# Popular Science Monthly/Volume 85/November 1914/Phenomena of Inheritance VI

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* PHENOMENA OF INHERITANCE
By Professor EDWIN GRANT CONKLIN
PRINCETON UNIVERSITY
II. Modifications and Extensions of Mendelian Principles
IT is a common experience that natural phenomena are found to be more complex the more thoroughly they are investigated. Nature is always greater than our theories, and with few exceptions hypotheses which were satisfactory at one stage of knowledge have to be extended, modified or abandoned as knowledge increases. This observation is well illustrated in the case of the Mendelian theory. The principles proposed by Mendel were relatively simple, but in attempting to apply them to the many phenomena of inheritance now known it has become necessary to modify or extend them in many ways. And yet the general and fundamental truth of these principles has been established in a surprisingly large number of cases, and they have been extended to forms of inheritance where at first it was supposed that they could not apply.
1. The Principle of Unit Characters and Inheritance Factors.—There has been much criticism on the part of some biologists of the principle of unit characters. It is said that unit characters can not be independent and discrete things; the organism itself is a unity and every one of its parts, every one of its characters, must influence more or less every other part and every other character. Certainly unit characters can not be absolutely independent of one another; the various parts and organs of the body and even the organism, as a whole, is not absolutely independent, and yet there are varying degrees of independence in organisms, organs, cells, parts of cells, hereditary units and characters which make it possible for purposes of analysis to deal with these things as if they were really independent, though we know they are not.
Of course characters of adult individuals do not exist as such in germ cells, but there is no escape from the conclusion that in the case of inherent differences between mature organisms there must have been differences in the constitution of the germ cells from which they developed. For every inherited character there must have been a germinal cause in the fertilized egg. This germinal cause, whatever it may be, is often spoken of as a determiner of a character. But the character in question is not to be thought of as the result of a single cause nor ​as the product of the development of a single determiner; undoubtedly many causes are involved in the development of every character, but the differential cause or combination of causes is that which is peculiar to the development of each particular character.
Again it is not necessary to suppose that every developed character is represented in the germ by a distinct determiner, or inheritance unit, just as it is not necessary to suppose that every chemical compound contains a peculiar chemical element; but it is necessary to suppose that each hereditary character is caused by some particular combination of inheritance units and that each compound is produced by some particular combination of chemical elements. An enormous number of chemical compounds exists as the result of various combinations of some eighty different elements, and an almost endless number of words and combinations of words—indeed, whole literatures—may be made with the twenty-six letters of the alphabet. It is quite probable that the kinds of inheritance units are few in number as compared with the multitudes of adult characters, and that different combinations of the units give rise to different adult characters; but it is certain that every inherited difference in adult organization must have had some differential cause or factor in germinal organization.
Mendel did not speculate about the nature of hereditary units, though he evidently conceived that there was something in the germ which corresponded to each character of the plant. Weismann postulated a determinant in the germ for every character which is independently heritable, and many recent students of heredity hold a similar view.
But it is evident that there is not an exact one-to-one correspondence of inheritance units and adult characters. Many characters may be decided by a single unit or factor; for example, all the numerous secondary sexual characters which distinguish males from females are decided by the original factor which determines whether the germ cells shall be ova or spermatozoa.
On the other hand, two or more factors may be concerned in the production of a' single character. In many cases among both plants and animals the development of color appears to depend upon the presence in the germ cells and the cooperation in development of at least two factors, viz. (1) a pigment factor P (for black B, for brown Br, for yellow Y, for red R, etc.), and (2) a color developer C. When both of these factors are present color develops; when either one is absent no color appears.
