Mendelian inheritance (or Mendelian genetics or Mendelism) is a set of primary tenets relating to the transmission of hereditary characteristics from parent organisms to their offspring; it underlies much of genetics. They were initially derived from the work of Gregor Mendel published in 1865 and 1866 which was "re-discovered" in 1900, and were initially very controversial. When they were integrated with the chromosome theory of inheritance by Thomas Hunt Morgan in 1915, they became the core of classical genetics.
The laws of inheritance were derived by Johann Gregor Mendel, a 19th century  monk conducting hybridization experiments in garden peas (Pisum sativum). Between 1856 and 1863, he cultivated and tested some 29,000 pea plants. From these experiments he deduced two generalizations which later became known as Mendel's Laws of Heredity or Mendelian inheritance. He described these laws in a two part paper, Experiments on Plant Hybridization that he read to the Natural History Society of Brno on February 8 and March 8, 1865, and which was published in 1866.
Mendel's conclusions were largely ignored. Although they were not completely unknown to biologists of the time, they were not seen as generally applicable, even by Mendel himself, who thought they only applied to certain categories of species or traits. A major block to understanding their significance was the importance attached by 19th Century biologists to the apparent blending of inherited traits in the overall appearance of the progeny, now known to be due to multigene interactions, in contrast to the organ-specific binary characters studied by Mendel. In 1900, however, his work was "re-discovered" by three European scientists, Hugo de Vries, Carl Correns, and Erich von Tschermak. The exact nature of the "re-discovery" has been somewhat debated: De Vries published first on the subject, mentioning Mendel in a footnote, while Correns pointed out Mendel's priority after having read De Vries's paper and realizing that he himself did not have priority. De Vries may not have acknowledged truthfully how much of his knowledge of the laws came from his own work, or came only after reading Mendel's paper. Later scholars have accused Von Tschermak of not truly understanding the results at all.
Regardless, the "re-discovery" made Mendelism an important but controversial theory. Its most vigorous promoter in Europe was William Bateson, who coined the term "genetics", "gene", and "allele" to describe many of its tenets. The model of heredity was highly contested by other biologists because it implied that heredity was discontinuous, in opposition to the apparently continuous variation observable for many traits. Many biologists also dismissed the theory because they were not sure it would apply to all species, and there seemed to be very few true Mendelian characters in nature. However later work by biologists and statisticians such as R.A. Fisher showed that if multiple Mendelian factors were involved in the expression of an individual trait, they could produce the diverse results observed. Thomas Hunt Morgan and his assistants later integrated the theoretical model of Mendel with the chromosome theory of inheritance, in which the chromosomes of cells were thought to hold the actual hereditary material, and create what is now known as classical genetics, which was extremely successful and cemented Mendel's place in history.
Mendel's findings allowed other scientists to predict the expression of traits on the basis of mathematical probabilities. A large contribution to Mendel's success can be traced to his decision to start his crosses only with plants he demonstrated were true-breeding. He also only measured absolute (binary) characteristics, such as color, shape, and position of the offspring, rather than quantitative characteristics. He expressed his results numerically and subjected them to statistical analysis. His method of data analysis and his large sample size gave credibility to his data. He also had the foresight to follow several successive generations (f2, f3) of his pea plants and record their variations. Finally, he performed "test crosses" (back-crossing descendants of the initial hybridization to the initial true-breeding lines) to reveal the presence and proportion of recessive characters. Without his hard work and careful attention to procedure and detail, Mendel's work could not have had the impact it made on the world of genetics.
The principles of heredity were written by the Austrian monk Gregor Mendel in 1865. Mendel discovered that by crossing white flower and purple flower plants, the result was not a blend. Rather than being a mix of the two, the offspring was purple flowered. He then conceived the idea of heredity units, which he called "factors", one of which is a recessive characteristic and the other dominant. Mendel said that factors, later called genes, normally occur in pairs in ordinary body cells, yet segregate during the formation of sex cells. Each member of the pair becomes part of the separate sex cell. The dominant gene, such as the purple flower in Mendel's plants, will hide the recessive gene, the white flower. After Mendel self-fertilized the F1 generation and obtained the 3:1 ratio, he correctly theorized that genes can be paired in three different ways for each trait: AA, aa, and Aa. The capital "A" represents the dominant factor and lowercase "a" represents the recessive. (The last combination listed above, Aa, will occur roughly twice as often as each of the other two, as it can be made in two different ways, Aa or aA.)
