HEREDITY IN PLANTS Flower Colour.—Flower colour has been more extensively studied than any other character. Thus in Primula sinensis, in which more genes are known than in any other flower (28 in all), 11 affect flower colour, while 3 others determine the size of the "eye" or central coloured patch, the remainder affecting struc tural characters, such as leaf shape and habit of the plant. Many of the genes affect several parts. Thus the same gene causes crimp ing of the leaves and petals, and the light-coloured flowers due to several recessive genes are associated with light stems and leaves.
Flower colour may be due to red or blue anthocyanin in the sap, to yellow pigment in the plastids, or to both. The combi nation is generally purple or orange. Some plants, e.g., the prim rose, have no anthocyanin, others, such as Primula sinensis, no yellow pigment, while many, e.g., the rose, tulip and stock, have both. Usually each pigment is governed by a separate set of genes. Loss of the principal gene or genes determining anthocyanin formation gives a yellow plant if plastid pigments are present, otherwise a white. Similarly loss of the principal gene for yellow may convert a bronze into a red. In addition either the anthocyanin or the plastids may be whitened by a dominant gene. A gene may alter the colour of the anthocyanin either by altering the reaction of the cell-sap (for blue antho cyanin is generally reddened by acid) or by altering its molec ular structure. The genes which cause large changes in flower colour (e.g., turn a white into a coloured plant) often affect leaf and stem colour too. They may also affect structural characters. Thus whiteness in stocks may be due to the loss of either of two genes. A white stock cannot have hairy leaves. The hairiness or otherwise of the coloured plants is determined by two further dominant genes.
Maize has been very thoroughly analysed, and the inheritance of many economically important characters is under stood. Thus the sugary endosperm of the sweet corn is due to a recessive gene, and another recessive gene determines waxy endosperm. But most cultivated wheats and oats are hexaploids, though a few are tetraploids. They are moreover allohexaploids. Hence while certain characters behave in a Mendelian manner, the same character may be due to a gene in either of the three sets of chromosomes, and will therefore exhibit different linkages in different races. Moreover, as usual in allopolyploids, characters which behave as recessives in F, do not always reappear in
The characters known to be Mendelian are not in general those of the greatest economic importance. Resistance to disease varies in its inheritance with the species of rust or mildew concerned. Thus resistance to yellow rust, Puccinia glumarum, is recessive, but that to brown rust, Puccinia triticina, and mildew, Erisyphe graminis, are dominant. But the matter is greatly complicated by the fact that a wheat which is immune in one environment may be attacked in another, owing to the existence of different races of the same parasite species.
Among the structural and physiological char acters which have been found to obey Mendel's laws are size, habit, leaf shape, flower shape, time of flowering, heterostylism, sterility of either male or female organs, hairiness, and a variety of ab normalities. An occasional complication of plant genetics is anisogeny, i.e., the ovules and pollen grains are of different genetical composition. This may be due to differences in the cytoplasm or plastids which are maternally inherited, to only half the pollen grains being functional or, in dioecious plants to sex-linkage. Thus in the stock, Matthioln incana, a race exists which, though single, gives slightly more than 50% of double flowered plants when self-fertilized. When used as a female with a normal single, half the
produce doubles; when used as a male all the
do so. But doubleness now behaves as an ordinary recessive. This is generally explained on the view that the domi nant gene S for singleness is closely linked with a gene p which prevents the proper functioning of pollen grains bearing it. The ever-sporting type is –. All its functional pollen is sP, so that SP when selfed, about half the seeds are
and give doubles, the sp other half perpetuating the parental type. The gene inhibiting plastid colour is carried in the same chromosome.
