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Genetic correlations between purebred and crossbred performance and purebred and crossbred heritabilities were estimated for egg production traits of laying chickens using a multivariate sire model accounting for additive relationships between sires. Two sire lines, denoted lines 1 and 2, were crossed to one dam line to produce crossbred progeny. Records for egg weight, egg specific gravity, and egg number were collected on purebred and crossbred hens. In total, 99 sires in line 1 and 292 sires in line 2 were used in the analysis, each sire producing on average 45 purebred and 105 crossbred daughters. Estimates of purebred heritability in lines 1 and 2 were in range of .54 to .74 for egg number traits, .52 to .91 for egg weight traits, and .41 to .83 for egg specific gravity traits. Estimates of crossbred heritability were .04 to .51 for egg numbers, .23 to .45 for egg weight, and .13 to .31 for egg specific gravity. The sire component in crossbreds differed up to 78% from the sire component in purebreds depending on traits. The estimate of genetic correlation ([r.sub.pc]) between purebred and crossbred performance was .56 to .73 for egg number, .69 to .99 for egg weight, and .72 to .82 for egg specific gravity. Although crossbred parameters were strongly affected by environmental factors, the results tend to agree with the theory that traits with a larger dominance variation and a larger difference between sire components in purebreds and crossbreds show a lower [r.sub.pc]. Because the estimates of [r.sub.pc] were significantly lower than 1 for egg number and egg specific gravity, an optimal selection strategy should combine purebred and crossbred information for crossbred progress. Key Words: Crossbreeding, Egg Production, Genetic Parameters, Multivariate Analysis, Poultry

An animal model analysis was used to estimate simultaneously additive |Mathematical Expression Omitted~ and dominance |Mathematical Expression Omitted~ variances for egg production traits within three White Leghorn lines. The data consisted of information for three generations on egg number (EN) produced at 18 to 25 (EN1), 26 to 65 (EN2), and 18 to 65 wk of age (EN3); egg weight (EW) measured at 30 to 35 (EW1) and 40 to 45 wk (EW2); and egg specific gravity (ESG) measured at 30 to 35 (ESG1) and 40 to 45 wk (ESG2). A transformation was used for EN2 and EN3 because of a skewed distribution. In total, 813 sires, 2,575 dams, and 28,649 daughters were involved in the analyses. Three genetic models (sire-dam, additive, and dominance) were compared in estimating heritability (|h.sup.2~). The sire-dam model underestimated |h.sup.2~ because it ignored animal relationships. The |h.sup.2~ estimates from the additive model were approximately 9 to 52% higher for EN and 2 to 18% higher for EW and ESG than those from the dominance model. The differences between the |h.sup.2~ estimates from the additive and dominance models were increased for larger dominance variance |Mathematical Expression Omitted~. Ratios of |Mathematical Expression Omitted~ to total variance were high for EN (10 to 20%) and low for EW and ESG (1 to 13%). Ratios of |Mathematical Expression Omitted~ to total genetic variance for EN1, EN2, EN3, EW1, EW2, ESG1, and ESG2 were 18 to 36, 29 to 43, 29 to 56, 1 to 26, 3 to 8, 20 to 27, and 2 to 14%, respectively. The results on dominance were in good agreement with heterosis and inbreeding depression for these egg production traits described previously.