Inheritance is not directly from the parent but from the germ cells, which may carry characteristics not found in the parent but in some of the other ancestors. An individual does not inherit what his parents are but what is in the two germ cells, one from the mother, one from the father, that unite to form that individual.
With the union of the two germ cells the inborn characteristics of the individual are determined, “the gate of gifts is closed.” Environment and training may increase the strength, or minimize the force of inborn characteristics, or even suppress some of them, but it cannot add to them, or increase their force beyond their inherent limitations.
Some few characteristics are inherited only through the mother, or only through the father, or are transmitted only to the sons or only to the daughters; most characteristics are not thus limited, but may be transmitted by either parent to either son or daughter.
Acquired characteristics are not inherited. If a man loses his hand in an accident, his descendants cannot inherit one-handedness; if he masters a foreign tongue, his descendants cannot inherit his knowledge of that language.
No disease germ is inherited, in the genetic sense of being conveyed in the special germ cells. A child may be infected with a disease before its birth; this is not, strictly speaking, heredity but congenital (or prenatal) infection. Tuberculosis is sometimes thus conveyed from the mother, and syphilis very frequently when either the mother or the father has this disease even in latent form. What may be inherited is a tendency toward a disease, a weakness of specific organs or tissues, a lack of resistance to a specific disease.
Variations sometimes appear apparently spontaneously, as the result of some accident to the germ plasm, or an unusual combination in the two germ cells; such variations may be inherited.
Some characteristics are apparently persistent, and in the process of inheritance tend to predominate over their complementary characteristics. The former are called dominant, the latter recessive characteristics. The law by which dominant and recessive traits are inherited was first formulated by Mendel, an Austrian monk, less than half a century ago. Biological research is being devoted at present to discovering what traits of human significance are subject to this Mendelian law, as it is called.
A characteristic found in both parents, or in both families, has a double possibility of appearing in their descendants, and some mental defects and abilities tend to appear with greater force and at an earlier age, in the descendants.
Every individual is born with all the germ cells he will ever possess.
These germ cells are highly susceptible to poisons in the circulation, especially to: