GENE. The word gene (Gr. -yivos, race) has come to be used, more particularly in the United States, as the equivalent of the word gen. The latter was first coined by Johannsen (1909) to signify the unit in inheritance. As a prefix, gen appears in such words as genetic and genealogical, but used by itself it stands for a purely hypothetical unit in inheritance. In England the word "factor" is more frequently used, but factor has also a wider significance ; gene is equivalent to genetic factor. The word is used almost exclusively in Mendelian literature, and has come to have a more specific application to the elementary unit carried by chromosomes. The functional significance of the gene in de velopment, in determining the character of the individual, is the subject at present of much speculation, and diverse views have been expressed as to how the genes are related to the characters for which they are, in a sense, the sponsors in the germinal ma terial.
The assumption of fundamental units in heredity is a familiar procedure in biological literature. Herbert Spencer postulated physiological units out of which the body is built, somewhat as are crystals from molecules of a salt in solution. Darwin's theory of pangensis called for "gemmules," supposed to be specific par ticles set free from all parts of the body, which uniting with similar particles in the eggs and sperm-cells render the inheritance of acquired characters possible. Haeckel used the word "plasti dule" for the ultimate particles of the protoplasm. De Vries postu lated intracellular pangenes, which, being set free from the nu cleus, determine the character of the cell but do not pass beyond the cell boundaries. Weismann invented an elaborate system of hierarchies of elements consisting of biophors (the simplest) determinants, ids and idants (chromosomes). The determinants were supposed to be sorted out during the earlier embryonic cell-divisions in an orderly fashion, each cell finally getting a particular kind that determined its functional behaviour. None of Weismann's units were supposed to pass beyond the limits of the cell. In all these earlier hypotheses the units were purely fictitious in the sense that they were devised to carry out certain imaginary processes. To the units were assigned arbitrarily such properties as the particular end in view required.
The idea of units in Mendelian work has a different derivation. The results of experimental breeding have shown that when cer tain contrasted characters are brought together in a hybrid (Fi), the two original characters appear in the next generation in definite proportions. Mendel pointed out that these numerical relations could be understood if for any one pair it is assumed that the reproductive cells of the first two parents that were crossed each introduced into the hybrid an element representative of the character in question. He further assumed that these ele ments are sorted out in the germ-cells of the hybrid (without having contaminated each other) in such a way that each germ cell (egg or sperm) carries one or the other of these elements. Chance fertilization of any egg by any sperm will give the numer ical ratios found in the next generation. No special functions or characteristics are assigned to these elements (genes) other than their power to divide, or at least to reproduce themselves and each to retain in the hybrid its specificity. All later work in genetics rests on these Mendelian postulates, but in the further develop ment of the subject evidence has been found that makes it pos sible to locate the genes in the chromosomes and even to deter mine their relation there with respect to one another. For example, in the vinegar (or fruit) fly, Drosophila melanogaster, which has been more thoroughly examined than any other form, it has been found that the genes are inherited in great groups. They are said to be linked together. There are as many of these linkage groups as there are chromosome pairs. The members of one group usu ally remain united, but there may be occasional interchanges between the members of a group, always in an orderly fashion (crossing over). Such interchanges take place only between like linkage groups, i.e., between each group derived from the father with that from the mother—never between different linkage groups. Crossing over has made it possible to determine the se quence of the genes within each group, and to get a rough measure of their distances apart, on the assumption that this distance apart determines the chance of crossing over between them. The mapping of the genes on this assumption has made it possible to predict accurately the behaviour of the genes in any known situ ation.
The localization of the genes in the chromosomes (which are material bodies easily visible under the microscope) has undoubt edly led to the conception of the genes as material particles ly ing in a single line in each chromosome. Whether they are so conceived or not does not affect the theory that the characters of the individual have representatives in the chromosomes that permit formulation of definite laws of heredity.
There is some further evidence concerning the relation of the genes of characters that is significant. While it is true that our knowledge concerning the gene has been determined by following the reappearance of single contrasted pairs of characters from one generation to the next, nevertheless there is abundant evidence that each gene may affect many characters at the same time ; a study of any pair of which will lead to the same result. The genes then are thought of as fundamentally different from Weis mann's determinants. In fact, there is reason to infer that every gene contributes to every part of the body, affecting some parts more than others, and the latter are only those that are picked out for convenience in studying heredity. As a corollary to this view there is the further inference, based on sufficient evidence, that the character of the individual is the result of a definite balance (or interaction) between the activities of the genes. If this balance is changed the end result is affected. This is most evident in the case of sex determination where the female is the result of one kind of balance and the male of another in animals and plants with separate sexes. Alterations in this balance, if not too extreme, may give rise to intersexes that stand in certain respects between the typical males and females. The balance also carries with it the tacit assumption of a normal environment. If the environment is altered the end result may be different. Thus in certain hermaphroditic plants, and perhaps also in animals, it is possible to suppress the development of one set of sexual or gans by changing the outside influences (light, temperature, etc.). It is more questionable whether in the higher animals with sepa rate sexes it is possible to change one sex into the other by alter ation in the environment, although it has been demonstrated that by changing the internal environment (especially by altering the relation of the endocrine substances produced by the gonad or other glands having internal secretions), it is possible to change the secondary sexual organs of one sex into those of the other.
(T. H. M.)