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
 
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