Agriculture Reference
In-Depth Information
smallest seed weights (26.4 g/100 seeds). From this Sax
introduced the concept that the quantitative loci deter-
mining seed weight were linked to the single gene locus
for seed colour.
The potential of expanding this concept in plant
breeding attracted the attention of many researchers
after Sax's work was published. However, few advan-
tages were achieved because plant breeders were forced
to work with mainly morphologically visible single gene
traits and major-gene mutants. These were not the most
suitable for investigating QTL's because:
The process involved in QTL's will be illustrated
using a simple, simulated, example where two homozy-
gous parents are hybridized to produce F
1
plants.
One parent was homozygous
AABBCC
at the A-, B-
and C-bands, respectively, while the other parent was
homozygous
aabbcc
. It should be noted that in this
example,
A
is not dominant to
a
, etc.
Thirty two homozygous lines were derived from
the F
1
family using double-haploidy techniques (see
Chapter 8). These lines were grown in a four replicate
field trial to determine yield of each line. In addition,
the lines were polymorphic for three loci that appeared
to be located on the same chromosome. The molecular
marker banding at the three molecular markers (iden-
tified simply as A, B and C-bands,
AA
,
BB
, and
CC
,
respectively) along with the yield of each line is shown
in Table 5.10. We use doubled haploids in this exam-
ple for simplicity as there will be no heterozygotes in
the population. This makes some of the calculations
simpler as dominance effects can be ignored. However,
the principle is the same and can be carried out using
any segregating population resulting from a two-parent
cross.
Mapping of the three qualitative loci is done accord-
ing to the method described earlier, and the map is
as follows:
•
They were relatively few in number
•
Were usually recessive, and their expression masked
in the phenotype by dominant alleles
•
Often had deleterious effects (or pleitropic effects)
on the quantitative trait of interest (i.e. albinism,
dwarfism, etc.)
These defects have been corrected by the intro-
duction of molecular markers, which tend to be
numerous, do not affect the plant phenotype, and are
often co-dominant allowing the heterozygotes to be
differentiated from the homozygotes parental types.
In plant breeding, QTL's have greatest potential in
marker assisted selection for quantitatively inherited
traits which have low heritability or that are difficult
or expensive to screen or evaluate.
µ
µ
A-------31.3 m
-------B----18m
----C
Table 5.10
Yield of 32 double haploid lines, and genotype of each line at the A-, B-, and C-bands.
Line
A-band
B-band
C-band
Yield
Line
A-band
B-band
C-band
Yield
1
AA
BB
CC
107.80
17
aa
BB
cc
112.41
2
AA
BB
CC
113.57
18
aa
bb
cc
104.93
3
AA
BB
cc
111.68
19
aa
bb
cc
104.62
4
aa
bb
CC
101.09
20
AA
BB
CC
114.68
5
aa
bb
cc
91.29
21
AA
BB
CC
110.79
6
aa
bb
cc
112.24
22
AA
bb
cc
101.47
7
aa
bb
cc
97.17
23
AA
BB
cc
116.61
8
aa
bb
cc
95.75
24
aa
bb
CC
101.95
9
aa
BB
CC
113.52
25
aa
bb
cc
106.33
10
aa
BB
CC
119.27
26
aa
bb
cc
95.42
11
AA
bb
cc
98.40
27
AA
BB
CC
121.85
12
AA
BB
CC
106.82
28
AA
BB
CC
111.94
13
AA
BB
CC
117.61
29
AA
bb
cc
105.45
14
AA
BB
CC
112.88
30
AA
bb
cc
99.15
15
AA
bb
CC
101.58
31
aa
BB
CC
116.49
16
aa
BB
CC
119.27
32
aa
bb
cc
100.21