Mendel stated that each individual has two factors for each trait, one from each parent. The two factors may or may not contain the same information. If the two factors are identical, the individual is called homozygous for the trait. If the two factors have different information, the individual is called heterozygous. The alternative forms of a factor are called alleles. The genotype of an individual is made up of the many alleles it possesses. An individual's physical appearance, or phenotype, is determined by its alleles as well as by its environment. An individual possesses two alleles for each trait; one allele is given by the female parent and the other by the male parent. They are passed on when an individual matures and produces gametes: egg and sperm. When gametes form, the paired alleles separate randomly so that each gamete receives a copy of one of the two alleles. The presence of an allele doesn't promise that the trait will be expressed in the individual that possesses it. In heterozygous individuals the only allele that is expressed is the dominant. The recessive allele is present but its expression is hidden.
Mendel summarized his findings in two laws; the Law of Segregation and the Law of Independent Assortment.
The Law of Segregation states that when any individual produces gametes, the copies of a gene separate, so that each gamete receives only one copy. A gamete will receive one allele or the other. The direct proof of this was later found when the process of meiosis came to be known. In meiosis the paternal and maternal chromosomes get separated and the alleles with the characters are segregated into two different gametes.
The Law of Independent Assortment, also known as "Inheritance Law", states that alleles of different genes assort independently of one another during gamete formation. While Mendel's experiments with mixing one trait always resulted in a 3:1 ratio (Fig. 1) between dominant and recessive phenotypes, his experiments with mixing two traits (dihybrid cross) showed 9:3:3:1 ratios (Fig. 2). But the 9:3:3:1 table shows that each of the two genes are independently inherited with a 3:1 ratio. Mendel concluded that different traits are inherited independently of each other, so that there is no relation, for example, between a cat's color and tail length. This is actually only true for genes that are not linked to each other.
Independent assortment occurs during meiosis I in eukaryotic organisms, specifically metaphase I of meiosis, to produce a gamete with a mixture of the organism's maternal and paternal chromosomes. Along with chromosomal crossover, this process aids in increasing genetic diversity by producing novel genetic combinations.
Of the 46 chromosomes in a normal diploid human cell, half are maternally-derived (from the mother's egg) and half are paternally-derived (from the father's sperm). This occurs as sexual reproduction involves the fusion of two haploid gametes (the egg and sperm) to produce a new organism having the full complement of chromosomes. During gametogenesis - the production of new gametes by an adult - the normal complement of 46 chromosomes needs to be halved to 23 to ensure that the resulting haploid gamete can join with another gamete to produce a diploid organism. An error in the number of chromosomes, such as those caused by a diploid gamete joining with a haploid gamete, is termed aneuploidy.
In independent assortment the chromosomes that end up in a newly-formed gamete are randomly sorted from all possible combinations of maternal and paternal chromosomes. Because gametes end up with a random mix instead of a pre-defined "set" from either parent, gametes are therefore considered assorted independently. As such, the gamete can end up with any combination of paternal or maternal chromosomes. Any of the possible combinations of gametes formed from maternal and paternal chromosomes will occur with equal frequency. For human gametes, with 23 pairs of chromosomes, the number of possibilities is 223 or 8,388,608 possible combinations. The gametes will normally end up with 23 chromosomes, but the origin of any particular one will be randomly selected from paternal or maternal chromosomes. This contributes to the genetic variability of progeny.
