Agriculture Reference
In-Depth Information
tion will be 0.1250 for 1 sire, 0.0625 for 2 sires, 0.0250
for 5 sires, and 0.0125 for 10 sires.
The detrimental infl uence of inbreeding has been dem-
onstrated in a number of studies. The consequence is a
reduction in the overall vigor of the offspring, with a
greater infl uence on traits associated with fi tness, such as
conception, fertility, and survival.
Half-sib pedigree
C
Common ancestor
n
n'
+1
A
X
D
C
1
1
1
3
= 1/8 = 0.125 or 12.5%
C
F
X
(C) = (½)
B
E
Phenotypic, Genetic, and Environmental Variation
In a population, the variance is a measure of the average
squared deviations from the mean for any measurable mor-
phological characteristic and production performance. In
goat breeding there is interest in the components of vari-
ance starting with the phenotypic variance (
Full-sib pedigree
C
Common ancestor
n
n'
+1
A
C
1
1
1
D
X
D
1
1
1
σ
2
P
) which
constitutes the sum of the genetic (
σ
2
G
), environmental
3
C
F
X
(C) = (½)
= 1/8 = 0.125 or 12.5%
(
σ
2
E
), and genotype
×
environment interaction (
σ
2
G × E
) vari-
B
F
X
(D) = (½)
3
= 1/8 = 0.125 or 12.5%
ance components. The genetic variance (
σ
2
G
) can be further
D
Σ
C+D = 0.125 + 0.125 = 0.25 or 25%
subdivided into additive genetic (
σ
2
A
), dominance (
σ
2
D
),
and epistatic (
2
I
) variance components. This has been
described in the following:
σ
Parent offspring pedigree
D
Common ancestor
n
n'
+1
A
A
0
1
1
2
2
2
2
σσσσ
σσσσ
=++
=++
P
G
E
G
×
E
E
X
(A) = (½)
2
= 0.25 or 25%
2
2
2
2
X
In selection programs, the genetic effect of interest is
based on additive genetic variance (
G
A
D
I
A
2
A
) arising from the
average effects of genes, which determines the transmit-
ting ability from the parents to offspring. The nonadditive
genetic variance comprises dominance variance (
σ
B
C
Figure 4.1
Coeffi cient of inbreeding for full-sib,
half-sib and parent-offspring pedigree.
2
D
)
associated with effects due to the combination of alleles at
a locus, and epistatic variance (
σ
2
I
) associated with com-
binations of alleles at more than one locus. Allele combi-
nations that produce dominance and epistatic effects on
traits are lost at meiosis, when only one allele of each pair
for each gene forms a gamete. Thus offspring do not have
the same allele combinations as their parents. It is possible,
however, for breeding programs to take advantage of the
nonadditive genetic effects through the use of specialized
mating systems.
There is clear evidence to suggest environment is a
signifi cant source of variation in many morphological
characteristics and production performance. The environ-
mental variation is associated with nongenetic sources of
variation arising from differences among years, age of doe,
season, sexes, types of birth and rearing, management,
diets, housing, and pens. The infl uence of year and season
is random in nature; therefore, it is diffi cult to predict their
performance. Sex is of course a genetic effect, but it can
be considered as a hormonal and developmental environ-
ment in which a trait is expressed. For example male kids
σ
Where n = number of generations from the sire to the
common ancestor, n
= number of generations from the
dam to the common ancestor,
′
= summation of the various
paths (there may be more than one common ancestor, and
thus more than one path connecting the sire or dam to the
common ancestor), 1/2 = probability of receiving one
allele or the other at each segregation, and F
A
= inbreeding
of the common ancestor.
As noted earlier, it is possible to compute the inbreeding
rate (
Δ F
) per generation in a closed population from the
effective number of breeding males
N
m
and females
N
f
in
the population, as follows:
Σ
1
1
1
Δ
F
=
+
.
Δ
F
=
,
8
NN
m
8
8
N
m
f
1
8
N
f
when
N
f
>
12 because
is negligible and can be
ignored. It is possible to estimate the increase in inbreeding
per generation from the effective number of sires used as
parents. The estimate of increase in inbreeding per genera-
