Genetic (co)variance components (1) [[sigma].sup.2.sub.a] c [[sigma].sup.2.sub.e] Birth 0.004 [+ or -] 0.016 0.164 [+ or -] 0.247 [+ or -] 0.02 weight 0.024 Genetic parameters (2) [h.sup.2.sub.a] [c.sup.2] [e.sup.2] Birth weight 0.011 [+ or -] 0.04 0.394 [+ or -] 0.595 [+ or -] 0.053 0.045 [[sigma].sup.2.sub.p] Birth 0.416 [+ or -] 0.025 weight Birth weight (1) [[sigma].sup.2.sub.a]: Direct additive genetic variance
; c: Variance attributed to the permanent maternal environment; Residual variance; [[sigma].sup.2.sub.e]: Phenotypic variance.
The estimates of additive genetic variance
and heritability from this study are not precise because of the small numbers of families and parents in this trial.
This is because most of the genetic variance expressed by structural genes is additive and because genes with epistatic effects influence the response to individual selection through their contribution to the additive genetic variance
(Crow and Kimura 1970).
The models, explaining within a between variances (Table 3) on the basis of two genetic components and two environmental components, viz., additive genetic variance
(DR), dominance (HR), within family environment ([E.sub.1]), and between family environment ([E.sub.2]) were applied to data on twins reared together having nonsignificant G x E interaction (Table 4).
The estimated additive genetic variance
for development time was higher in females than in males (Table 2).
Additive genetic variance
had increasing trend during the lactation trajectory.
where A is the Wright's additive relationship matrix  among all animals in the pedigree, [[sigma].sup.2.sub.a] is the direct additive genetic variance
, [[sigma].sup.2.sub.m] is the maternal additive genetic variance
, [[sigma].sup.2.sub.c] is the maternal permanent environmental variance, [[sigma].sup.2.sub.e] is the residual variance and [I.sub.c] and [I.sub.n] are identity matrices of size equal to the number of dams and the number of observations, respectively.
Epistasis as a source of increased additive genetic variance
as population bottlenecks.
The phenotypic variance ([V.sub.P]) was separated into the additive genetic variance
([V.sub.A]), non-additive genetic variance ([V.sub.N] and environmental variance ([V.sub.E]), and the environmental variance ([V.sub.E]) was separated into the common environmental variance ([V.sub.E]) and the specific environmental variance ([V.sub.ES) using the standard separation of variance components (Falconer 1989).
The square root of direct (i.e., Mendelian) heritability ([h.sub.o]), path coefficient 1, represents the square root of the fraction of the total phenotypic variance in the trait of interest [Mathematical Expression Omitted] that is determined by the direct additive genetic variance
[Mathematical Expression Omitted], whereas the path coefficient 2 represents the proportion accounted for by random environmental variance ([Mathematical Expression Omitted]).