Such cases have been described for mice, guinea-pigs and rabbits as well as for several species of plants. Bateson and Punnett found two varieties of white sweet peas which were apparently alike in every respect except the shapes of their pollen grains, one of them having long ​and the other round pollen. But when these were crossed a remarkable thing occurred, for the progeny, "instead of being white, were purple, like the wild Sicilian plant from which our cultivated sweet peas are descended." This is apparently a typical case of reversion and its cause was found in the fact that at least two factors are necessary in this case for the production of color, a pigment factor R and a color developer C. One of these was lacking in each of the white parents, their gametic formulæ being Cr and cR, but when these two factors came together in the offspring a purple-flowered type was produced with the gametic formula Cc Rr. These F$_3$ plants produced colored and white F$_3$ plants in the proportion of 9 colored: 7 white and the colored forms were of six different kinds (Fig. 57). For the production Fig. 57. Results of Crossing two Different Races (A) and (B) of White Sweet Peas; all the F$_1$ hybrids (C) are purple with blue wings like the wild ancestral stock; in F$_2$ six colored varieties are formed ranging from purple with blue wings (D) to tinged white (I) and several kinds (genotypes) of white varieties (K). (After Punnett.) of these six colored forms five different factors must be present in the gametes, according to Punnett, viz.: (1) a color base (R), (2) a color developer C, (3) a purple factor B, (4) a light wing factor L, (5) a factor for intense color I. When all of these factors are present the result is the purple wild form with blue wings, while the omission of one or more of these factors leads to the production of six forms of colored and various types of white flowered plants of the F$_2$ generation.
Castle found that eight different factors may be involved in producing the coat colors of rabbits; these are:
C a common color factor necessary to produce any color.
B a factor acting on C to produce black.
Br a factor acting on C to produce brown.
Y a factor acting on C to produce yellow.
I a factor which determines intensity of color.
U a factor which determines uniformity of color.
A a factor for agouti, or wild gray pattern, in which the tip of every hair is black, below which is a band of yellow, while the basal part of the hair is gray. ​E a factor for the extension of black or brown but not of yellow.
Plate found that all of these factors except the last, E, are also involved in the production of the coat colors of mice. Baur has recognized more than twenty different factors for the color and form of flowers in the snap-dragon, Antirrhinum.
These factors are probably complex chemical substances which preserve their individuality in various combinations, just as groups of atoms or radicals do in chemical reactions; they may be dropped out or added, substituted or transposed, just as chemical radicals may be in chemical compounds. To this extent they maintain continuity and independence, but they are not absolutely independent, for they react upon one another as well as to environmental changes, so that the characters of the developed organism are the results of all these reactions and interactions.
Inheritance Factors and Germinal Units
It is obvious that there must be things in germ cells which correspond to the inheritance factors; furthermore, these things must be material particles even though they be only atoms or molecules and their combinations or dissociations. And yet there are many students of the phenomena of heredity who know little about germ cells and to whom all parts of a cell are hypothetical structures, to whom "chromosomes are articles of faith," and who protest rather violently against any attempt to find the factors of inheritance in any of the structures of the germ cells. And yet it is perfectly evident that if there are inheritance units they must exist in the germ cells as discrete particles, even if they are only molecules, by whose associations or dissociations in response to intrinsic or extrinsic conditions the various characters of the developed organism arise. It is certainly legitimate to ask what the germinal elements are which correspond to inheritance factors.
There was a time when the cell was the ultima thule of biological analysis and when the contents of cells were supposed to be "perfectly homogeneous, diaphanous, structureless slime." Then the nucleus was discovered within the cell, then the chromosomes within the nucleus, then the chromomeres within the chromosomes, and there is no reason to suppose that organization ceases with the powers of our present microscope. With every improvement of the microscope and of microscopical technique, structures have been found in cells which were undreamed of before, and it is not probable that the end has been reached in this regard. We know that cells contain nuclei and chromosomes and chromomeres, centrosomes and plastosomes and microsomes, and we know that some of these parts differ in function as well as in structure. And there is no reason to doubt that if we had sufficiently powerful microscopes we should find still smaller and smaller units until we came at last to molecules and atoms.
