Agriculture Reference
In-Depth Information
change in gene frequency as a result of selection
under different levels of dominance is described in
Table 4.3 .
Example: In a random breeding population of Angus
cattle suppose all
red calves
are destroyed. If 100
red
calves
are born each generation out of a total of 10,000
births, the distribution of the three genotypes are shown in
Table 4.4 .
Therefore:
1
11
−( )
−−
sq
q
2
Δ
(
1
−
q
) =−
(
1
s
)
(
1
−
q
) +
=
009
.
2
s
(
q
)
It is important to note that with selection against homo-
zygous recessive genotypes (
bb
), the allelic frequency of
b
changed by 0.01, from 0.1 to 0.09. This indicates the
diffi culty associated with selection against recessive geno-
types, such as lethal genes, color, and markings.
(
q
=
09
.
and
1
−
q
) =
01
. ,
and
s
=
1
Table 4.3
Genotypes and their frequency, and genetic contribution with various levels of dominance
for fi tness.
Genotype
BB
Bb
bb
Δ
(1
−
q
)
2
Genotypic frequency
q
2
2
q
(1
−
q
)
(1
−
q
)
2
1
11
−( )
−−
sq
q
2
s
(
q
)
Complete dominance
1
1
(1 − s)
(selection against
bb
genotypes)
No dominance
1
1
−
( ½ )s
(1
−
s )
1
2
1
11
2
(
)
−
sq
−
q
(selection against
b
allele)
s
(
q
)
−−
sq
(1 −
q
)
2
Complete dominance
(1 − s)
(1 − s )
1
(selection against
B
allele)
(
)
(
)
Overdominance
(1 − s
1
)
1
(1 − s
2
)
qqs qs
1 1
111
−
{
−−
q
}
1
2
{
}
(selection against
BB
and
bb
genotypes)
2
(
−−−
s
q
)
Note:
q
and (1
−
q
) are the frequencies of
B
and
b
alleles, (1
−
s), 1
−
( ½ )s, (1
−
s
1
) and (1
−
s
2
) are the proportion
of parents of the specifi c genotype which reproduce in each generation.
Table 4.4
Genotypes and their initial frequency and number, genotypic frequency, fi tness, and
genetic contribution.
Genotype
BB
Bb
bb
Total
q
2
(1 −
q
)
2
Initial frequency
2
q
(1 −
q
)
Initial number
8,100
1,800
100
10,000
Genotypic frequency
0.81
0.18
0.01
1.0
Fitness
1
1
0
1
2
Genetic contribution
q
2
2
q
(1
−
q
)
(1
−
s
)(1
−
q
)
2
−
( )
−−
1
11
sq
q
2
(
)
s
q
Note:
q
and (1
−
q
) are the frequencies of
B
and
b
alleles, (1
−
s
) is the proportion of the specifi c
bb
genotype
parents which reproduce in each generation.
1
Since (s = 1).
