Agriculture Reference
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plants. There are very few instances where heterozy-
gous advantage
per se
has been shown to result from
over-dominance. The counterpart to heterosis, inbreed-
ing depression, is generally attributed to the fixation
of unfavorable recessive alleles and so it is argued
that heterosis should simply reflect the converse effect.
Therefore unfavourable recessive alleles in one line
would be masked, in the cross between them, by dom-
inant alleles from the other. If this is all that there is
to it, then heterosis should be fixable in true breed-
ing lines. In general it has been found that this simple
explanation does not explain all the observed effects. So
the question is whether this breakdown in the explana-
tion is related to a statistical problem of the behaviour
of a large number of dominant/recessive alleles each
with small effect; whether the failure to detect over-
dominance is simply a technical failure rather than a
lack of biological reality; or whether a more complex
explanation needs to be invoked. Dominance can be
regarded as the interaction between alleles at the same
locus, their interaction giving rise to only one of their
products being observed (dominance is expressed) or
a mixture of the products of the two (equal mixing
giving no dominance, and inequality of mixing giving
different levels of incomplete dominance). But another,
well established, type of interaction of alleles can occur,
that between alleles at different loci (called non-allelic
interaction or epistasis). In addition we cannot simply
ignore the fact that linkage between loci is a recognized
physical reality of the genetic system that we now regard
as being the basis of inheritance. It has been shown that
the combination of these two well established geneti-
cal phenomena can produce effects that are capable of
mimicking the presence of over-dominance.
To examine the effect of having many loci, showing
dominance and recessivity, determining the expression
of a character differing between the parents of a single
cross, let us examine the case of two genetically con-
trasting parental lines that differ by dominant alleles at
only five loci.
Consider the two following cases where we assume
that: capital letters represent '
increasing
' alleles, lower
case '
decreasing
' ones; each locus contributes in additive
fashion to the expression of the character (i.e. the phe-
notype, let us consider yield); each increasing allele adds
the same amount to the yield (2 units) while the decreas-
ing adds nothing to yield; and that dominance is
complete and for increasing expression.
Case 1. Parent 1
=
AABBCCDDEE and Parent 2
=
aabbccddee then:
Parent 1
Parent 2
×
Genotype
AABBCCDDEE
aabbccddee
+
+
+
+
=
+
+
+
+
=
Phenotype
2
2
2
2
2
10
0
0
0
0
0
0
(yield)
F
1
Genotype
AaBbCcDdEe
+
+
+
+
=
Phenotype
2
2
2
2
2
10
(yield)
=
=
Case 2. Parent 1
AAbbCCddEE and Parent 2
aaBBccDDee then:
Parent 1
Parent 2
×
Genotype
AAbbCCddEE
aaBBccDDee
+
+
+
+
=
+
+
+
+
=
Phenotype
2
0
2
0
2
6
0
2
0
2
0
4
(yield)
F
1
Genotype
AaBbCcDdEe
Phenotype
2
+
2
+
2
+
2
+
2
=
10
(yield)