​The manner in which inheritance units from the two parents unite in fertilization and later segregate in the formation of gametes, so that the latter are pure with respect to any character, is a familiar part of Mendelian inheritance (Fig. 58). What are these units in terms of
Fig. 58. Diagram Showing Union of Factors in Fertilization and their Segregation in the Formation of Germ Cells. With 4 pairs of factors (Aa, Bb, Cc, Dd), 16 types of gametes are possible, as shown in the two series of small circles at the right. (From Wilson.)
cell structures and where are they located in the cell? We have in the chromosomes, as Wilson especially has emphasized, an apparatus which fulfils all the requirements of carriers of Mendelian factors (Fig. 59). Both factors and chromosomes come in equal numbers from both parents; both material and paternal factors and chromosomes pair in the zygote and separate in the gamete, as shown in diagrams 58 and 59; and so far as known the chromosomes are the only portion of the germ cells which fulfil these conditions. Furthermore, there is much additional evidence that the chromosomes are especially concerned in heredity, as was pointed out in the last lecture, and it is not reasonable to suppose that this remarkable coincidence between the distribution of Mendelian factors and of chromosomes is without significance.
Of course Mendelian factors are not all the factors of development, but merely the differential factors which cause, for example, one guinea pig to be white and its brother to be black. Very many factors are involved in the production of white or black color, but there is at least one differential factor for every unit character, and this alone is the Mendelian factor. Of course there is no such thing as a "sex-producing chromosome," sex being the result of the interaction of the X-chromosome upon other chromosomes, and of all of these upon the cytoplasm. The X-chromosome is only one factor in the determination of sex, but ​if it is a factor which differs in the ease of the two sexes it is a "sex-determining factor." There are many parts of a germ cell, all of which may be concerned in heredity and development, but the chromosomes appear to be the seat of the differential factors for Mendelian characters.
Fig. 59. Cellular Diagram Corresponding to Fig. 58, Showing the Union of Maternal Chromosomes (ABCD) and Paternal Ones (abcd) in Fertilization, their distribution in cleavage, their union into 4 pairs (Aa, Bb, Cc, Dd), in synapsis and the separation of the pairs in the reduction division. Only 2 of the 16 possible types of germ cells are shown. (From Wilson.)
2. Modifications of the Principle of Dominance.—A great number of animal and plant hybrids show one contrasting character completely dominant over the other one, as Mendel observed in the case of his peas. But in a considerable number of cases this dominance is incomplete or imperfect. When white-flowered strains of four-o'clocks are crossed with red-flowered ones the F$_1$ plants bear neither white nor red flowers, but pink ones, and the F$_2$ plants bear white, red and pink flowers. The whites and reds are always homozygous, the pinks heterozygous; pure white and pure red are produced only when their factors are duplex (WW), (RR); when they are simplex (WR) pink is produced. In this case red is not completely dominant over white, but the hybrid is more or less intermediate between the two parents (Fig. 56).
It has long been known that the breed of fowls called Blue Andalusian does not breed true, but in each generation produces a certain number of blacks and whites as well as blues. Bateson found that the blues are really hybrids between blacks and whites in which neither of ​the latter is completely dominant. Black and white appear only when they are pure (homozygous), blue only when both black and white are present (heterozygous).
Again, a cross of red and white cattle produces roan offspring, but the latter when interbred give rise to reds, roans and whites in the proportion of 1 : 2 : 1, showing that the roans are heterozygotes in which red is not completely dominant over white, while the reds and whites are homozygotes and consequently breed true.
Lang found that when snails with uniformly colored shells were crossed with snails having bands of color on the shells the hybrids were faintly banded, thus being more or less intermediate between the two parents; but when these hybrids were interbred they produced banded, faintly banded and uniformly colored snails in the ratio of 1:2:1, thus proving that Mendelian segregation takes place in the F$_2$ generation, and that dominance is incomplete in the heterozygotes. Many other similar cases of incomplete dominance are known.
