Some possible bearings of genetics on pathology

by Thomas Hunt Morgan

Unions of	I	and	I	give	I
	I		II	}	I, II
	II		II	}
	I		III	}	I, III
	III		III	}
Unions of	II	and	III	give	I, II, III, IV.
	IV		I		I, II, III, IV.
	IV		II		I, II, III, IV.
	IV		III		I, II, III, IV.
	IV		IV		I, II, III, IV.

Two actual pedigrees, one of them carried through three generations, will serve to illustrate particular cases, [Fig. 9].

From a knowledge of the blood group to which the child belongs it is possible to predict to what groups its parents may have belonged, and in certain cases it is possible to state that an individual of a certain group could not have been the parent of a particular child.

Fig. 9. The upper pedigree gives the children from the family in which types I and IV were the parents. The offspring belong to types II and III (two of the four possible kinds of offspring).

The lower pedigree represents three generations. The grandparents are I and II and I and III, respectively, while the parents are II and III.

In the transfusion of blood from one individual to another, that is sometimes necessary, it is essential that the blood corpuscles of the donor are not agglutinated by the serum of the recipient. Thus it is a matter of great importance to select a donor that does not bring about such a catastrophe. The simple rules are that individuals belonging to the same blood group (I, II, III, or IV) do not agglutinate each other’s blood, but the blood corpuscles of an individual represented by AA or Aa will be precipitated if the donor contains the agglutinin represented by aa, and conversely the blood corpuscles of an individual represented by BB or Bb will be precipitated if the donor contains the agglutinin represented by bb. Inspection of the diagram will show that group II (with serum bb) precipitates III and IV, and group III (with serum aa) precipitates II and IV. Further the serum of group I (aa bb) precipitates all of the other groups; while the serum of group IV precipitates none of the others.

My fourth illustration has probably in some cases a glandular basis, and in this sense has probably also a quantitative chemical background. Height or stature in man is, in part, an hereditary trait. It is sometimes said that short is dominant to tall, because short parents may have both tall and short children, but tall parents produce only tall children. This is probably an overstatement, or at least a rather loose generalization. Height may be due to long legs, or to a long body, or to a long neck or to time of reaching maturity or to any combination of these; and these differences may themselves be due to independent factors in inheritance. The best that we can do with height at present is to refer it to a multiple factor basis, the actual factors being little understood.

In addition to these differences in stature, all of which we call normal differences, there are certain extreme conditions superimposed on these as a background, in which the endocrine glands probably play an important rôle. While it may well be that many of these cases are caused by tumors of one of the glands, more especially of the pituitary, thyroid, or testis, it is quite possible that there may be actual inherited differences in the size and activity of these glands.