In Protista, including Bacteria and certain algae, any cell can reproduce, and there is no clear distinction between growth and reproduction. Where reproduc tion is asexual we have a condition parallel to that of a clone in the higher plants, and variations are rather feebly inherited. Nevertheless selection has sometimes been at least temporarily effective within such a clone. When sexual reproduction occurs, there, is often a marked outbreak of variation. Acquired char acters, such as those produced by certain poisons in trypano somes, are inherited at least for a considerable period but are generally lost on sexual reproduction, and sometimes without it. Adaptive characters acquired by a strain of micro-organisms, e.g., a capacity for fermenting sugars not usually attacked, may be genuine adaptations inherited, but are sometimes believed to be due to the selection of mutants.
When two species with different chromosome numbers are successfully crossed the reduction divisions of the hybrid are generally irregular, and it is nearly or quite sterile. Occasionally in such cases unreduced gametes may be formed, in which case there is no segregation of characters, and allopoly ploidy may result in later generations. When however the chromo some numbers are equal, the normal segregation mechanism may be able to function, though it does not always do so. In this case the
generation, and the results of back-crossing
to either parent, are usually polymorphic, and may include mon strous or more or less inviable forms. In certain cases, owing to the inviability of most gametic and zygotic combinations, the F2 consists entirely of types resembling one or other parent fairly closely. Sometimes however a partial Mendelian analysis is possible. For example from the cross between the primrose, Primula acaulis, and the blue Asiatic primrose, Primula Juliae, it appears that the former possesses a gene for yellow pigment, the latter for anthocyanin, so that whites appear in F2. If the oxlip, P. elatior, is crossed with Juliae it is found to carry a gene inhibit ing anthocyanin formation and one for the umbellate habit as well as those of acaulis. At least two genes are concerned in determin ing hairiness in these crosses, one being linked with a colour gene. Besides genes, species may differ in cytoplasmic factors, as do Geranium striatum and G. Endressii, and the commonness of variegation as the result of species crosses suggests that the plas tids may also differ. Mendelian analysis has been carried some way in specific crosses of the moths of the sub-family Bistoninae, and some other animals, but it is complicated not only by sterility and upsets of sex (q.v.), but by the effects of heterosis, which cause increase in size and vigour.
Where species cannot be crossed their genetics and chromosomal architecture can be compared; thus the Norway rat, Rattus nor vegicus, has three linked autosomal genes, C, R and P whose loss causes albinism, red-eyed yellowness and pink-eyed yellowness respectively. C and P have also been lost in different mouse races, and are linked with somewhat greater intensity, R has been lost in the black rat Rattus rattus, C and R in the Californian deer-mouse, Peromyscus maniculatus, where they are also linked. Clearly the architecture of the germ-plasm is similar in these species. On the other hand the arrangement of the genes in the chromosomes differs appreciably in different species of Drosophila, and sectors of a chromosome have been reversed in certain geographical races of D. melanogaster. In the rodents certain species and varieties differ by colour genes which are multiple allelomorphs of those producing well marked changes such as yellowness ; that is to say that these particular genes differ less between species than domes tic varieties.
Early thought on this subject was mainly speculative, and has left very little mark on modern theories. On the other hand a considerable empirical knowledge of the subject has long been current among practical breeders of animals, and to a less extent of plants. Among the principles clearly recognized, and formulated among others by C. Darwin in Variation of Ani mals and Plants under Domestication, were the facts that when two widely divergent varieties were crossed the
tended to resemble the wild form of the species (atavism, q.v.) and that in such a case, or a species cross, the F2 were very variable. Ata vism is a consequence of the fact that most variants are recessive to the wild type. Various theories were put forward in the 19th century, mostly based on the assumption that acquired characters were inherited. Thus Darwin supposed that particles were carried by the blood from adult organs to the gonads, and affected the hereditary characters transmitted.