Sometimes dominance is incomplete in early stages of development, but becomes complete in adult stages. Davenport found that when pure white and pure black Leghorn fowls are crossed the chicks are speckled white and black, but in the adult fowl dominance is complete and the plumage is black. Similar conditions of delayed dominance are well known in the color of the hair and eyes of children, though dominance may become complete when they have reached adult life.
In a few instances a character may be dominant at one time and recessive at another. Thus Davenport found that an extra toe in fowls is dominant under certain circumstances and recessive under others. Tennent found that characters which are usually dominant in hybrid echinoderms may be made recessive if the chemical or physical nature of the pea water is changed. Such cases seem to show that dominance may sometimes depend upon environmental conditions, sometimes upon a particular combination of hereditary units.
Sex and Sex-limited Inheritance
Sex and sex-limited inheritance may be considered here, since they involve questions of dominance. There is good evidence, as was shown in the last lecture, that sex is a Mendelian character, in which the female has a double dose of the determiner for sex, whereas the male has only a single dose. Consequently in the formation of the gametes every egg receives one sex-determiner, while only one half of the spermatozoa receive such a determiner, the other half of them being without it. If. then, an egg is fertilized by a sperm without one of these determiners, a male results; but if an egg is fertilized by a sperm with one of these determiners, a female is produced. This is graphically represented in diagram 60, in which X represents the sex determiner, which is duplex ​in the female and simplex in the male, and the chance anions of male and female gametes yield females (XX) and males (XO) in equal numbers.
Fig. 60. Diagram Showing Sex as a Mendelian character, the Female being homozygous, the Male heterozygous for Sex. The female forms gametes all of which contain the x-chromosome; the male forms two sorts of gametes one half of which contain the X-chromosome and the other half lack it. All possible combinations of these gametes give a 2: 2 or 1: 1 ratio of females to males.
In either sex many secondary sexual characters of the other sex are present during development, and traces of these may persist in the adult; but one set of these characters develops in the male and another in the female, so that they may be called sex-limited. The development of the secondary sex characters is usually determined by the ovaries or testes, which are the primary sex characters, though in some instances they may develop in animals which have lost their ovaries or testes, but in the last analysis both primary and secondary sex characters are dependent upon the sex determiner. Sex and sex-limited inheritance are only special cases of Mendelian inheritance in which conditions of dominance differ in the two sexes, depending upon whether the factor for sex is duplex or simplex.
Sex-linked Inheritance
In this connection we may consider another class of characters, which are linked with sex but are in no wise connected with sexual reproduction. Such characters are not necessarily limited to one sex or the other, as are many primary and secondary sexual characters, but they may appear in either sex, though they are usually transmitted from fathers to daughters, or from mothers to sons ("criss-cross" inheritance) in exactly the way in which the sex chromosomes (X) are transmitted. Morgan has therefore concluded that the factors for these characters are carried by the sex chromosomes and has named them sex-linked characters. In the fruit fly, Drosophila, he has discovered more than twenty-five such characters, applying to the color of the eyes and of the body, to the length of the wings, etc. A typical case is shown in Figs. 61 and 62. The eye color of this fly is normally red, but mutations have arisen in which the eye is white. Such a mutation always appears in males, though it may later be transferred to females, as we shall see. If now a white-eyed male and a red-eyed female are crossed all the F$_1$s are red eyed, but if these F$_1$s are interbred all the females of F$_2$ have ​red eyes while half of the males have red eyes and the other half have white eyes (Fig. 61). On the other hand, if one of the F$_1$females of this cross is bred with a white-eyed male half of the females of F$_2$ are
Fig. 61. Sex-linked Inheritance of White and Red Eyes in Drosophila. Parents, white-eyed male and red-eyed female; F$_1$, red-eyed males and females; F$_2$, red eyed females and equal numbers of red-eyed and white-eyed males. The distribution of sex chromosomes is shown to right of flies; X carries the factor for red eyes, X the factor for white eyes, O stands for absence of X. (After Morgan.)
red eyed and half are white eyed, and half of the males are red eyed and half are white eyed.
