Divergent selection for yolk cholesterol in laying hens

1. Lines were selected for divergence in yolk-cholesterol concentration for three and four generations in the Cornell Controll (Leghorn) and Athens-Canadian (AC) (meat-type) randombred populations respectively. Selection was based on both individual and family records. 2. Cholesterol values were obtained from two consecutively laid eggs when birds were 28 weeks of age. The yolks of eggs from individual hens were pooled and analysed for cholesterol content by the colorimetric assay of Zlatkis et al. (1953) on the "fat extract" obtained by a modification of the procedure of Folch et al. (1956). 3. The largest differences between high and low lines (1-27 and 1-38 mg cholesterol/g yolk) were observed in the last generation of selection. There was evidence of a lack of response in the low lines. Realized heritability estimates obtained by dividing the difference between high and low lines by half the cumulative selection differential ranged from 0-11 to 0-25 in the AC population and from 0-21 to 0-25 in the Leghorn population. Realised estimates on a within-line bassis suggest that selection for yolk cholesterol is effective only in the upward direction.