

According to the assumption that most of the inheritable mental disorders, are, like the trait of blue eyes, transmitted in the manner of Mendelian recessives, theoretical expectation would be as follows:

1. Both parents being neuropathic, all children will be neuropathic.

2. One parent being normal but with the neuropathic taint from one grandparent, and the other parent being neuropathic, half the children will be neuropathic and half will be normal, but capable of transmitting the neuropathic make-up to their progeny.

3. One parent being normal and of pure normal ancestry, and the other parent being neuropathic, all the children will be normal but capable of transmitting the neuropathic make-up to their progeny.

4. Both parents being normal, but each with the neuropathic taint from one grandparent, one-fourth of the children will be normal and not capable of transmitting the neuropathic make-up to their progeny, one-half will be normal but capable of transmitting the neuropathic make-up, and the remaining one-fourth will be neuropathic.

5. Both parents being normal, one of pure normal ancestry and the other with the neuropathic taint from one grandparent, all the children will be normal; half of them will be capable and half not capable of transmitting the neuropathic make-up to their progeny.

6. Both parents being normal and of pure normal ancestry, all the children will be normal and not capable of transmitting the neuropathic make-up to their progeny.

Table 1 (from Rosanoff and Orr, loc. cit.) gives actual findings alongside of theoretical expectation, and it will be seen that the correspondence between the two sets of figures is very close.

Table 1

Types of Mating.

Number of Matings.

Total Number of Offspring.

Died in Childhood.

Data Unascertained.

Neuropathic Offspring.

Normal

Offspring.

Actual Findings.

Theoretical Expectation.

Actual Findings.

Theoretical. Expectation

1. RRXRR = RR.............

17

75

11

0

54

64

10

0

2. DRXRR = DR+RR........

93

500

66

5

190

2141/2

239

214 1/2

3. DDXRR = DR............

14

61

13

3

0

0

45

45

4. DRXDR = DD+2DR+RR .

62

369

44

3

107

801/2

215

2411/2

5. DDXDR = DD+DR.......

20

92

12

3

0

0

77

77

6. DDXDD = DD............

0

0

0

0

0

0

0

0

Totals..................

206

1097

146

14

351

359

586

578

The more important mental disorders which are supposed to develop on a hereditary basis are: arrests of development, epileptic psychoses, constitutional psychopathic states, dementia praecox, paranoia, manic-depressive psychoses, involutional melancholia, psychoneuroses, and Huntington's chorea. Of 8700 cases admitted to the New York state hospitals during the year ending July 30, .1918, 5420, or 62.3%, belonged to these groups.1