1) The document discusses the genetic model for quantitative traits, which partitions an individual's phenotypic value (P) into components including breeding value (BV), genotypic value (G), additive genetic value (A), dominance (D), epistasis (I), and environmental effects (E). 2) It explains that an individual's breeding value (BV) is equal to their additive genetic value (A) but not their full genotypic value (G), since G includes non-additive effects from gene combinations like dominance and epistasis that are not transmitted to offspring. 3) An example is given comparing the weaning weights of three calves, explaining that Calf 3 has the highest breeding value despite a

1. (4 Points) Construct to scale a diagram (like the one used in lecture for calf weaning weights)
showing the following hypothetical sample of records for live (market) weight in lambs (assume
= 130 lb ). Be sure to lobel completely. Lambs 1 and 3 have similar market weights, but for
different reasons. Briefly, explain. Appendix B Partitioning the Genotypic Value Where is the
breeding value? As defined in the body of this section, the basic genetic model for quantitative
traits is: P = + G + E where P = the phenotypic value or performance of an individual animal for
a particular trait. = (Greek letter m u ) the population mean or average phenotypic value for the
trait for all animals in the population. G = the genotypic value of the individual for the trait. E =
the environmental effect on the individual's performance for the trait. We said this model would
help us understand breeding value, which we defined as the value of an individual as a
contributor of genes to the next generation. But, where is breeding value (BV) in the model? The
only genetic component of the model is the genotypic value (G). Does G equal BV? The answer
is no. To explain why the answer is no, let's look at G in more detail. It represents the overall
effects of an individual's genes, either singly or in combination, on that individual's performance
for a trait. As a result, it can be partitioned into additive (A), dominance (D), and epistatic (i)
genetic components. The basic genetic model can then be rewritten as: P = + A + D + I G First,
consider A. Additive gene action is the most important type of gene action in the inheritance of,
and selection for, quantitative traits. When gene action is additive, each allele in the genotype has
a specific metric value it adds to the phenotype. The additive genetic effect of an allele can be
either positive or negative. Also, the effect of that allele is independent of the effect of the other
allele at that locus as well as the effects of alleles at all other loci. This means additive
geneticeifects are independent gene effects. Only independent gene effects are directly
transmitted to the next generation. [Recall our discussion of Mendelian genetics in Section II.]
Thus, the additive genetic value (A) is equal to the breeding value (BV). B V is the portion of G
that is transmitted to offspring; it is due to independent gene effects. Additive gene effects are
independent. Thus, A = B V . Now consider D and I. Both these components of G have non-
additive genetic effects. The effects of dominance and epistasis are due to gene combinations
and, as such, are not directly transmitted to the next generation. Recall that dominance is the
interaction between alleles at a single locus and epistasis is the interaction of alleles at different
loci (Section Ii). Thus, neither have independent gene effects. Figure 5. Contributions of
components of the genetic model for repeated traits for tw o records on a single dairy cow for
305-d lactation yield. Calf #1 weighs 600 lb (100 lb above average), has a higher than average
genotypic value ( l ), and experienced a better than average environmental effect (E). Calf 12
weighs 450 lb (50 lb below average), has a lower than average genotypic value, and experienced
a worse than average environmental effect. Calf 13 weighs 460 lb . His genotypic value for
weaning weight is higher than average, but his actual performance is below average due to a very
poor environment. A New Model The basic genetic model for quantitative traits can now be
expanded to include breeding value (BV) and gene combination value (GCV): P = + B V + GC
V + E The weaning weights of the three calves depicted in Figure 1 are shown again in Figure 4,
this time with their (hypothetical) breeding values and gene combination values included. Calf
#1 has the heaviest weaning weight, but note that most of his superior performance is due to
either environment or genetic effects that cannot be transmitted to offspring. If heavier weaning
weights are desirable, the best breeding animal should be Calf #3. Despite his mediocre
individual performance, he has the highest breeding value. Figure 4. Contributions of breeding
value, gene combination value, and environmental effect to the weaning weights of the three
calves depicted in Figure 1. The new model for quantitative traits retains many of the

2. characteristics of the basic model. Like genotypic value, breeding value and gene combination
value are expressed as positive and negative deviations from a population mean. The average of
breeding values and the average of gene combination values across an entire population are,
therefore, zero. Furthermore, breeding values and gene combination values are considered
independent of environment effects and each other. It is possible to have highly inbred animals
that suffer inbreeding depression (unfavorable gene combination value) and,