Agriculture Reference
In-Depth Information
Table 4.1
Frequency of alleles from bucks and does and
genotypic frequency of their offspring.
the condition is detrimental (for example, polled bucks
with hypoplasia of the testes). True lethal conditions result
in death before or shortly after birth, delayed lethal condi-
tion is expressed later in the life, and partial lethal condi-
tion becomes fatal under certain conditions. Hutt (1964)
has described a number of lethal and semilethal conditions
in chickens.
Muscular hypertrophy in cattle, also known as double
muscling, is an example of a gene that can have detrimen-
tal effects as well as favorable effects. Calves homozygous
for the gene have an increased incidence of diffi cult calving
and thus increased calf mortality at birth but possess up to
20% more muscle mass than normal animals that are
homozygous. The molecular basis for a lethal condition
arising from an inherited disorder in more than 135 traits
of animal species other than the human and the mouse can
be found in a database of genes available online describing
Mendelian Inheritance in animals (Nicholas, 1987).
Mutation rates are generally very small and cause little
change in the population unless its effects are accumulated
over many generations. Mutation in combination with
other forces acting on gene frequencies, however, can be
a very important force. Selection, for example, can rapidly
increase the frequency of a rare mutant allele.
In breeding populations where bucks are to be widely
used to produce large numbers of kids across many herds
it is important to establish if the animal is a carrier of
alleles for lethal or deleterious conditions. This can be
achieved by test mating to mates known to carry a reces-
sive allele for a condition, or to close relatives such as
daughters. DNA tests are available for a few conditions.
When an abnormal condition conforming to a well-known
syndrome appears, the condition may be genetic in origin.
When there is a genotype
Alleles from bucks
Allele
B
(
q
)
b
(1
−
q
)
Alleles from
B
(
q
)
BB
(
q
2
)
Bb
{
q
(1
−
q
)}
does
b
(1
−
q
)
Bb
{
q
(1
−
q
)}
Bb
{(1
−
q
)
2
}
Note:
q
and (1 −
q
) are the frequencies of
B
and
b
alleles, and
BB
,
Bb
and
bb
are genotypes.
above, the genotype frequencies in offspring are expected
to be
F
BB
=
q
2
= 0.36,
F
Bb
= 2
q
(1
−
q
) = 0.48 and
F
bb
= (1
−
q
)
2
= 0.16.
Hardy - Weinberg Equilibrium
A large random mating population of goats in the absence
of mutation, migration, selection, or drift will have gene
and genotype frequencies that remain unchanged from
generation to generation. This state is referred to as Hardy-
Weinberg equilibrium.
An application of the relationship that exists between
allele frequencies and genotype frequencies is as follows:
consider
black coat color
to be recessive to
white coat
color
in goats. A large herd of
white
goats gives birth
to one
black
kid out of every 100 kids. The genotypic
frequency of the
black color
is expected to represent
1
100
2
1
(
−
q
) ==
0 01
.
. It is important to note that this is
true in the absence of mutation, migration, selection or
drift acting to change gene frequencies. The allelic fre-
quency can thus be calculated as the square root of the
frequency of the recessive genotype as
environment interaction, the
condition would occur in specifi c environments. The det-
rimental condition may be environmental in origin. The
condition may be a result of lack of amino acid, vitamin,
or mineral and in some cases, from the consumption of
toxic plants. Detrimental conditions introduced by envi-
ronment do not recur when the environment is changed.
×
2
(
(
q q
. .
It is then possible to calculate the frequency of the
allele for
white color
of the kid
q
= 1.0
1
−
) =−
1
)
=
0 01
=
0 10
−
0.1 = 0.9.
Similarly, frequency of the
white color
(
BB
) will be
q
2
= 0.9
×
0.9 = 0.81 or 81%, heterozygote (
Bb
) will be 2
q
(1
0.9 = 0.18 or 18%, and those of the
black color
(
bb
) will be (1
−
q
) = 2
×
0.1
×
−
q
)
2
= 0.1
×
0.1 = 0.01 or 1%.
M
IGRATION
In a large population, in each generation the proportion of
migrants and natives are
m
and (1
Forces That Change Gene Frequencies
m
), respectively. Let
the frequency of the allele in the natives and migrants be
q
0
and
q
m
, respectively, and after one generation those of
the mixed (migrants and natives) population be
q
1
. The
frequency in the mixed population written as the propor-
tion of migrants and their allelic frequency together with
the proportion of natives and their allele frequency can be
−
M
UTATION
There are a large number of detrimental and lethal condi-
tions controlled by a single recessive gene with a mutation.
In some cases when the goat suffers from an inherent
disorder, the animal is able to survive and reproduce but