If now one of these white-eyed females is bred with a red-eyed male all the females of the F$_1$ generation are red eyed and all the males white eyed ("criss-cross" inheritance) and if these are interbred there are produced in the F$_2$ generation equal numbers of red-eyed and white-eyed males and females (Fig. 62).
The distribution of the maternal and paternal sex chromosomes (X) exactly parallels this distribution of this sex-linked character, as is shown in the right half of each of the figures, 61 and 62, and this is certainly very strong evidence that the differential factors for these characters are carried in these chromosomes.
Another case of sex-linked inheritance is found in an abnormal condition in man known as hæmophilia, which is characterized by a deficiency in the clotting power of the blood, and consequently by excessive bleeding after injury. "Bleeders" are almost always males, though the defect is always transmitted to a son from his mother, who does not usually show the defect because it appears in females only when both parents were affected. The manner of inheritance of this character is exactly similar to the inheritance of white eyes in Drosophila and is in ​all probability associated with the distribution of the maternal and paternal sex chromosomes.
One of the most striking cases of sex-linked inheritance is that form of color-blindness known as Daltonism, in which the affected person is unable to distinguish between red and green. It is known that males are more frequently affected than females, and that color-blindness is in some way associated with sex. It requires two determiners for colorblindness, one from the father, the other from the mother, to produce a color-blind female, whereas only a single determiner is necessary to
Fig. 62. Reciprocal, Cross of Fig. 61. Parents, white-eyed ♀ and red-eyed ♂, F$_1$, red eyed ♀ and white-eyed ♂ ("Criss-cross inheritance"), F$_2$, equal numbers of red-eyed ♀ and ♂ and white-eyed ♀ and ♂. The distribution of sex chromosomes is shown on the right, as in Fig. 61.
produce a color-blind male, just as is true of sex. The accompanying diagrams illustrate the method of inheritance of color-blindness. As in the previous diagrams X represents the sex determiner, O its absence, and X the sex determiner which carries the factor for color-blindness. (Diagrams from Morgan.) It will be seen that a color-blind father and a normal mother have only normal children, but the father transmits to his daughters and not to his sons the sex determiner which carries the factor for color-blindness. But since color-blindness does not develop in females unless it is duplex (i. e., comes from both father and mother), whereas it develops in males if it is simplex (i. e., comes from either parent) all the daughters will appear normal although carrying one determiner for color-blindness, while all the sons will be normal because they carry no determiner for color-blindness. But these daughters transmit to one half of their children the single determiner for color-blindness, and if any of those receiving this determiner are males they will ​be color-blind. Consequently we have the curious phenomenon of simplex color-blindness appearing only in males and being transmitted to them only through apparently normal females.
Fig. 63. Diagram of Inheritance of Color-blindness through the Male A color-blind male (here black) transmits his defect to his grandsons only. The corresponding distribution of the sex chromosomes is shown on the right, the one carrying the factor for color-blindness being black. (After Morgan.)
On the other hand, if a female is color-blind she has inherited it from both father and mother, i. e., the character in her is duplex, and in all of her children by a normal male the character will be simplex: accordingly, all of her sons will be color-blind and all of her daughters will be normal, though carrying the simplex determiner for color-blindness.
In all cases dominance means merely the development in offspring of certain characters of one parent, while contrasting characters of the other parent remain undeveloped. The appearance of any developed character in an organism depends upon many complicated reactions of germinal units to one another and to the environment. Under certain conditions of the germ or of the environment some characters may develop in hybrids to the exclusion of their opposites, whereas under other condition-these results may be reversed or the characters may be intermediate. The principle of dominance is not a fundamental part of Mendelian inheritance. Even when the characters of hybrids are intermediate between those of their parents, if the parental types reappear in the F$_2$ generation we may be certain that we are dealing with cases of Mendelian inheritance.