The reason for these laws is found in the nature of the cell nucleus. It is made up of several chromosomes carrying the genetic traits. In a normal cell, each of these chromosomes has two parts, the chromatids. A reproductive cell, which is created in a process called meiosis, usually contains only one of those chromatids of each chromosome. By merging two of these cells (usually one male and one female), the full set is restored and the genes are mixed. The resulting cell becomes a new embryo. The fact that this new life has half the genes of each parent (23 from mother, 23 from father for total of 46) is one reason for the Mendelian laws. The second most important reason is the varying dominance of different genes, causing some traits to appear unevenly instead of averaging out (whereby dominant doesn't mean more likely to reproduce - recessive genes can become the most common, too).
There are several advantages of this method (sexual reproduction) over reproduction without genetic exchange:
A Mendelian trait is one that is controlled by a single locus and shows a simple Mendelian inheritance pattern. In such cases, a mutation in a single gene can cause a disease that is inherited according to Mendel's laws. Examples include sickle-cell anemia, Tay-Sachs disease, cystic fibrosis and xeroderma pigmentosa. A disease controlled by a single gene contrasts with a multi-factorial disease, like arthritis, which is affected by several loci (and the environment) as well as those diseases inherited in a non-Mendelian fashion. The Mendelian Inheritance in Man database is a catalog of, among other things, genes in which Mendelian traits causes disease.
MENDELISM. To define what some biologists call Mendelism briefly is not possible. Within recent years there has come to biologists a new idea of the nature of living things,. a new conception of their potentialities and of their limitations; and for this we are primarily indebted to the work of Gregor Mendel. Peasant boy, monk, and abbot of Briinn, this remarkable man at one time interested himself in the workings of heredity, and the experiments devised by him and carried out in his cloister garden are to-day the foundation of that exact knowledge of the physiological process of heredity which biologists are rapidly extending in various directions. This extension. Mendel never saw. Born in 1822 he published the account of his experiments in 1865, but it was not until 1900, eighteen years after his death, that:biologists came to appreciate what he had accomplished. That year marked the simultaneous rediscovery of his work by three distinguished botanists: Hugo de Vries, C. Correns and E. Tschermak. Thenceforward Mendel's ideas have steadily gained ground, and, as the already strong body of evidence in their favour grows, they must come: to exert upon biological conceptions an influence not less than. those associated with the name of Darwin.
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Mendel chose the common pea. (Pisum sativum) as a subject for experiment, and investigated the effects of crossing different varieties. In his method he differed from previous investigators in concentrating his attention on the mode of inheritance of a single pair of alternative characters at a time. Thus on crossing a tall with a dwarf and paying attention to this pair of characters alone, he found that the hybrids (or F 1 generation) were all tall and that no intermediates appeared. Accordingly he termed the tall character dominant and the dwarf character recessive. On allowing these hybrids to fertilize themselves in the ordinary B. Absence of dominance, the heterozygote being more or less intermediate in form.
Black and white splashed plumage (Andalusian fowls). Lax and dense ears (wheat).
Six rowed and two rowed ears (barley).
The meaning of this phenomenon is at present obscure, and we can make no suggestion as to why it should be complete in one case, partial in another, and entirely absent in a third. When found it is as a rule definite and orderly, but there are cases known where irregularity exists. The extra toe characteristic of certain breeds of fowls, such as Dorkings, behaves generally as a dominant character, but in certain cases it has been ascertained that a fowl without an extra toe may yet carry the extra toe character. It is possible that in some cases dominance may be conditioned by the presence of other features, and certain crosses in sheep lend colour to the supposition that sex may be such a feature. A cross between the polled Suffolk and the horned Dorset breeds results in horned rams and polled ewes only, though in the F2 generation both sexes appear with and without horns. At present the simplest hypothesis which fits the facts is that horns are dominant in the male and recessive in the female. It is important not to confuse cases of apparent reversal of dominance such as the above with cases in which a given visible character may be the result of two entirely different causes. One white hen may give only colour chicks by a coloured cock, whilst the same cock with another white hen, indistinguishable in appearance from the former, will give only white chickens containing a few dark ticks. There is here no reversal of dominance, but, as has been abundantly proved by experiment, there are two entirely distinct classes of white fowls, of which one is dominant and the other recessive to colour.
