QUESTION No. 68
What do you mean by 'heredity,' and what influence has it on mental
disease?
ANSWER
1) Heredity is the transmission of qualities of like kind with those of
parents or ancestors to their offspring.
The material basis of inheritance in all ordinary cases of sexual
reproduction is the union of the reproductive cells of the female (ova) and
the reproductive cells of the male (spermatozoa).
When the ova are fertilized by the spermatozoa, the physical
characteristics of both parental families are mingled and the inheriting
must be dual.
There is a complete hereditary equipment of paternal origin and another of
maternal origin, and these form the warp and woof of the organism, though
it does not follow that both will be equally expressed. The offspring may
exhibit an intimate mixture of the characters of its two parents.
It frequently happens that an hereditable characteristic of a parent is
unexpressed in the development of the offspring but reappears in the third
generation.
It should be remembered that all that is inherited is a predisposition
towards the disease, and not the mental disease itself. Bacterial disease,
such as tuberculosis, cannot from its very nature be transmitted as such,
yet constitutional predisposition towards tuberculosis is certainly
hereditable.
The offspring of near relations tend to inherit the characteristics of both
parents in an exaggerated form, but there is no evidence that the children
of near relations (such as first cousins) are specially predisposed to
insanity, if both stocks are healthy.
Hereditary influence is said to be:-
a. Direct when the father of the patient has been mentally afflicted.
b. Collateral when mental disease appears only amongst brothers, sisters,
cousins, aunts, and uncles of the patient.
c. Atavistic or Reversive when any of the ancestors but not the patient's
parents have been mentally afflicted; this means skipping a generation.
d. Similar when the inherited mental disease is of the same type, in
members of the same family.
e. Dissimilar when the mental disorders in a family bear little resemblance
to the diseases of the ancestors.
Hereditary influence may be antedating in character, that is, the
appearance of the mental disorder at an earlier age in the offspring than
in the parents; thus a parent who develops manic-depressive insanity at the
age of thirty years, may beget a child who develops dementia praecox at the
age of puberty.
2. Among the general predisposing causes of insanity, heredity is
undoubtedly the first in importance, and there seems little doubt that
nervous disorders have their roots in some germinal defect and are
therefore in various degrees transmissible.
In the case of female mammals it is improbable that a seriously impaired
nervous system will admit of normal nutrition of the foetus, and thus a
nervous disorder due to arrest of development may reappear in the
offspring.
It is generally believed that a father is more likely to pass his mental
abnormalities on to his daughters, and a mother to her sons; the insanity
of the mother, however, is considered more dangerous.