In the late 19th century, apart from Mendel's work, two main lines of enquiry were pursued. A. Weissman (q.v.) and others showed that characters acquired by the adult are not in general transmitted to the offspring (see LAMARCKISM). It is of course possible that this occurs in exceptional cases, or so slowly that the process is only effective on a geological time-scale. He stressed the continuity of the germ-plasm from one generation to another, and developed a theory of heredity by the chromosomes which has since been abandoned, but which contained the essential idea that inheritance was not of characters, but of determinants which caused them to appear under the right conditions. F. Galton began an enquiry into heredity by statistical methods which enabled him to formulate a law of ancestral heredity which, in a modified form, holds good for large mixed populations. His work on human heredity was particularly important, and he was able to demon strate the relative importance of heredity and environment by a study of identical twins. The statistical method was later de veloped by K. Pearson and his colleagues into an instrument of research which is indispensable in the investigation of quantitative characters. H. de Vries began an investigation of heredity on Oenothera and its mutant derivatives and stressed the importance of unit characters, i.e., the somatic expressions of single genes or groups of linked genes.
In 190o G. Mendel's paper on inheritance in peas, published in 1865, was rediscovered by de Vries, Correns and Tschermak, who were able to illustrate the principles there enun ciated from other plants. The core of Mendel's discovery was that the factors determining hereditary characters segregate into the gametes of a hybrid according to definite and quantitative laws. In the next few years W. M. Bateson and his pupils (Saun ders, Punnett, Hurst, Doncaster, Durham, Gregory, Marryat, Wheldale and Sollas) published a mass of work extending Men delism to other plants, and to man, other mammals, birds and insects. In the course of this work Doncaster, Durham and Marryat discovered sex-linkage in the moth, Abraxas grossulariata, and the canary, while Bateson, Saunders and Punnett discovered linkage between genes. About the same time Cuenot discovered multiple allelomorphism in the mouse. A large amount of detailed Mendelian work on a variety of organisms was carried out by Correns, Lang, Tschermak and others in Germany, and Castle and Davenport in America. Biffen began the Mendelian analysis of cereals, later carried on by Engledow, Watkins and others, and Nettleship demonstrated the Mendelian inheritance of a number of human defects. This work met with determined opposition from the statistical or biometrical school of Pearson, who laid stress on deviations from Mendel's laws, now known to be due partly to differential viability of zygotes and gametes which distort the theoretical ratios, partly to the fact that varieties generally differ by a number of minor genes as well as those which are readily observed. The Mendelians retorted that the statistical method ignored genetical differences which exist between individ uals of the same phenotypical characters and could not account for segregation. Pearson and his school, however, continued to investi gate the inheritance of human physical and psychical characters with great success by statistical methods. Meanwhile Johanssen, working with beans, developed the concept of a pure line, which has been fundamental in all subsequent genetics. McClung, E. B. Wilson, T. H. Morgan and others discovered X and Y chromo somes, and R. R. Gates discovered the first case of a hereditary character associated with an abnormal chromosome number. Such was the position about 1910.
In that year Morgan put forward the view that sex-linked genes were carried by the X chromosome, and in 1913 his pupil Sturtevant enunciated the theory of the linear arrange ment of the genes in the chromosome. These theories, which had been foreshadowed by Correns and Sutton, have since been tested on a vast scale on Drosophila by Morgan and his school, especially Bridges, H. J. Muller and Sturtevant. Bridges in particular in vestigated the genetics of cytologically abnormal individuals, and Muller balanced lethals and the frequency of mutation. Thanks to this fundamental work the chromosome theory is now pretty generally accepted, although for some time Bateson and R. C. Punnett regarded linkage as due to differential multiplication of different types of cell. Among the more notable investigations of inheritance on orthodox Mendelian lines carried out in plants of late years are those of Nilsson-Ehle and Kajanus on inheritance in the polyploid cereals, of East, Emerson and Hayes on maize, Goodspeed and Clausen on tobacco, Baur on Antirrhinum, Gregory, de Winton and Bateson on Primula sinensis, of Punnett on the sweet pea, Andersson on ferns and Wettstein on mosses, where individual gametophytes can be studied. In animals special mention should be made of the work on various rodents by Castle and his colleagues, the rabbit and fowl by Punnett and his col leagues, the fowl by Dunn, Paratettix and Apotettix by Nabours, and on other Drosophilae by Metz and Lutz.