Whether the phenomenon of dominance occur or not, the unit-characters exist in pairs, of which the members are seemingly interchangeable. In virtue of this behaviour the unit-characters forming such a pair have been termed allelomorphic to one another, and the question arises as to what is the nature of the relation between two allelomorphs. The fact that such cases of heredity as have been fully worked out can all be formulated in terms of allelomorphic pairs is suggestive, and has led to what may be called the "presence and absence" hypothesis. An allelomorphic pair represents the only two possible states of any given unit-character in its relation to the gamete, viz. its presence or its absence. When the unit-character is present the quality for which it stands is manifested in the zygote: when it is absent some other quality previously concealed is able to appear. When the unit-character for yellowness is present in a pea the seeds are yellow, when it is absent the seeds are green. The green character is underlying in all yellow seeds, but can only appear in the absence of the unit-character for yellowness, and greenness is allelomorphic to yellowness because it is the expression of absence of yellowness.
The instances hitherto considered are all simple cases in which the individuals crossed differ only in one pair of unit-characters. Mendel himself worked out cases in which the parents differed in more than one allelomorphic pair, and he pointed out that the principles involved were capable of indefinite extension. The inheritance of the various allelomorphic pairs is to be regarded as entirely independent. For example, when two individuals AA and aa are crossed the composition of the F2 generation must be AA + 2Aa +aa. If we suppose that the two parents differ also in the allelomorphic pair B - b, the composition of the F2 generation for this pair will be BB + 2Bb -}- bb. Hence of the zygotes which are homozygous for AA one quarter will carry also BB, one quarter bb, and one half Bb. And similarly for the zygotes which carry Aa or aa. The various combinations possible together with the relative frequencies of their occurrence may be gathered from fig. 3. Of the 16 zygotes there are: 9 containing A and B 3 containing B but not A 3 „ A but not B I „ neither A nor B In a case of dihybridism the F 1 zygote must be heterozygous for the two allelomorphic pairs, i.e. must be of the constitution Aa Bb. It is obvious that such a result may be produced in two ways, either by the union of two gametes, Ab and aB, or of two gametes AB and ab. In the former case each parent must be homozygous for one dominant and one recessive character; in the latter case one parent must be homozygous for both the dominant and the other for both recessive characters. The results of a cross involving. dihybridism may be complicated in several ways by the reaction upon one another of the unit-characters belonging to the separate allelomorphic pairs, and it will be convenient to consider the various possibilities apart.
I. The simplest case is that in which the two allelomorphic pairs affect entirely distinct characters. In the pea tallness is dominant to dwarfness and yellow seeds are dominant to green. When a yellow tall is crossed with a green dwarf the F 1 generation consists entirely of tall yellows. Precisely the same result is obtained by crossing a tall green with a dwarf yellow. In either case all the four characters involved are visible in one or other of the parents. Of every 16 plants produced by the tall yellow F 1, 9 are tall yellows, 3 are tall greens, 3 are dwarf yellows, and i is a dwarf green. If we denote the tall and dwarf characters by A and a, and the yellow FIG. 4.
The four types of comb referred to in the text are shown here. All the drawings were made from male birds. In the hens the combs are smaller. All four types of comb are liable to a certain amount of minor variation, and the walnut especially so. The presence of minute bristles on its posterior portion, however, serves at once to distinguish it from any other comb.
and green seed characters by B and b respectively, then the constitution of the F2 generation can be readily gathered from fig. 3.
p E A
2. When the two allelomorphic pairs affect the same structure we may get the phenomenon of novelties appearing in F 1 and F2. Certain breeds of fowls have a "rose" and others a "pea" comb (fig. 4). On crossing the two a "walnut" comb results, and the offspring of such walnuts bred together consist of 9 walnuts, 3 roses, 3 peas, and z single comb in every 16 birds. This case may be brought into line with the scheme in fig. 3 if we consider the allelomorphic pairs concerned to Rose Fig. 3.
be rose (A) and absence of rose (a), and pea (B) and absence of pea (b). The zygotic constitution of a rose is therefore AAbb, and of a pea aaBB. A zygote containing both rose and pea is a walnut: a zygote containing neither rose nor pea is a single. The peculiar feature of such a case lies in the fact that absence of rose and absence of pea are the same thing, i.e. single; and this is doubtless owing to the fact that the characters rose and pea both affect the same structure, the comb.
