Page:The New International Encyclopædia 1st ed. v. 09.djvu/880

* HEKEDITY. 810 HEREDITY. ikviate from the mode to an extent which can be predicted, or which we inaj" hope some day to be able to prediel. The subject will here be treated under the following heads: (1) The classes of heredity; (2) the mathematical laws of normal inheritance; (3) special eases of inheritance not following the normal law, and (4) the limits of inheritance. ( I ) Cl.S8ES of Heredity. Heredity deals, as we have seen, with the qualities of organisms and their parts. Now there are, in the case of two parents and their offspring, four general methods of inheritance of qualities: {a) The qualities of the parents may blend in the offspring. This case of blending fwritage is illustrated by human stature. Let us imagine the wholly supposable case of a family of children all of whose maternal ancestors for five generations back had been two inches taller than the mode of females, and all of whose paternal ancestors had been four inches taller than the mode of males ; then the stature of the male offspring or of the female offspring would be found close to three inches above the mode of male or female stature, as the case might be. The tendencies in the two parents have precisely blended, and the offspring are interme- diate. Another illustration of blending heritage is skin-color in man. The mulatto is a half tint, the quadroon a fourth tint, and so on. (6) The quality may be inherited from one parent wholly, and not at all from the other parent. This case is known as alternative hrritage. This sort of inheritance is illustrated by hiunan eye- color. Galton {Xatiiral Inheritance, p. 139) says: "If one parent has a light eye-color and the other a dark eye-color, some of the children will, as a rule, be light and the rest dark; they will seldom >e medium eye-colored, like the children of medium eye-colored parents." Cases of alterna- tive heritage have also been described for ))lant hybrids. Alternative heritage is exhibited in the hair-color of some mammals. For when a white and yellow mouse are crossed, some of the off- spring may be pure white (albinos), like the one parent, and the others may be yellow, like the other parent, with no sign of the yellow becoming lighter, (c) In one part of the body the quality ma,y be exactly lik? that of one parent; in an- other part, exactly like that of the other parent : in other words, there may be a patchwork inter- mingling of parental qualities. This case is known as particulate inheritance. Particulate inheritance is common in hair-color of horses, dogs, mice, and other mammals. When w-e cross two plants in which the leaf-hairs differ, this sort of inheritance often occurs, so that the two kinds of hairs of typical form are scattered over the leaf, (d) Reversion. This is inheritance from a remote ancestor, or, perhaps, more truly, the ab- sence of typical inheritance. Reversion is often illustrated when two domesticated races of very dissimilar color arc crossed. The result may be viffspring which resemble neither parent, but are like the ancestral, wild species. It is at present impossible to predict what kind of inheritance will be shown by the qualities of any new cross. (2) Mathematical Laws of Normal Inherit- ance. Man. like most other animals, has two parents, four grandparents, eight great-grand- parents, sixteen ancestors of the foirth genera- tion, and, generally, In ancestors of the nth ancestral generation. We inherit not only from our parents, but also from our remote ancestors. The law of inheritance in the case of blending heritage has been worked out by Galton as fol- lows: To the total heritage the parents together contribute 50 per cent. : the four grandparents together, 25 per cent. ; the third ancestral gen- eration, (^A)" or 12.5 per cent.; and. generally, with the (ith ancestral generation V-n- per cent, of the total heritage. It is a corollary of this law that the offspring of an exceptional pair will, in the long run. be less exceptional than their ])arents, unless, which is extremely improbable, llieir ancestors for six or seven generations have all been as exceptional as the parents. It is because the vast population of one's ancestors is pretty sure to be mediocre that the sons of exceptional parents tend to be less exceptional than they. There is a regression of the progeny toward niediocrity. We can measure this regres- sion (on the average), and say the deviation of the son's stature from the mean will be only 446 -f the deviation of the father's stature from the mean, or less than half. From the law of ancestral inheritance we may indeed calculate the relative relationship between two blood relatives in respect to any blending character. This has been done by Pearson in the following table: Parent 3000 First cousins 0750 firandparent 1500 First cousiuB, once re- ( Jreat-grandparent 0750 moved 0344 Great-great-grandpar- Second couBin.s 0172 ent 0375 Second cousins, once Brothers 4000 removed 0082 rncles and nephew 1500 Third cousins 0041 Great-uncle and nephew .0625 (3) Special Cases of Inheritance Not Following the Normal Law. The normal law has so far been demonstrated only for cases of Idending inheritance. For the other cases our knowle(lge is almost wholly qualitative. The nor- mal condition of inheritance in these cases may be said to be equal inheritance of- any quality from father and mother; or, in the case of alter- native heritage, an equal proportion of offspring affected by the two parents. The first deviation from this law is the phenomenon of pre])otency, or the excessive inheritance from one parent or one race. It often happens that a certain family characteristic crops out again and again, and in a larger percentage of cases than is called for by the normal law of inheritance. This is most marked when the parents belong to distinct races. Tims, when a common house mouse and a domes- ticated white mouse are crossed, the offspring are more like the house mouse in color than they are like the tame albinos. Ewart, in crossing horses, found that the male was strongly prepotent. The fact of prepotency is akso familiar to plant- breeders who have tried to formulate some of its laws. Thus, it is asserted that the older species, or the more vigorous, is prepotent. 'Sports,' or considerable deviations from the typical condition, are apt to be prepotent. Bell has shown that deaf-mutism is a prepotent qual- ity. The following case of inheritance of syndac- tylism in man is given by Smith and Norwell, in tiie Briti.fh Medical Journal for .July 7, 1894. S^^ldactylism. or the growing together of two or more fingers, is a rather rare abnormality. In the diagram (J' stands for male. 9 for female, N for normal, and S for syndactylic. Of two par- ents, the male precedes the female. When the law of normal inheritance calls for 20 per cent, of syndactylic individuals in the latest generations, observation gives 09 per cent.