The relationships between egg weight and yolk weight

Figures on egg weight and yolk weight from a commercial strain of White Leghorn pullets obtained during the early part of their laying year have been analysed to obtain estimates of heritability and phenotypic, genotypic and environmental correlations.

The heritability figure of 0.50 for egg weight agrees with the majority of those in the literature. The larger value of 0.71 derived from the sire component compared with that from the dam component of 0.29 is in agreement with a number of other reports suggesting the existence of sex linkage.

The heritability of yolk weight was estimated to be 0.43 with no difference between estimates derived from sire and dam components. Attention is drawn to the lower precision of the figures based on dam components because of the small number of daughters per dam available.

Correlations between egg weight and yolk weight are high. The phenotypic correlation is 0.58 and the genetic correlation 0.82, whereas the environmental correlation is only 0.33.

Continuing selection for increased egg weight is therefore expected to lead to a concomitant increase in yolk weight nearly as great as that expected from direct selection for yolk weight.

The heritability of the yolk weight/egg weight ratio is low, of the order of 0.20. The negative phenotypic correlation of —0.42 of this ratio with egg weight is the joint effect of a large negative genetic correlation of — 1.00 and a small positive environmental one (0.02). Direct selection for a higher yolk ratio would therefore be tantamount to selecting for small eggs.