3. Cases exist in which the characters due to one allelomorphic pair can only become manifest in the presence of a particular member of the other pair. If in fig. 3 the characters due to B - b can only manifest themselves in the presence of A, it is obvious that this can happen in twelve cases out of sixteen, but not in the remaining four, which are homozygous for aa. An example of this is to be found in the inheritance of coat colour in rabbits, rats and mice where the allelomorphic pairs concerned are wild grey colour (B) dominant to black (b) and pigmentation (A) dominant to albinism (a). Certain albinos (aaBB) crossed with blacks (AAbb) give only greys (AaBb), and when these are bred together they give 9 greys, 3 blacks and 4 albinos. Of the 4 albinos 3 carry the grey character and i does not, but in the absence of the pigmentation factor (A) this is not visible. The ratio 9: 3: 4 must be regarded as a 9: 3: 3: I ratio, in which the last two terms are visibly indistinguishable owing to the impossibility of telling by the eye whether an albino carries the character for grey or not.
4. The appearance of a zygotic character may depend upon the coexistence in the zygote of two unit-characters belonging to different allelomorphic pairs.. If in the scheme shown in fig. 3 the manifestation of a given character depends upon the simultaneous presence of A and B, it is obvious that 9 of the 16 zygotes will present this character, whilst the remaining 7 will be without it. This is shown graphically in fig. 5, where the 9 squares have been shaded and the 7 left plain. The sweet pea offers an example of this phenomenon. White sweet peas breed true to whiteness, but when certain strains of whites are crossed the offspring are all coloured. In the next generation (F 2) these F 1 plants give rise to 9 coloured and 7 whites in every 16 plants. Colour here is a compound character whose manifestation depends upon the co-existence of two factors in the zygote, and each of the original parents was homozygous for one of the two factors necessary to the production of colour. The ratio 9: 7 is in reality a 9: 3: 3: i ratio in which, owing to special conditions, the zygotes represented by the last three terms are indistinguishable from one another by the eye.
The phenomena of dihybridism, as illustrated by the four examples given above, have been worked out in many other cases for plants and animals. Emphasis must be laid upon the fact that, although the unit-characters belonging to two pairs may react upon one another in the zygote and affect its character, their inheritance is yet entirely independent. Neither grey nor black can appear in the rabbit unless the pigmentation factor is also present; nevertheless, gametic segregation of this pair of characters takes place in the normal way among albino rabbits, though its effects are never visible until a suitable cross is made. In cases of trihybridism the Mendelian ratio for the forms appearing in F2 is 27: 9 :9: 9 3: 3: 3: 1, i.e. 2 7 showing dominance of three characters, three groups of 9 each showing dominance of two characters, three groups of 3 each showing dominance of one character, and a single individual out of 64 which is homozygous for all three recessive characters. It is obvious that the system can be indefinitely extended to embrace any number of allelomorphic pairs.
Facts such as those just dealt with in connexion with certain cases of dihybridism throw an entirely new light upon the phenomenon known as reversion on crossing. This is now seen to consist in the meeting of factors which had in some way or other become separated in phylogeny. The albino rabbit when crossed with the black "reverts" to the wild grey colour, because each parent supplies one of the two factors upon which the manifestation of the wild colour depends. So also the wild purple sweet pea may come as a reversion on crossing two whites. In such cases the reversion appears in the F 1 generation, because the two factors upon which it depends are the dominants of their respective allelomorphic pairs. Where the reversion depends upon the simultaneous absence of two characters it cannot appear until the F2 generation. When fowls with rose and pea combs are crossed the reversionary single comb characteristic of the wild Gallus bankiva first appears in the F2 generation.
