INBREEDING AND OUTBREEDING In numerous animal and plant species prolonged inbreeding may lead to the production of defective and sterile individuals. When members of two inbred lines are mated, there is usually a considerable gain in vigour. But more careful work has shown that inbreeding is not always harmful. Many plants, such as the pea, are normally self-fertilized, and rats have been successfully bred from brother to sister matings for more than 20 generations. If a plant heterozygous for a number of genes is self-fertilized, only half its offspring will be heterozygous for each of them, and the average plant will be heterozygous for only half the original number. The same process occurs, only more slowly, with in breeding. In either case nearly complete homozygous is reached in about six generations. Experience shows that most of the harmful effects of inbreeding, if any, appear within the first five to ten generations. If rigid selection is exercised during that period, a vigorous inbreeding race may often be built up, which does not vary appreciably in a constant environment, and cannot, of course, be further altered by selection.
Two distinct views are held as to the advantages of outcross ing. On the one hand it is thought that the bad effects of in breeding are merely due to the appearance of recessive char acters, and that it should be theoretically possible to eliminate these. Of course if the desirable dominant genes were very strongly repelled the practical difficulties would be immense.
Other workers believe that heterozygous is beneficial as such. It is certainly of interest that the more successful types of asex ually propagated plants often give evidence of extreme hetero zygosis when self-fertilized. For example, the raspberry "Super lative" gives white plants which die at an early stage, plants sterile on the male side, others sterile on the female side, besides several recessive colour varieties; seedlings from it, which are homozygous for the dominant characters allelomorphic to these, are less satisfactory as fruit plants than their parent. It is pos sible therefore that it may be advantageous to be heterozygous for one or more lethal genes.
The condition of heterosis or hybrid vigour is of considerable interest as a character which is genetically determined, but yet not inherited. In particular the first generation of a cross be tween species is often larger and healthier than either parent, though they may be sterile or yield very unsatisfactory offspring. When, however, a cross is made between tetraploid species, or the chromosome number of a hybrid is doubled, an allotetraploid is produced in which there is no segregation, and it is suggested that the good effects of heterosis are not annulled by sexual reproduc tion in such case though the matter has not as yet been definitely decided.