Agriculture Reference
In-Depth Information
You note that each successive generation of selfing
reduces the proportion of heterozygotes by half (i.e. 1/2
Tt
at F
2
, 2/8
Tt
at F
3
, 2/16
Tt
at F
4
and 2/32
Tt
at F
5
)
Collecting the like genotypes we get the following
frequency of genotypes:
.
Let us now consider how these two different pairs of
alleles behave with respect to each other in inheritance.
One way to study this is to cross individuals that differ
in both characteristics:
tall, 6-row
/
/
/
1
16
TTSS
;2
16
TTSs
;1
16
TTss
/
/
/
2
16
TtSS
;
4
16
TtSs
;
2
16
Ttss
/
/
/
1
16
ttSS
;
2
16
ttSs
;
1
16
ttss
and with the frequency of phenotypes:
×
dwarf, 2-row
/
=
9
16
T
_
S
_:
tall and 6-row
(
TTSS
×
ttss
)
/
=
3
16
T
_
ss
:
tall and 2-row
When this cross is made, the F
1
shows both dominant
characteristics, tall and 6-row. Plant breeder's interests
in genetics mainly relate to selection and as no selection
takes place at the F
1
stage so the interest begins when
the self-pollinated progeny of the F
1
is considered. F
1
individuals are assumed to produce equal frequencies of
four kinds of gametes during meiosis (
TS, Ts, tS
and
ts)
.
An easy way to illustrate the possible combination of
F
2
progeny is using a
Punnett square
, where the four
gamete types from the male parent are listed in a row
along the top, and the four kinds from the female parent
are listed in a column down the left hand side. The 16
possible genotype combinations are then obtained by
filling in the square, that is:
/
=
3
16
ttS
_:
short and 6-row
/
=
1
16
ttss
short and 2-row
As with the single gene case above, if the alleles do
not show dominance then there would indeed be nine
different phenotypes in the ratio:
1
TTSS
:2
TTSs
:1
TTss
2
TtSS
:4
TtSs
:2
Ttss
1
ttSS
:2
ttSs
:1
ttss
In most cases when developing inbred cultivars, plant
breeders carry out selection based on plant phenotype
amongst early generation (F
2
,F
3
,F
4
, etc.), segregating
populations. It is obvious that 75% of the F
2
plants
will be heterozygous at one, or both, loci, and domi-
nance effects can mask the true genotypes that are to be
selected. Single F
2
plant selections for the recessive traits
(short stature and 2-row) allows for identification of the
desired genotype (
ttss
) but only 1/16th of F
2
plants will
be of this type, while the recessive expression of the trait
in most plants is not expressed due to dominance.
The effects of heterozygosity on selection can be
reduced through successive rounds of self-pollination.
Plant breeders, therefore, do not only select for sin-
gle gene characters at the F
2
stage. Consider now what
would happen if a sample of F
2
plants from the above
example were selfed, what then would be the resulting
segregation expected at the F
3
stage?
There are nine different genotypes at the F
2
stage,
TTSS, TTSs, TTss, TtSS, TtSs, Ttss, ttSS, ttSs
and
ttss
and that they occur in the ratio 1:2:1:2:4:2:1:2:1,
respectively. Obviously if any genotype is homozygous
at a locus then these plants will not segregate at that
locus. For example plants with the genetic constitu-
tion of
TTSS
will always produce plants with the
TTSS
genotype. F
2
plants with a genotype of
TTSs
will not
Gametes from
female parent
Gametes from male parent
TS
Ts
tS
ts
TS
TSTS
TSTs
TStS
TSts
Ts
TsTS
TsTs
TstS
Tsts
tS
tSTS
tSTs
tStS
tSts
ts
tsTS
tsTs
tstS
tsts
This also can be written as:
Gametes from
female parent
Gametes from male parent
TS
Ts
tS
ts
TS
TTSS
TTSs
TtSS
TtSs
Ts
TTSs
TTss
TtSs
Ttss
tS
TtSS
TtSs
ttSS
ttSs
ts
TtSs
Ttss
ttSs
ttss