In certain cases the distribution of characters in heredity is complicated by the fact that particular unit-characters tend to become associated or coupled together during gametogenesis. In no case have we yet a complete explanation of the phenomenon, but in view of the important FIG. 6.
bearing which these facts must eventually have on our ideas of the gametogenic process an illustration may be given. The case in which two white sweet peas gave a coloured on crossing has already been described, and it was seen that the production of colour was dependent upon the meeting of two factors, of which one was brought in by each parent. If the allelomorphic pairs be denoted by C - c and R - r, then the zygotic constitution of the two parents must have been CCrr and ccRR respectively. The F 1 plant may be either purple or red, two characters which form an allelomorphic pair in which the former is dominant, and which may be denoted by the letters B - b. If B is brought in by one parent only the F1 plant will be heterozygous for all three allelomorphic pairs, and therefore of the constitution Cc Rr Bb. In the F2 generation the ratio of coloured to white must be 9 : 7, and of purple to red 3: 1; and experiment has shown that this generation is composed on the average of 27 purples, 9 reds and 28 whites out of every 64 plants. The exact composition of such a family may be gathered from the accompanying table (fig. 6). So far the case is perfectly smooth, and it is only on the introduction of another character that the phenomenon of partial coupling is witnessed. Two kinds of pollen grain occur in the sweet pea. In some plants they are oblong in shape, whilst in others they are round, the latter condition being recessive to the former. If the original white parents were homozygous for long and round respectively the F 1 purple must be heterozygous, and in the F2 generation, as experiment has shown, the ratio of longs to rounds for the whole family is 3: I. But among the purples there are about twelve longs to each round, the excess of longs here being balanced by the reds, where the proportion crB crb crb crB crb crb p - at sB B! FIG. 5.
aa bb bs is i long to about 3.5 rounds. There is partial coupling of long pollen with the purple colour and a complementary coupling of the red colour with round pollen. This result would be brought about if it were supposed that seven out of every eight purple gametes produced by the F 1 plant carried the long pollen character, and that seven out of every eight red gametes carried the round pollen character. The facts observed fit in with the supposition that the gametes are produced in series of sixteen, but how such result could be brought about is a question which for the present must remain open.
Instances of association between characters are known in which the connection is between the dominant member of one pair and the recessive of another. In many sweet peas the standard folds over towards the wings, and the flower is said to be hooded. This "hooding" behaves as a recessive towards the erect standard. Red sap colour is also recessive to purple. In families where purples and reds as well as erect and hooded standards occur it has been found, as might be expected, that erect standards are to hooded ones, and that purples are to reds as 3:r. Were the case one of simple dihybridism the F2 generation should be composed of 9 erect purples, 3 hooded purples, 3 erect reds and i hooded red in every 16. Actually it is composed of 8 erect purples, 4 hooded purples and 4 erect reds. The hood will not associate with the red, but occurs only on the purples. Cases like this are best interpreted on the assumption that during gametogenesis there is some form of repulsion between the members of the different pairs - in the present instance between the factor for purple and that for the erect standard - so that all the gametes which contain the purple factor are free from the factor for the erect standard. To the process involved in this assumption the term spurious allelomorphism has been applied.
On the existing evidence it is probable that the inheritance of sex runs upon the same determinate lines as that of other characters. Indeed, there occurs in the sweet pea what may be regarded as an instance of sex inheritance of the simplest kind. Most sweet peas are hermaphrodite, but some are found in which the anthers are sterile and the plants function only as females. This latter condition is recessive to the hermaphrodite one and segregates from it in the ordinary way. Most cases of sex inheritance, however, are complicated, and it is further possible that the phenomena may be of a different order in plants and animals. Instructive in this connexion are certain cases in which one of the characters of an allelomorphic pair may be coupled with a particular sex. The pale lacticolor variety of the currant moth (Abraxas grossulariata) is recessive to the normal form, and in families produced by heterozygous parents one quarter of the offspring are of the variety. Though the sexes occur in approximately equal numbers, all the lacticolor in such families are females; and the association of sex with a character exhibiting normal segregation is strongly suggestive of a similar process obtaining for sex also. Castle has worked out similar cases in other Lepidoptera and has put forward an hypothesis of sex inheritance on the basis of the Mendelian segregation of sex determinants. An ovum or spermatozoon can carry either the male or the female character, but it is essential to Castle's hypothesis that a male spermatozoon should fertilize only a female ovum and vice versa, and consequently on his view all zygotes are heterozygous in respect of sex. Whether any such gametic selection as that postulated by Castle occurs here or elsewhere must for the present remain unanswered. Little evidence exists for it at present, but the possibility of its occurrence should not be ignored.
