Agriculture Reference
In-Depth Information
complete. Nevertheless, the ability to identify animals with
high genetic merit is greater with the Selection Index as
long as genetic parameter estimates of performance traits
included in the selection criterion and their economic
values are accurate.
Best Linear Unbiased Prediction (BLUP) or Selection is
a mixed model methodology for predicting the breeding
value of individuals that are candidates for selection. A
mixed model has fi xed and random effects (see Boldman
et al., 1995) that account for the environmental and genetic
effects that are parts of each phenotypic expression of a
trait by an animal. In this procedure, a system of equations
based on fi xed (often environmental effects such as herd,
year, and age of the animal) and random effects (including
animal breeding values, permanent environment effects),
affecting one or more traits is used. This procedure requires
genetic parameter estimates and makes use of the relation-
ship among individuals to give the best possible estimates
of the animal's breeding values, given the information that
is available.
In the U.S., G. Wiggins has contributed toward the
development of national programs for evaluation of dairy
goats. National goat evaluation programs have used animal
models to predict the breeding values of bucks, does, and
their offspring for milk yield and composition, as well as
linear body measurements. Estimated transmitting ability
(ETA) or predicted transmitting ability (PTA) of each
parent is an estimate of the genetic merit that is transmitted
to their offspring, which is equal to one-half the breeding
value. Genetic comparisons are made among individuals
in a contemporary group, which may include goats of the
same age group or lactation, within year, herd, and breed.
Furthermore genetic comparisons are made among indi-
viduals not in a contemporary group relative to a rolling
genetic base. Animals of a given breed with superior
genetic merit can be found in different herds and years,
because of widespread use of artifi cial insemination espe-
cially in dairy goats. Similarly, most probable producing
ability (MPPA) provides an estimate of the milk produc-
tion potential in subsequent lactations. Often body confor-
mation measurements have been included in the evaluation
of dairy goats resulting in a production type index (PTI).
genome. Quantitative trait loci (QTL) are regions in the
genome closely linked to the genes associated with a quan-
titative trait. Breeding strategies based on QTLs require
phenotype, genotype and pedigree records from a large
number of individuals that vary in performance. In prac-
tice, a QTL refers to a region on a chromosome with an
effect large enough to be detected and mapped in the
genome. This may be considered as an early step in iden-
tifying and sequencing the actual gene that could be geno-
typed for selection purposes. Marker-Assisted selection
exploits major genes by genotyping animals for genetic
markers linked to QTLs.
There are several types of markers. An indirect marker
is closely linked to the gene that has the effect of interest.
A direct marker is the gene itself, or the portion of the gene
that contains the mutation producing the effect of interest.
The value of Marker-Assisted selection depends on the
nature of traits associated with increased production effi -
ciency as follows: when marker information is available to
combine with phenotypic measurements to give an esti-
mate of breeding value for an animal, the marker informa-
tion can improve the accuracy with which the breeding
value of an animal is estimated. The value of the marker
information depends upon the heritability of the trait. With
lowly heritable traits, the marker information improves the
accuracy of the estimated breeding value, whereas with
highly heritable traits the marker information is less valu-
able. The use of markers can also be important with sex-
limited traits, for example where dairy bulls are selected
initially on the basis of the value of their parents. A more
accurate selection of these young bulls could be made
using marker information on the males themselves.
Another situation where marker information may prove
valuable is if the trait can only be measured after the
slaughter of the animal. In this case, marker information
could be used instead to estimate the genetic merit of
potential breeding animals for carcass traits.
Crossbreeding
Crossbreeding is a common practice used in the commer-
cial production of meat from many species of farm live-
stock. This may be due to the fact that effi cient commercial
meat production requires the superior performance in
reproductive traits of adult animals as well as superior
performance of young animals reared to market age.
Getting superior performance in these different abilities
generally requires the use of more than one breed, as well
as use of a genetic phenomenon called hybrid vigor or
heterosis. On the other hand, with dairying in cattle and
other species, crossbreeding is much less common. Often
Quantitative Trait Loci and
Marker - Assisted Selection
In the 1990s, considerable strides were achieved in
mapping the goat genome based on highly polymorphic
microsatellite markers and mitochondrial DNA. Further-
more, single nucleotide polymorphisms (SNP) and
DNA chips have been developed to rapidly screen the