3. The Principle of Segregation.—The individuality of inheritance units, and their segregation or separation in the sex cells and recombination in the zygote are fundamental principles of the Mendelian doctrine. ​Indeed, the evidence for the individuality and continuity of inheritance units is based entirely upon such segregation and recombination, so that the entire Mendelian theory may be said to rest upon the principle of segregation. If there are cases in which such segregation does not take place they belong to other forms of inheritance than the Mendelian: if segregation occurs in every instance there is no other type of inheritance than that discovered by Mendel. Are there cases which do not segregate according to Mendelian expectation?
Fig. 64. Diagram of Inheritance of Color-blindness through Female. A color-blind female transmits her defect to all her sons, to half of her granddaughters and to half of her grandsons. Corresponding distribution of sex chromosomes on right. (After Morgan.)
When the Mendelian theory was new it was generally supposed that there were forms of inheritance which differed materially from the Mendelian type; indeed, it was supposed that the latter was one of the less common forms of heredity and that blending of parental traits and not segregation was the rule. All cases in which the characters of the parents appeared to blend in the offspring, or in which there was not a clear segregation of the parental types in the F$_2$ generation or in which the ratio for a monohybrid differed from the well known 3:1 ratio, were supposed to be non-Mendelian.
However, further work has shown that some of these are really Mendelian. Sometimes offspring are intermediate between their parents owing to incompleteness of dominance, rather than to incompleteness of segregation; in such cases the parental types reappear in the F$_2$ generation as in the cross between red and white four-o'clocks. Sometimes departures from the 3:1 ratio are caused by the fact that two or more factors of the same sort are involved in the production of a single character. Nilsson-Ehle found that when oats with black glumes were ​crossed with varieties having white glumes the ratio of 3 white to 1 black was usually found in the second generation; but one variety of black oats when crossed with white gave in the second generation approximately 15 blacks to 1 white, which is the dihybrid ratio. From this and other evidence he concludes that in this variety of oats two hereditarily separable factors are involved in the production of black. In crosses between red-grained and white-grained wheat he usually got in the second generation the monohybrid ratio of 3 red: 1 white, but three strains gave the dihybrid ratio of 15 : 1 and two gave the trihybrid ratio of 63 : 1. Consequently he concludes that while the red color of wheat grains is usually due to one factor for red, it may in some cases be due to two or even three factors; notable departures from expected ratios may thus be explained.
Blending Inheritance
But the most serious objections which can be presented against the universality of the Mendelian doctrine are found in phenomena of "blending" inheritance. In some instances contrasting characters of parents appear to blend in offspring and even in the F$_2$ in subsequent generations the descendants remain more or less intermediate between the parents. One of the best known illustrations of this is found in the skin color of the mulatto, which is intermediate between the white parent and the black one, and even in the F$_2$ and in subsequent generations mulattoes do not usually, if ever, produce pure white or pure black children, though the children of mulattoes show considerable variation in color. Here there is an apparent failure of the Mendelian principle of segregation.
But white skin is not really white nor is black skin ever perfectly black. Davenport has shown that there is a mixture of black, yellow and red pigment in both white and black skins, though the amount of each of these pigments varies greatly in negroes and whites. A white person may have a skin color composed of black (b) 8 per cent., yellow (y) 9 per cent., red (r) 50 per cent., and absence of pigment or white (w) 33 per cent. On the other hand a very black negro may have b 68 per cent., y 2 per cent., r 26 per cent., w 4 per cent. The nine children of two mulattoes, the father having 13 per cent, of black and the mother 45 per cent., ranged all the way from 46 per cent, to 6 per cent, of black—the latter so far as skin color is concerned being virtually white. On the other hand, where both parents have about the same degree of pigmentation the children are more nearly uniform in color; thus seven children of two mulattoes, the father having 36 per cent, and the mother 30 per cent, of black, ranged only from 27 per cent, of 39 per cent, of black.