More recently evidence has been brought forward by Bateson and others (3) which supports the view that the inheritance of sex is on Mendelian lines. The analysis of cases where there is a closer association between a Mendelian character and a particular sex has suggested that femaleness is here dominant to maleness, and that the latter sex is homozygous while the former is heterozygous.
Breeding experiments have established the conception of definite unit-characters existing in the cells of an organism: in the cell histology has demonstrated the existence of small definite bodies - the chromosomes. During gametogenesis there takes place what many histologists regard as a differentiating division of the chromosomes: at the same period occurs the segregation of the unit-characters. Is there a relation between the postulated unit-character and the visible chromosome, and if so what is this relation? The researches of E. B. Wilson and others have shown that in certain Hemiptera the character of sex is definitely associated with a particular chromosome. The males of Protenor possess thirteen chromosomes, and the qualitative division on gametogenesis results in the production of equal numbers of spermatozoa having six and seven chromosomes. The somatic number of chromosomes in the female is fourteen, and consequently all the mature ova have seven chromosomes. When a spermatozoon with seven chromosomes meets an ovum the resulting zygote has fourteen chromosomes and is a female; when a spermatozoon with six chromosomes meets an ovum the resulting zygote has thirteen chromosomes and is a male. In no other instance has any such definite relation been established, and in many cases at any rate it is certain that it could not be a simple one. The gametic number of chromosomes in wheat is eight, whereas the work of R. H. Biffen and others has shown that the number of unit-characters in this species is considerably greater. If therefore there exists a definite relation between the two it must be supposed that a chromosome can carry more than a single unit-character. It is not impossible that future work on gametic coupling may throw light upon the matter.
It has long been realized that the problems of heredity and variation are closely interwoven, and that whatever throws light upon the one may be expected to illuminate the other. Recent as has been the rise of the study of genetics, it has, nevertheless, profoundly influenced our views as to the nature of these phenomena. Heredity we now perceive to be a method of analysis, and the facts of heredity constitute a series of reactions which enable us to argue towards the constitution of living matter. And essential to any method of analysis is the recognition of the individuality of the individual. Constitutional differences of a radical nature may be concealed beneath apparent identity of external form. Purple sweet peas from the same pod, indistinguishable in appearance and of identical ancestry, may yet be fundamentally different in their constitution. From one may come purples, reds and whites, from another only purples and reds, from another purples and whites alone, whilst a fourth will breed true to purple. Any method of investigation which fails to take account of the radical differences in constitution which may underlie external similarity must necessarily be doomed to failure. Conversely, we realize to-day that individuals identical in constitution may yet have an entirely different ancestral history. From the cross between two fowls with rose and pea combs, each of irreproachable pedigree for generations, come single combs in the second generation, and these singles are precisely similar in their behaviour to singles bred from strains of unblemished ancestry. In the ancestry of the one is to be found no single over a long series of years, in the ancestry of the other nothing but singles occurred. The creature of given constitution may often be built up in many ways, but once formed it will behave like others of the same constitution. The one sure test of the constitution of a living thing lies in the nature of the gametes which it carries, and it is the analysis of these gametes which forms the province of heredity.
