Agriculture Reference
In-Depth Information
Let us return again to the canola cross, and continue
to assume the relationship between uppercase alleles
adding to a base yield and lowercase alleles adding noth-
ing. Previously, we did not consider dominant alleles
and their effect on the distribution.
Assume a two loci and two alleles per locus model of
inheritance for yield. Assume also that
A
is dominant
to
a
, but
B
and
b
are additive. Therefore
Aa
7 possible phenotypes. Assume, that
A
is dominant to
a
, but that
B
,
b
,
C
and
c
are all additive, and uppercase
alleles add 20 kg/plot to the base yield of 500 kg/plot.
We now have, the
F
1
=
=
AaBbCc
580 kg/plot (as
=
=
+
=
Aa
40 kg/plot). Again we see that
the F
1
performance is higher than the mid-parent value
(
m
AA
20
20
)
indicating dominance.
As with the two loci case, the distribution of pheno-
types with three genes is similarly skewed to the right.
In this instance, the average (mean) performance of the
F
2
generation would be 570 kg/plot.
=
AA
,so
we have:
P
2
×
P
1
AABB
×
aabb
AABBcc
AABbcC
AABbCc
aabBCC AAbBCc
aaBBcC AAbBcC
aaBBCc AAbbCC
aaBbCC aAbBcC
AAbbcC aAbBCc AABBcC
AAbbCc aAbbCC AABBCc
AAbBcc
One (or two)
A
alleles would add 60 kg/plot to the
base weight.
Therefore
AA
adds
60
kg/plot,
Aa
=
AA
(dominance
)
=
60 kg/plot, and
B
adds 30 kg/plot. The
F
1
=
]=
(
P
1
−
P
2
)/
AaBb
=
590 kg/plot. Now
[
a
2
=
AabbCC AAbBCC
=
P
2
+[
F
1
is not
60 kg, so
m
a
]=
560, clearly the
aabbCC AABbcc
AabBcC AABbCC
equal to
m
, and we have a case of dominance.
aabBcC
aAbbcC
AabBCc
AaBBCc
aabBCc
aAbbCc
AaBbcC
AaBBcC
P
2
F
1
P
1
aaBbcC
aAbBcc
AaBbCc
AaBbCC
aaBbCc
aABbcc
AaBBcc
AabBCC
500
590
620
aabbcC aaBBcc
AabbcC
aABbcC
aABBCc
←−
−→
m
aabbCc AAbbcc
AabbCc
aABbCc
aABBcC AABBCC
←− [
]−→←−[
]−→
a
a
aabBcc
aAbbcc
AabBcc
aABBcc
aABbCC aABBCC
aabbcc aaBbcc
Aabcc
AaBbcc
aaBBCC aAbBCC AaBBCC
500
520
540
560
580
600
620
When the F
2
population is examined we see that
the basic bell-shape curve has now been skewed to
the right (below), as a greater frequency of progeny
have higher yield due to the effect of the dominant
A
allele. The average (mean) performance of the F
2
is
now 575 kg/plot.
↑
The keen observer will have noted two points:
The F
1
performance was proportionally higher in
the two gene case compared to the three gene case,
because a higher proportion of alleles in the three gene
case were showing non-dominance. In Figure 5.3, the
degree of skewness of a six loci two allele system are
shown for no dominance, one dominant loci, three
dominant loci and five dominant loci.
•
AABb
AAbb
AaBb
Aabb
aABb
aABB
aabB
aAbb
AabB
AaBB
aabb
aaBb
aaBB
aAbB
AABB
•
There was a relationship between the parent perfor-
mance and the performance of the F
1
and F
2
. The
mean performance of the F
2
is midway between
m
,
the mid-parent value (560 kg/plot) and the mean
of the F
1
family (580 kg/plot). Not surprisingly the
mean of the F
3
family would be 565 kg/plot, half-way
between the mid-parent and the F
2
values. The mean
500
530
560
590
620
↑
Expand this idea on to a three loci, two allele exam-
ple as before and we have 64 possible genotypes with