Agriculture Reference
In-Depth Information
Effectively, dominance-recessiveness are allelic inter-
actions whereby the phenotypic expression of a
character does not depend solely on the additive
effects of the different alleles at the same locus, and
the mean expression of the heterozygote is not the
same as the mid-parent value. As we note earlier,
a similar phenomenon can occur between alleles at
different loci, and this is known as
non-allelic inter-
action
or
epistasis
. An example of epistasis and how it
might occur was presented in the qualitative genetics
section. A further example might be:
QUANTITATIVE TRIAL LOCI
The concept of linkage, between different loci located
on the same chromosome, was introduced in the
qualitative genetics section. Quantitatively inherited
characters are controlled by alleles at multiple loci.
Yield, for example, is a highly complex character which
is related to a multitude of other characters like, seedling
germination and emergence, flowering times, partition,
photosynthesis efficiency, nitrogen uptake efficiency,
etc., plus a susceptibility or resistance to a wide range
of stresses including diseases and pests. Even if a single
gene were to be responsible for all the individual factors
that are involved in yield potential (which they are not),
then it is easy to see that there will be hundreds or even
thousands of genes which influence yield. Given that
the number of chromosomes in crop species is small
(2
n
AABB
=
24;
AAbb
=
12;
aaBB
=
12;
aabb
=
8
In the presence of
BB
, the difference between the
AA
and
aa
genotypes is 24
12 units. How-
ever, in the presence of
bb
, the difference between
AA
and
aa
is 12
−
12
=
4 units. Of course, another
way of looking at the matter might be to say that
the difference between
BB
and
bb
is 24
−
8
=
=
=
=
=
2
x
34 in sunflower, 2
n
2
x
18 in let-
=
=
=
=
tuce, 2
n
4
x
38 in rapeseed, 2
n
2
x
20
−
12
=
12
in maize, 2
n
=
6
x
=
42 in wheat, 2
n
=
2
x
=
14
−
=
units in the presence of
AA
, but 12
4 units
in the presence of
aa
. Either way, it can be seen
that there is
interaction
between the alleles at dif-
ferent loci and that an additive-dominant model of
inheritance cannot be adequate. In fact, it is possi-
ble to add epistasis (usually symbolized by
aa
, for
interaction between loci which are homozygous,
ad
for those between loci where one is heterozygous
and one homozygous and
dd
8
in barley, 2
n
=
2
x
=
24 in rice, 2
n
=
2
x
=
22 in
bean, and 2
n
48 in potato), then linkage will
always be a major factor in the inheritance of quantita-
tively inherited traits. So this, as with other quantitative
effects, adds another level of complexity. In general, the
complexity of the genetics has meant that many ques-
tions remain unanswered. Some questions that might
be asked are:
=
4
x
=
for loci which are
heterozygous).
Whether all loci have equal effect on the quantitative
trial expression or whether there are some of the loci
that have major effects while others have minor effects
•
In general it is actually quite straightforward to take
into account other genetic phenomena by inclusion of
appropriate parameters in the basic additive-dominant
model of inheritance and thus increasingly account for
more complex genetic inheritance.
Although you should be aware of the existence of
these complications, they will not be taken any fur-
ther in this topic. Moreover, it is often found that, for
most metrical characters of interest to plant breeders
the additive-dominant model is adequate - if it fails
we are then aware that the situation is more complex
and act accordingly. Also, since what is of primary
practical interest is the ratio of the additive genetic vari-
ance in a generation to the variance attributable to all
causes (environmental, additive, dominant and all other
genetic phenomena), it is often unnecessary to itemize
them individually.
Whether the multiple loci are distributed evenly
throughout the genome, or whether they are clus-
tered on specific chromosomes, or in specific regions
of the genome ('hot spots')
•
The concept of quantitative trait loci (QTL) was first
raised by Sax in 1923. Sax reported examining yield
on a segregating F
2
progeny from a cross between two
homozygous common bean (
Phaseolus vulgaris
) lines.
One parent was homozygous for coloured seed while the
other had white seed. A single gene at the P-locus deter-
mined seed colour, with
PP
alleles for coloured seed and
pp
for white seed. On inspection of seed weights, Sax
found that
PP
lines produced seeds with an average
weight of 30.7 g/100 seeds, heterozygotes (
Pp
)pro-
duced seed with 28.3 g/100 seeds, while
pp
lines had