The special problems arising in autopolyploids were attacked by Gregory, Blakeslee and Winkler, while Winge, Heribert-Nils son, Newton and Pellew, Crane and Darlington dealt mainly with allopolyploids, and Lotsy with the effects of hybridization in general. Little, Crew and Mohr studied lethal genes in animals, and Heribert-Nilsson discovered certation between pollen grains. Bateson, Chittenden, Pellew and Correns studied non-Mendelian inheritance in plants, while Renner brought the genetics of Oeno thera into line with those of other organisms. Lehmann analysed sterility in Veronica, and East and his colleagues in Nicotiana.
On the physiological side Onslow studied the chemistry of pigmentation, R. Goldschmidt and J. S. Huxley the detailed effects of genes on growth-rates, and Goldschmidt and F. A. E. Crew the genetics of sexual abnormality (see SEx) while Brink began a study of the effect of genes on the physiology of the pollen grain, and Pearl of the genetics of longevity. Muller, W. H. Harrison and others showed that mutation could be produced by appropriate chemical (metallic salts in the food) or physical (X-rays) stimuli to the germ cells.
On the theoretical side R. A. Fisher showed that the results of Pearson and his colleagues followed from the assumption that such characters as stature were due to multiple genes acted on by natural selection, and thus healed the breach between the Men delian and biometric schools.
In the last part of the i9th century genetics was studied mainly with a view to elucidating the method of evolution. In the 2oth century, it has mainly been studied for its own sake and that of its practical applications. This independence has been of great advantage, but in the last few years the problem of evolution has been taken up again by geneticists. Among the outstanding work is that of Sumner on geographical races of deer-mice, and that of the Russian school. Vaviloff has studied the geographical distri bution of the cereals, Serebrovsky that of the poultry genes, and Tschetwerikoff the genetical nature of the wild populations of Drosophila.
The facts so far arrived at have had a considerable practical importance in the breeding of cereals, and some in horticulture. They are beginning to influence the breeders of live-stock, espe cially poultry, and attempts are being made to apply them to the human race (see EUGENICS).
F. Galton, Natural Inheritance (188q), Bibliography.-Classics: F. Galton, Natural Inheritance (188q), Hereditary Genius (1914) ; C. R. Darwin, The Variation of Animals and Plants under Domestication (ed. F. Darwin, Igor) ; W. Bateson, Mendel's Principles of Heredity (1909), Problems of Genetics (1913). General text books: E. W. Endicott and L. C. Dunn, Principles of Genetics (1925) ; T. H. Morgan, The Theory of the Gene (1926) ; R. C. Punnett, Mendetisnc (4th ed., 1927) ; E. B. Babcock and R. E. Clausen, Genetics in Relation to Agriculture (2nd ed., 1927) ; E. Baur and F. Hartmann, Handbuch der Vererbungswissenschaft (1927). Human Heredity: E. Baur, E. Fischer and F. Lenz, Grundriss der Menschlichen Erblichkeitslehre (Munich, 1923, etc.) ; F. A. E. Crew, Organic Inheritance in Man (1927) ; see also K. Pearson and A. Lee, "Laws of Inheritance in Man," in Biometrika (1903) and many articles throughout Biometrika. Animal Heredity: V. E. Castle, Genetics, Heredity and Eugenics (1916, 2nd ed., 1924) ; R. C. Punnett, Heredity in Poultry (1923) ; F. A. E. Crew, Animal Genetics, the Science of Animal Breeding (1925) . See also the journals Bibliotheca Genetica, ed. E. Baur (Leipzig), Bibliographia Genetica; Resumptio Genetica, consisting of abstracts of current work; The Journal of Genetics, Genetics, Journal of Heredity, Zeitschrift fur induktive Abstammungs and Vererbungslehre, Hereditas, Genetica, Archiv der Julius-Klaus Stiftung. Other Journals containing many genetic papers are: Biome trika, Annals of Eugenics, British Journal of Experimental Biology, The American Naturalist. (J. B. S. H.)
