A study, extending over a 10-year period, has been made of the effect of inbreeding in the White Leghorn breed of domestic fowl. The major object of this investigation was to study the effect of various intensities of inbreeding on the following characters: Fertility, hatchability, viability, days to first egg, egg production, egg size and body size.

In the present investigation the degree of inbreeding was, in general, less intense than that from brother and sister or parent and offspring matings. One intensely inbred family, however, with the equivalent of brother and sister mating, was maintained successfully for nine generations. Six more or less distinct families, each with a somewhat different type and intensity of inbreeding but with a similar foundation ancestry, were developed.

All individuals were selected primarily on the basis of high hatchability of their eggs, upon general vigor of the individual bird and of the offspring of each pair of birds. Other characters besides vigor and hatchability also were considered whenever practical.

The ancestry of all the birds in the present generation of these 6 families may be traced to four males and seven females. One of the four original males (No. 823), however, has a greater degree o£ relationship to the present generation than any of the other foundation birds. This relationship still averages approximately 55 percent, with the lowest degree of relationship being 29 percent and the highest 61 percent. The relationship of the present generation to the other three foundation males does not exceed 5 percent.

The relationship between brothers and sisters in the present generation chicks of the separate families ranges from 74 to 94 percent. The average inter-se relationship between the present chicks which are not sibs is above 50 percent.

The inbreeding coefficients of the present birds range from 41 percent for the least inbred to 82 percent for the most intensely inbred family.

There was no general decrease in percent of eggs fertile as the degree of inbreeding increased ; in fact some inbred families have shown an increase in percent of eggs fertile.

There was a slow but gradual decline in the average percent hatchability of fertile eggs set for all inbreds as the inbreeding increased. The average hatchability for all inbreds, however, was in most cases well above 60 percent. In six of the families studied there was no general decrease in hatchability, which demonstrates that it is possible to maintain a reasonably safe level of hatchability under a system of intense inbreeding.

There was a significant decrease in the number of days to first egg. The most intensely inbred birds on the average matured sexually 16 days earlier than the birds of the original non-inbred foundation stock.

There was a general but not a consistent decrease in number of eggs laid during a given period as the inbreeding increased. An exception was observed for the 70 percent inbred group which compared favorably with the original non-inbred foundation birds in egg production. There was no marked decrease in 200-day egg production for anyone of the six separate families as a result of the inbreeding.

The data on egg weight suggest no general decrease or increase in average egg weight as a result of the inbreeding.

The results indicate that intensive inbreeding did not decrease materially the growth rate or adult body weight of the birds used in this experiment.

With the exception of the 80 percent group, there was no increase in mortality in any of the groups up to 24 weeks of age. The pullet year mortality, however, showed a marked rise for the more intensely inbred birds with the exception of that group of birds having an inbreeding coefficient of 70 percent.