Such variations in color in the F$_2$ and in subsequent generations is ​exactly what one would expect in a Mendelian character in which more than one factor is involved, as, for example, in the case of the color of the sweet peas shown in Fig. 59. Davenport, who has made an extended study of this case concludes that "there are two double factors (AA BB) for black pigmentation in the full-blooded negro of the west coast of Africa and these are separably inheritable." These factors are lacking in white persons (this being indicated by aa bb). Since the germ cells carry only single factors and not double ones, the cross between negro and white would have only one set of these factors for black color, as shown by the formula $AB\times ab=ABab$; hence the color of the F$_1$ generation is intermediate between that of the two parents. In the F$_2$ generation there should be a variety of colors ranging all the way from white to black, though pure white or pure black would be expected in only a small proportion of the offspring. As a matter of fact it is known that the children of mulattoes vary considerably in
Fig. 65. Blending Inheritance of Size in Rabbits; the skulls of two parents are shown in 1 and 3, of their intermediate offspring in 2. (From Castle.)
color, and in some cases a child may be darker or lighter than either parent, which would indicate that segregation does actually occur. It is very probable that this classical case of "blending" inheritance is really Mendelian inheritance in which several factors for skin color are involved.
Similar blending inheritance is found in certain other cases where the parents differ in form or size. Thus Castle found that when long-eared rabbits were crossed with short-eared ones the offspring have ears of intermediate length, and in all subsequent generations the ear ​length remained intermediate between that of the parents. He found the same thing true of length and breadth of the skull (Fig. 65) and of the size of other portions of the skeleton, and he concluded that such quantitative characters are not inherited in Mendelian fashion.
Quite recently MacDowelL working on the inheritance of size in rabbits, concludes that this, as well as other quantitative differences between parents which appear to blend in the offspring, such as Castle's case of ear length in rabbits, is due not to a single factor, as in the case of Mendel's tall and dwarf peas, but to several factors. Consequently, in the formation of the germ cells there is not a clear segregation of all the factors for tallness, or large size or long ears, in half the germ cells, and their total absence in the other half of those cells, but some of these factors go into certain cells and others into others, as in the case of dihybrids, trihybrids or polyhybrids. As a result offspring appear more or less intermediate in size between their parents.
Thus it is possible to explain even "blending" inheritance as due not to the real fusion or blending of inheritance factors, but to varying combinations of numerous or multiple factors, according to the Mendelian rules. The Mendelian principle of segregation has been found to be of such general occurrence that there is a strong inclination among Mendelians of the stricter sort to make it universal, and to explain all cases of blending inheritance as due to incomplete dominance and to multiple factors. Whether or not such attempts may prove completely successful it is still too soon to say.
III. Mendelian Inheritance in Man
The study of human inheritance must always be less satisfactory and conclusions less secure than in the study of lower animals for the following reasons: In the first place there are no "pure lines," but the most complicated intermixture of different lines. In the second place experiments are out of the question and one must rely upon observation and statistics. There have been less than 60 generations of men since the beginning of the Christian era, whereas Jennings gets as many generations of Paramecium within two months and Morgan almost as many generations of Drosophila within two years. Finally the number of offspring are so few in man that it is difficult to determine what all the hereditary possibilities of a family may he. Bearing in mind these serious handicaps to an exact study of inheritance, it is not surprising that the method of inheritance of many human characters is still uncertain.
