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This study of the relative effects of inbreeding and outbreeding on reproduction in the domestic fowl is based on two years’ observations of triallel matings of hens having an inbred, outbred, and crossbred origin. (P. 529.) Among the many relations established by these observations and reported in the following pages, the data with respect to inbreeding and outbreeding show that: Fertility was influenced indirectly but not directly by the breeding system. Hens of an inbred origin were definitely less fertile than those of an outbred origin, irrespective of the kind of male to which they were mated. (P. 535.) Hatchability decreased with inbreeding of outbred females (F = 0 to F = .25) but did not decrease with further inbreeding of inbred females. Inbred females showed lower hatchability than outbred females whatever the system of mating, but the difference was not statistically significant. (P. 547.) Mean blastoderm diameter of unincubated eggs and number of somites in embryos after 38 hours incubation were smaller under inbreeding than outbreeding, and greater under outcrossing than crossbreeding. (P. 560.) The general fertility-hatchability relationship observed was not altered by origin or genetic relation, or by system of mating. Closely inbred matings of birds with an outbred origin showed a negative correlation between fertility and hatchability which supports the view that fertility is distinct from hatchability. (P. 611.)

The relative effects of inbreeding and outbreeding on reproduction in the domestic fowl