A large intercross between the domestic White Leghorn chicken and the wild ancestor, the red junglefowl, has been used in a Quantitative Trait Loci (QTL) study of growth and egg production. The linkage map based on 105 marker loci was in good agreement with the chicken consensus map. The growth of the 851 F2 individuals was lower than both parental lines prior to 46 days of age and intermediate to the two parental lines thereafter. The QTL analysis of growth traits revealed 13 loci that showed genome-wide significance. The four major growth QTLs explained 50 and 80% of the difference in adult body weight between the founder populations for females and males, respectively. A major QTL for growth, located on chromosome 1 appears to have pleiotropic effects on feed consumption, egg production and behaviour. There was a strong positive correlation between adult body weight and average egg weight. However, three QTLs affecting average egg weight but not body weight were identified. An interesting observation was that the estimated effects for the four major growth QTLs all indicated a codominant inheritance.

Selection can be a useful way to alter yolk proportion and thereby egg dry matter which, owing to its economic importance, is a trait of substantial importance for the egg-processing industry. However, the egg is primarily the chamber of embryonic development. The main purpose of this study was to estimate genetic correlations between the maternal effect on chick weight at hatching and the direct effect on different egg composition traits, in particular, yolk proportion. Additionally, genetic parameters were estimated for egg composition traits. To create a data set suitable for estimation of genetic parameters, a three-round selection experiment was set up. Birds were selected based on their predicted breeding values for the genetic maternal effect on chick weight and the direct genetic effect on yolk proportion according to the theory of elliptical selection. Genetic parameters were estimated using a multiple trait animal model and restricted maximum likelihood. The maternal heritability for chick weight was 0.5, whereas the direct heritability was dose to 0. The genetic correlations between the maternal effect on chick weight and the direct effect on yolk proportion, yolk weight, albumen weight, albumen dry matter concentration, and egg weight were 0.14, 0.76, 0.93, 0.14, and 0.99, respectively. The heritabilities for yolk proportion, yolk weight, albumen weight, albumen dry matter concentration, and egg weight were 0.33, 0.43, 0.57, 0.38, and 0.60, respectively. We conclude that breeding ought to be a useful way to increase egg dry matter with no expected unfavorable correlated effects on chick weight.

1. A striking change in egg consumption from shell eggs toward egg products is taking place. Consequently, internal egg quality traits must be seen in a different light, as they may affect the profitability of the egg processing industry. In order to study the possibility of using breeding as a means to alter yolk production, a one-generation divergent selection experiment was set up. The trait was yolk proportion. The effect of hen's age on egg composition was also studied. Genetic parameters for yolk proportion, yolk weight, egg weight and albumen weight were estimated, using a multiple trait animal model, and restricted maximum likelihood. 2. Selection response was obtained for yolk proportion and correlated selection responses for yolk weight and albumen weight. In the 'high line' a decrease in egg weight and an increase in yolk weight was established, whereas in the 'low line', egg weight remained more or less the same and yolk weight decreased slightly 3. Yolk proportion, yolk weight, egg weight and albumen weight all increased with increasing age of the hen in both selection lines. The difference between selection lines in yolk proportion, yolk weight and albumen weight persisted throughout the laying period. 4. Estimated heritabilities for yolk proportion, yolk weight, egg weight and albumen weight were 0.38, 0.22, 0.34 and 0.41 respectively. The genetic correlation between yolk proportion and yolk weight was 0.52, whereas the genetic correlations with egg weight and albumen weight were -0.51 and -0.74 respectively. 5. We conclude that selection can be used to alter yolk production. Selection can be at an early age with a sustained effect throughout the laying period.

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