An inherited trait or characteristic is one which is passed down from parents to children. Some of you are undoubtedly wondering how you possibly could have inherited your hay fever when neither your mother nor father are allergic. The answer to your wonderment lies in the laws of heredity. The rules or laws of heredity as they are called, were first evolved by Gregor Mendel, a priest, living at Brunn, Austria. Mendel experimented for eight years with varieties of garden peas. He inbred and cross-bred these peas by tampering with the stamens and pollens of the plants. As a result of his experiments he was able to learn that certain traits are passed from generation to generation. Traits that appeared more definitely and frequently than others he called dominant traits. Characteristics that appeared less definitely and less frequently he called recessive traits. Mendel observed that in crossing or mating these dominant and recessive traits in large numbers of plants they always appeared in the same ratio. And whenever a dominant trait was present with a recessive trait the plant would take the appearance of the dominant trait.
Since Mendel’s time, thousands of experiments with plants and animals have confirmed his results. By applying these observations to the study of family trees in humans we have been able to learn which traits are hereditary. Thus we have found that hair color, eye color, skin color, baldness, and some diseases are transmitted according to Mendel’s rules of heredity. Not only this, but we have been able to learn that some traits are dominant while others are recessive.
Through the knowledge of Mendel’s arithmetic rules governing the transmission of dominant and recessive traits we may be able to explain how it is possible for a child born of two allergic parents to be free of allergy or how a person may have severe allergies although neither of his parents appear to be allergic. This is easily demonstrated by showing the arithmetic ratios of traits resulting from the mating of two individuals who both contain a hereditary characteristic.
Let us for example use the condition of being allergic as the trait, and call it A. For this example we shall assume that it is a recessive trait. (Actually authorities are not yet agreed as to whether it is a dominant or recessive trait.) Let N stand for normalcy or the non-allergic condition. Now if your father and mother each had one allergic parent and one non-allergic parent, this would leave them both with the recessive A (allergy) trait and the N (normalcy) trait. Thus they are both potentially allergic, and have the allergy factor in their cells, but they may never show their allergy by symptoms. However, they will both pass on their recessive A trait. Let us now review the possibilities if they have four children.
It must not be implied from the above example that the allergic condition is actually transmitted on a simple recessive basis. It is possible that the trait may be dominant, incomplete dominant, or be based on multiple factors in the genes. Any conclusion as to whether allergies are inherited on a dominant, impure dominant, or recessive basis, must await further study and accumulation of information.