Using two inbred lines of Brown Leghorns, B and I, a backcrossing experiment was set up to test whether inherent rate of production was a simply inherited trait. Female progeny of each generation, selected for high rate, were mated to males of the B (lower rate) line. The pullet records now available, for 2 full years' production and to 31 March in the third generation, have yielded unexpected results.

In each backcross generation mean egg production has been at least as good as in the better parent, line I. This is not wholly accounted for by the behaviour of rate, since measurements of the trait in the cross approximate to the average of the two inbreds in each year. High viability and absence of very poor layers were also determining factors.

Backcross egg weight remained intermediate between the widely differing parent lines, and the cross produced the greatest total weight of egg material in each set of comparisons. Body weight was also intermediate, but sexual maturity was comparable with that of line B.

It was deduced that selecting the backcross breeding females for high rate had led to the retention of a greater proportion of the original I genotype than would normally have been expected, and so maintained continued heterozygosity in succeeding generations.

Practical implications of the results and interpretation are discussed.