Davenport and Plate have catalogued more than sixty human traits which seem to be inherited in Mendelian fashion. About fifty of these represent pathological or teratological conditions, while only a relatively small number are normal characters. This does not signify that the ​method of inheritance differs in the case of normal and abnormal characters, but rather that abnormal characters are more striking, more easily followed from generation to generation, and consequently statististics are more complete with regard to them than in the case of normal characters. In many cases statistics are not sufficiently complete to determine with certainty whether the character in question is dominant or recessive, and it must be understood that in some instances the classification in this respect is tentative. A partial list of these characters is given herewith:
Mendelian Inheritance in Man
normal characters
| Dominant | Recessive |
| Hair: |||
| Curly. | Straight. |
| Dark. | Light to red. |
| Eye Color: |||
| Brown. | Blue. |
| Skin Color: |||
| Dark. | Light. |
| Normal pigmentation. | Albinism. |
| Countenance: |||
| Hapsburg type (thick lower lip and prominent chin). | Normal. |
| German type. | Jewish type. |
| Temperament: |||
| Nervous. | Phlegmatic. |
| Intellectual Capacity: |||
| Average. | Very great. |
| Average. | Very small. |
| TERATOLOGICAL AND PATHOLOGICAL CHARACTERS |||
| General Size: |||
| Achondroplasy (dwarfs with short stout limbs but with bodies and heads of normal size). | Normal. |
| Normal size. | True Dwarfs (with all parts of the body reduced in proportion). |
| Hands and Feet: |||
| Brachydactyly (short fingers and toes). | Normal. |
| Syndactyly (webbed fingers and toes). | Normal. |
| Polydactyly (supernumerary digits). | Normal. |
| Skin: |
| Keratosis (thickening of epidermis). | Normal. |
| Epidermolysis (excessive formation of blisters). | Normal. ​ |
| Hypotrichosis (hairlessness associated with lack of teeth). | Normal. |
| Kidneys: |||
| Diabetes insipidus. | Normal. |
| Diabetes mellitus. | Normal. |
| Normal. | Alkaptonuria (urine dark after oxidation). |
| Nervous System: |||
| Normal condition. | General neuropathy, e. g., |
| | Hereditary epilepsy. |
| | Hereditary feeblemindedness. |
| | Hereditary insanity. |
| | Hereditary alcoholism. |
| | Hereditary criminality. |
| | Hereditary hysteria. |
| Normal. | Multiple sclerosis (diffuse degeneration of nerve tissue). |
| Normal. | Friedrieh's disease (degeneration of upper part of spinal cord). |
| Normal. | Meniere's disease (dizziness and roaring in ears). |
| Normal. | Chorea (St. Vitus dance). |
| Huntington's chorea. | Normal. |
| Muscular atrophy. | Normal. |
| Normal. | Thomsen 's disease (lack of muscular tone). |
| Eyes: |||
| Hereditary cataract. | Normal. |
| Pigmentary degeneration of retina. | Normal. |
| Glaucoma (internal pressure and swelling of eyeball). | Normal. |
| Coloboma (open suture in iris). | Normal. |
| Displaced lens. | Normal. |
| Ears: |||
| Normal. | Deaf-mutism. |
| Normal. | Otosclerosis (thickened tympanum with hardness of hearing). |
| SEX-LINKED CHARACTERS |||
| Recessive characters, appearing in male when simplex, in female only when duplex. |||
| Normal. | Gower 's muscular atrophy. |
| Normal. | Hæmophilia (slow clotting of blood). |
| Normal. | Color-blindness (Daltonism; inability to distinguish red from green). |
| Normal. | Night blindness (inability to see by faint light). |
| Normal. | Neuritis optica (progressive atrophy of optic nerve). |
Summary
The principles of heredity established by Mendel are almost as important for biology as the atomic theory of Dalton is for chemistry. By means of these principles particular dissociations and recombinations of characters can be made with almost the same certainty as particular dissociations and recombinations of atoms can be made in chemical reactions. By means of these principles the hereditary constitution of organisms can be analyzed and the real resemblances and differences of various organisms determined. By means of these principles the once mysterious and apparently capricious phenomena of prepotency, atavism and reversion, find a satisfactory explanation.
Before the establishment of Mendel's principles, heredity was, as Balzac said, "a maze in which science loses itself." Much still remains to be discovered about inheritance, but the principles of Mendel have served as an Ariadne thread to guide science through this maze of apparent contradictions and exceptions in which it was formerly lost.