The clear cut and definite mode of transmission of characters first revealed by Mendel leads inevitably to the conception of a definite and clear-cut basis for those characters. Upon this structural basis, the unit-character, are grounded certain of the phenomena now termed variation. Varieties exist as such in virtue of differing in one or more unit-characters from what is conventionally termed the type; and since these unitcharacters must from their behaviour in transmission be regarded as discontinuous in their nature, it follows that the variation must be discontinuous also. A present tendency of thought is to regard the discontinuous variation or mutation as the material upon which natural selection works, and to consider that the process of evolution takes place by definite steps. Darwin's opposition to this view rested partly upon the idea that the discontinuous variation or sport would, from the rarity of its occurrence, be unable to maintain itself against the swamping effects of intercrossing with the normal form. Mendel's work has shown that this objection is not valid, and the precision of the mode of inheritance of the discontinuous variation leads us to inquire if the small or fluctuating variation can be shown to have an equally definite physiological basis before it is admitted to play any part in the production of species. Until this has been shown it is possible to consider the discontinuous variation as the unit in all evolutionary change, and to regard the fluctuating variation as the uninherited effect of environmental accident.
In conclusion we may briefly allude to certain practical aspects of Mendel's discovery. Increased knowledge of heredity means increased power of control over the living thing, and as we come to understand more and more the architecture of the plant or animal we realize what can and what cannot be done towards modification or improvement. The experiments of Biffen on the cereals have demonstrated what may be done with our present knowledge in establishing new, stable and more profitable varieties of wheat and barley, and it is impossible to doubt that as this knowledge becomes more widely disseminated it will lead to considerable improvements in the methods of breeding animals and plants.
It is not, however, in the economic field, important as this may be, that Mendel's discovery is likely to have most meaning for us: rather it is in the new light in which man will come to view himself and his fellow creatures. To-day we are almost entirely ignorant of the unit-characters that go to make the difference between one man and another. A few diseases, such as alcaptonuria and congenital cataract, a digital malformation, and probably eye colour, are as yet the only cases in which inheritance has been shown to run upon Mendelian lines. The complexity of the subject must render investigation at once difficult and slow; but the little that we know to-day offers the hope of a great extension in our knowledge at no very distant time. If this hope is borne out, if it is shown that the qualities of man, his body and his intellect, his immunities and his diseases, even his very virtues and vices, are dependent upon the ascertainable presence or absence of definite unit-characters whose mode of transmission follows fixed laws, and if also man decides that his life shall be ordered in the light of this knowledge, it is obvious that the social system will have to undergo considerable changes.
Bibliography. - In the following short list are given the titles of papers dealing with experiments directly referred to in this article. References to most of the literature will be found in (I I), and a complete list to the date of publication in (3).
(I) W. Bateson, Mendel's Principles of Heredity (Cambridge, 1902), contains translation of Mendel's paper. (2) W. Bateson, An Address on Mendelian Heredity and its Application to Man, "Brain," pt. cxiv. (1906). (3) W. Bateson, Mendel's Principles of Heredity (1909). (4) R. H. Biffen, "Mendel's Laws of Inheritance and Wheat Breedings," Journ. Agr. Soc., vol. i. (1905) (5) W. E. Castle, "The Heredity of Sex," Bull. Mus. Comp. Zool. (Harvard, 1903). (6) L. Cuenot, "L'Heredite de la pigmentation chez les souris," Arch. Zool. Exp. (1903-1904). (7) H. de Vries, Die Mutationstheorie (Leipzig, 1901-1903). (8) L. Doncaster and G. H. Raynor, "Breeding Experiments with Lepidoptera," Proc. Zool. Soc. (London, 1906). (9) C. C. Hurst, "Experimental Studies on Heredity in Rabbits," Journ. Linn. Soc. (1905). (10) G. J. Mendel, Versuche fiber Pflanzen-Hybriden, Verh. natur. f. ver. in Briinn, Bd. IV. (1865). (II) Reports to the Evolution Committee of the Royal Society, vols. i. - iii. (London, 1902-1906, experiments by W. Bateson, E. R. Saunders, R. C. Punnett, C. C. Hurst and others). (12) E. B. Wilson, "Studies in Chromosomes," vols. i. - iii. Journ. Exp. Zool. (1905-1906). (13) T. B. Wood, "Note on the Inheritance of Horns and Face Colour in Sheep," Journ. Agr. Soc. vol. i. (1905). (R. C. P.)