INBREEDING is the term used to define any system of breeding which involves the mating of relatives. It has been de liberately used by animal breeders ever since the modern breeds of livestock were created : it is the sole method of reproduction of many self-fertilised plants, e.g., beans, barley, oats and peas, and it has been used extensively by plant breeders in order to "fix" varieties of certain naturally cross-fertilised species such as maize. Exceptionally it has been practised by man, as for example in the ancient royal families of Egypt. In spite of the evidence provided by experience, however, there has been and still is great diversity of opinion concerning the average results of this system of mating. It has been shown on the one hand that inbreeding in a population in which it is not the rule com monly leads to the production of offspring which exhibit a diminution in size, lowered powers of resistance and fertility, that in fact markedly defective types appear among the progeny of an inbred strain. On the other hand it is abundantly clear that inbreeding can and does promote uniformity of type amongst the individuals exposed to its action and to an increase in prepotency when an individual of an inbred line is mated with unrelated stocks. Among plants the self fertilised forms show no signs of being less vigorous than the normally cross-fertilised. It is not surprising therefore that a disagreement as to the effects and value of inbreeding exists. In different human societies there are to be found laws forbidding marriage save between relatives and laws forbidding the marriage of relatives altogether. Laws and practices vary because inbreeding has not always yielded the same results.
But during recent years it has been shown that the results of inbreeding, diverse as they are in different cases, can readily and consistently be explained by an appeal to the Mendelian theory. (See HEREDITY : Breeds and Breeding.) It has been demonstrated that the chief effect of inbreeding, the continued mating of close relatives, is to increase genetic purity or the proportion of homo zygous individuals or, conversely, to decrease heterozygosis among the progeny. To take an example; a recessive strain, such as that of dwarf peas, is crossed with a corresponding dominant strain, such as that of tall peas. All the first (filial) generation are hybrid (heterozygous) but show only the dominant character tallness. If these are allowed to become self-fertilised, 4 of the resulting generation are pure (homozygous) dwarfs, are heterozygous tails and a are homozygous tails. (See HEREDITY.) If all these plants are allowed in their turn to become self-fertilised the homozygous classes will continue to breed true while the heterozygous class will again break up into 25% homozygous dwarfs, 5o% heterozygous tails and 25% homozygous tails. If self-fertilisation of all individuals is continued generation after generation, the proportion of the heterozygous individuals be comes steadily reduced (being halved in each generation) and the proportion of the homozygous types steadily increases because these remain homozygous and are augmented in each self-fertilised generation. This progressive reduction in heterozygosis (the condition of being heterozygous or hybrid for one or more Mendelian characters) occurs automatically in all Mendelian factor-pairs, regardless of the number involved. The rate of reduction is determined by the intensity of the inbreeding practised. Reduction is most rapid in cases of self-fertilisation; theoretically it is calculated that for all practical purposes com plete homozygous (the condition of being homozygous for all characters the individual exhibits) should be attained in the loth generation in such cases. Reduction is less rapid in cases of continued brother and sister mating : it is estimated that starting with heterozygous individuals the proportion of heterozygosis after ten generations becomes reduced from 5o% to about 5%. This theoretical expectation is not likely to be realised in practice for the reason that the theory requires that mating shall be at random whereas in practice there is bound to be some certain selection, either natural or artificial, and this is more likely to preserve the heterozygous than the homozygous. The matings of relatives beyond the first cousin degree reduces heterozygosis so slightly they cannot really be considered as cases of inbreeding.
In breeding practice a system of mating commonly adopted is that of using a male upon a number of his half-sisters, these being half-sisters to each other. Such a system requires five generations to reduce the proportion of heterozygosis to about half the original value. Even after fifteen generations the proportion is and this system, therefore, is to be commended for all general purposes in cases in which a breeder wishes to avoid the dangers of too narrow inbreeding, to preserve the advantages of inbreeding and to keep wholly within his own stock. In the case of those breeds of livestock of which a study has been made, Shorthorn cattle, for example, it has been shown that the breeders who have contributed most notably to the advancement of the breeds have been those who used the system of inbreeding. They did so in most cases because they possessed males of such superior merit as sires that they wished to perpetuate and to concentrate the blood of these. Their methods earn the endorsement of science, for if is desired to fix a type, the best guarantee that the animals shall not only look alike but also be alike in hereditary constitution is close relationship.
Critical experimental analysis of the effects of inbreeding has shown that its action takes the form of isolating a number of types each homozygous for the characters it displays and differ ing one from the other in respect of hereditary constitution. If, as is often the case, some of the recessive factors carried in the original stock are deleterious in their effects, some of the homo zygous lines produced by inbreeding will be weakly or even in capable of life. Others, however, will have become purified of all harmful recessives and these will continue to flourish. But even these may show some decrease in general vigour, size and fertility ; by crossing the best of these inbred stocks the effects of hybrid vigour (see HYBRIDISM) can be brought into play and a purified stock of equal, or better, performance than the original will be produced.
In the case of man, as in that of his domesticated animals, consanguinity in marriage is not dangerous in itself, it is only dangerous for the reason that human stocks are heavily laden with undesirable recessives, hidden in the heterozygous hybrid. These are most frequently revealed in the children when closely related individuals marry. Inbreeding exerts its effects solely through the medium of inheritance and not through the blood relationship of the individuals concerned. Incest—inbreeding of the narrowest kind—is not harmful per se but only because of the recessives it may bring to light. It is usually more common among the biologically inferior : it is not surprising therefore that the offspring of such are themselves biologically unworthy, but this does not mean that incest itself is pernicious but only that it should be forbidden in the cases of unsound stock. There is no biological objection to the marriage of a man to his deceased wife's sister; there is some endorsement for the social attitude concerning the marriage of uncle and niece and of first cousins. If it can be shown that there is nothing but good in the pedigrees then cousinship is no barrier to marriage. This applies not only to first cousins but also to the case of the marriage of non-related individuals, for if in the pedigrees of such there appear records of the same disadvantageous character then on biological grounds marriage is undesirable, assuming of course that marriage implies parentage. It is because pedigrees have not been and still are not kept that in all cases of prospective marriage of related individuals it is necessary to advocate caution—it has to be assumed that all such marriages are fraught with danger.