Agriculture Reference
In-Depth Information
were propagating and probably the natural tendency
of some species to self-pollinate (e.g. wheat) at a high
frequency. However, this strategy has a limited potential
and so modern plant breeders have to generate genetic
variation and hence the three phase breeding schemes
were established to
create genetic variation
,
identify
desirable recombinant lines within the progeny
and
stabilize and increase the desired genotype
. It is inter-
esting to note, however, that recently a number of plant
breeders have returned to old landraces of wheat and
barley to examine their wealth of genetic diversity as well
as to testing combinations of lines in '
modern
' landrace
combinations (i.e. multilines). Unfortunately most of
the landraces that existed even 100 years ago are no
longer available and potentially valuable germplasm and
adapted combinations have been lost.
By far the most commonly used method of gen-
erating genetic variation within inbreeding lines is
via
sexual reproduction using artificial hybridization.
There are, of course, other ways to produce variation.
For example, variation can be produced by induced
mutation, somatic variation, somatic hybridization and
recombinant DNA techniques (all discussed in later
chapters).
After producing the variation plant breeders will
then traditionally screen the segregating population for
desirable '
segregants
' while continuing to self succes-
sive generations, to produce homozygous lines. Thus,
accomplishing the last two steps of the breeding scheme
(selection and stabilization) more or less simultaneously.
selfing generations. Consider the simple case of just one
locus A-a:
Parents
AA x aa
Frequency of
heterozygotes
F
1
Aa
100%
AA
¼
Aa
aa
F
2
Frequency
½
¼
50%
F
3
AA
AA
Aa
aa
aa
Frequency
1/8
¼
1/8 ¼
25%
¼
Consider just six of the loci that are involved, as set
out below. Of these six loci two, only loci A and f have
the same allele in both parents (which are both com-
pletely homozygous) and so the F
1
is homozygous at
these two loci. At the other 4 loci the parents have dif-
ferent alleles and so the F
1
is heterozygous at these loci
and these segregate in the F
2
.
Parents
AAbbCCDDeeff
×
AABBccddEEff
F
1
AABbCcDdEeff
F
2
AABBCcDdeeff, AABbCcDDEeff,
AAbbCCEeDdff, AABBccDdEeFF, etc
This can be generalized in mathematical terms as fol-
lows. Consider an F
1
that is heterozygous at
n
loci;
heterozygosity (
h
) at any single loci after
g
generations
(
g
Homozygosity
One of the difficulties in selecting desirable recombi-
nant lines in pure-line breeding is related to segregat-
ing populations and the masking of adverse character
expression as a result of the dominance/recessive nature
of the segregating alleles in the heterozygotes. Another
consideration is the relationship between genetical
homozygosity and '
commercial inbred lines
'. The def-
inition of complete homozygosity is that all the alleles
at all loci are identical by descent, that is, there is
not heterozygosity at
any
locus. However, for practical
exploitation, the level of homozygosity does not need to
be complete. Clearly the lines must basically breed '
true
to type
' but this is by no means absolute. The degree of
homozygosity can be directly related to the number of
=
0atF
1
)ofselfing will be:
h
g
=
(
1
/
2
)
The probability (
p
) of homozygosity at
n
loci will be:
n
=
(
−
)
p
1
h
Hence after
g
generations:
g
n
=[
−
(
/
)
]
p
1
1
2
This can also be written as:
p
2
g
2
g
n
=[
(
−
1
)/
]
The level of homogeneity required in an inbred cul-
tivar will depend to a varying extent on the personal
