Agriculture Reference
In-Depth Information
grow more rapidly than female kids and attain a heavier
body weight at maturity. Type of birth and rearing can be
important; for example, single-born kids grow more
rapidly than those born as twins, which in turn grow more
rapidly than triplets. Part of this effect is due to care and
milking ability of the dam divided amongst the one or
several offspring with the limiting factor being the milk
production ability of the dam. The impact on performance
of goats from management, diet, housing and pen are
important. Therefore, every attempt should be made to
minimize their infl uence. To obtain a clear estimate of the
genetic component, it is important to ensure the environ-
mental effects on production performance are similar for
a group of animals.
Some traits can be repeatedly expressed over an ani-
mal's lifetime. In making culling decisions, it is useful to
know whether an animal's performance measure is likely
to be repeated. This is a slightly different type of decision
than the selection of parents in a breeding program, but it
is important in the management and operation of breeding
and commercial herds.
Repeatability (
R
) is a measure of the similarity of per-
formance measurements on the same trait over the life of
an individual. For example, with milk production, a doe
will perform in a similar way from one lactation to the
next, not only because the same genes with all of the same
genetic effects are expressed, but also because there will
be some environmental effects such as presence of mastitis
that are permanent in nature that affect all lactations.
However, lactations from the same doe will differ due to
temporary environmental effects. A simple way to estimate
the repeatability of milk production in goats would be to
calculate the correlation between fi rst and second lactation
milk yield measurements for a sample of does. Though
there are better ways to determine repeatability, this dem-
onstrates what it represents. In terms of the variance com-
ponents discussed previously, repeatability is defi ned as
the ratio of genetic (
Heritabilities, Repeatabilities, and Correlations
The heritability for any measurable morphological charac-
teristic and production performance indicates the ability of
both the parents to transmit superior productivity to their
offspring. Heritability in the broad sense (
H
2
) is the ratio
of the genetic (
σ
2
G
) to total phenotypic variance (
σ
2
P
),
2
=
σ
σ
G
P
which is given by
H
2
. The genetic variance is not a
2
σ
2
G
) and permanent environment
useful measure in selection programs because not all
genetic effects are transmitted from parents to their off-
spring as mentioned previously.
Heritability (
h
2
) in the narrow sense is the ratio of
the additive genetic variance (
(
σ
2
PE
) variance for a trait to total phenotypic (
σ
2
P
) variance
2
2
σσ
σ
G
PE
as follows:
R
=
. Examples of traits that are
2
P
expressed repeatedly in a goat's life include number of
kids born, milk production, and weight of mohair. Estimates
of repeatability for multiple births and daily gain in goats
are presented in Table 4.8.
The phenotypic correlation (
r
P
) is an estimate of asso-
ciation between two traits measured on an individual,
σ
2
A
) to total phenotypic
2
σ
σ
A
P
variance (
σ
2
P
), which is given by
h
2
=
. Heritability can
2
be calculated in a number of ways. One method uses phe-
notypic measures for the same trait in parents and their
offspring. The slope of the line relating the performance
of offspring to that of their parents or midparent average
gives a measure of heritability in the narrow sense.
Heritability estimates have also been derived from regres-
sion using other types of relatives (daughter-dam, son-sire,
and offspring-parents), and from intra-class correlation
(using full siblings or half siblings). The choice of the
procedure depends, in part, on the nature of the trait (for
example, daughter-dam regression for milk production
because milk is only expressed in the female sex). Modern
techniques and computer programs based on mixed models
(Boldman et al., 1995) are now the methods of choice and
make use of information from all types of relatives in a
population to give estimates of heritability for a trait.
Estimates of heritability for reproductive traits, body
weight and daily gain in goats are presented in Table 4.8.
σ
σσ
XY
denoted by
r
P
=
22
. Where,
σ
XY
is an estimate of the
XY
phenotypic covariance between traits
X
and
Y
, and
σ
2
and
σ
2
are estimates of their respective phenotypic
variances.
The phenotypic correlation is due to genetic effects that
are in common for the two traits, as well as environmental
effects that affect both traits. There are thus genetic (
r
G
)
and environmental (
r
E
) correlations between the two traits
that play a role in the observed phenotypic correlation. As
mentioned, the genetic correlation between two traits is, in
part, due to genes that have effects on both traits. This
phenomenon is known as pleiotropy. If two traits are
affected by many of the same genes then the genetic cor-
relation will be high (+1 or
−
1). If different genes affect
