Agriculture Reference
In-Depth Information
segregate for the
Tt
locus and will only segregate for the
Ss
locus. Similarly, F
2
plants with a genotype of
TtSs
will segregate for both loci, with the same segregation
frequencies as the F
1
.
From this we have:
This would result in the same genotypic and phe-
notypic frequencies as shown above. It is, however,
necessary to be familiar with the more direct method for
situations where segregation is not independent (i.e. in
cases of linkage).
It should be noted, as in the single gene case,
increased selfing results in increased homozygosity.
Therefore with increased generations of selfing there
is an increase in the frequency of expression of recessive
traits. If, in this case, a breeder wished to retain the
6-row short plant type, then it would be expected that
3/16 (approximately 19%) of all F
2
plants will be of this
type (
ttS
_), and only 1/3 of these would at this stage be
homozygous for both traits. If selection were delayed
until the F
3
generation, then 15/64 (or just over 23%)
of the population would be of the desired type, and
now 60% of the selections would be homozygous for
both genes. Obviously, if selection is delayed until after
an infinite amount of selfing, then the frequency of the
desired types would be 25% (and all homozygous).
F
3
F
2
TTSS TTSs TTss TtSS
TtSs
Ttss
ttSS
ttSs
ttss
Total
4/64
8/64 4/64 8/64 16/64 8/64 4/64 8/64 4/64
TTss
4
2
−
2
1
−−−−
9
TTSs
−
4
−−
2
−−−−
6
TTss
−
24
−
1
2
−−−
9
TtSS
−−−
4
2
−−−−
6
TtSs
−−−−
4
−−−−
4
Ttss
−−−−
2
4
−−−
6
ttSS
−−−
2
1
−
42
−
9
ttSs
−−−−
2
−−
4
−
6
ttss
−−−−
1
2
−
24
9
64
Summing over rows, this results in a genotypic
segregation ratio of 9/64
TTSS
: 6/64
TTSs
: 9/64
TTss
: 6/64
TtSS
: 4/64
TtSs
: 6/64
Ttss
: 9/64
ttSS
: 6/64
ttSs
: 9/64
ttss
.
The phenotypic expectation at the F
3
would be:
T_S_
Qualitative linkage
The principle of independent assortment of alleles at
different loci is one of the corner stones on which an
understanding of qualitative genetics is based. Indepen-
dent assortment of alleles does not, however, always
occur. When certain different allelic pairs are involved
in crosses, deviation from independent assortment
regularly occurs if the loci are located on the same chro-
mosome. This will mean that there will be a tendency
of parental combinations to remain together, which is
expressed in the relative frequency of new combinations,
and is the phenomenon of
linkage
.
It is often desirable to have knowledge of linkage
in plant breeding to help predict segregation patterns
in various generations and to help in selection deci-
sions. All genetic linkages can be broken by successive
rounds of sexual reproduction, although it may take
many attempts of recombination before tight linkages
are broken. In general, however, if two traits of interest
are adversely linked (i.e. they appear jointly with lower
than expected frequency) then increased opportunities
for recombination need to take place before selection.
The ratio of F
2
progeny after selfing F
1
individuals
(i.e. say
AaBb
), of equal numbers of gametes of the four
possible genotypic combinations (
AB
,
Ab
,
aB
,
ab
), leads
=
/
=
25
64
Tall and 6-row
=
/
=
T_ss
15
64
Tall and 2-row
=
/
=
ttS_
15
64
short and 6-row
short and 2-row
This result could be obtained in a simpler manner by
considering the segregation ratio of each trait separately
and using these to form a Punnett square. For example,
the frequency of homozygous tall (
TT
), heterozygous
tall (
Tt
) and homozygous short (
tt
) at the F
2
is1:2:1,
respectively. Similarly, the frequency of
SS
,
Ss
and
ss
is
also 1:2:1. We can use these frequencies to construct
a Punnett square such as:
ttss
=
9
/
64
=
−
/
−
/
−
/
Gametes
TT
1
4
Tt
1
2
tt
1
4
SS
−
1
/
4
1/16
TTCC
2/16
TtCC
1/16
ttCC
Ss
−
1
/
2
2/16
TTCc
4/16
TtCc
2/16
ttCc
ss
−
1
/
4
1/16
TTcc
2/16
Ttcc
1/16
ttcc
