Agriculture Reference
In-Depth Information
(9) A new barley disease (yellow stripe rust, YSR)
has recently been identified in the Palouse region.
A single, completely dominant gene has been
found which confers complete and durable resis-
tance to this disease. A successful barley variety
should be dwarf (controlled by a single recessive
gene) and be shatter resistant (controlled by a
single recessive gene). Using the notation
Y
to
indicate a single YSR resistance gene and
y
to be a
susceptible recessive gene,
T
to be a single domi-
nant tall gene and
t
to be a single dwarf gene,
S
to
be a shatter susceptible gene and
s
to be a recessive
resistant gene. Two crosses were examined:
(10) Both parents (P
1
and P
2
), the F
1
and F
2
from a
cross between two homozygous wheat lines were
grown in a replicated field trial and the yield of a
number of individual plants was recorded. From
these data the total yield of all plants measured
(
x
2
) and the number of
plants assessed (
n
) are shown below:
x
), sum of squares (
P
1
P
2
F
1
F
2
x
324
166
305
591
x
2
10 000
30 000
9 100
21 678
(
)
=
×
(
)
=
×
1
YYTT
yytt
and
2
YYSS
yyss
n
11
11
11
21
The F
1
family from each cross was test crossed
to the recessive parent and a number of progeny
screened. From cross (1) the following results were
found:
From the data above, determine the mean
and variance of each family and then test if the
additive-dominance model is adequate to explain
the inheritance for yield in this cross. Suggest
three reasons why the additive-dominance model
of inheritance might not be adequate to explain
the inheritance of a character.
(11) Three different F
2
populations were grown out
from a cross between a dwarf susceptible parent
and a tall resistant parent (
ttrr
=
YSR resistant and tall
3240
=
YSR resistant and short
3100
=
YSR susceptible and tall
3400
=
YSR susceptible and short
3260
(a) Given that the F
1
family was selfed to produce
an F
2
and this in turn was selfed to produce
F
3
families (without selection) what would
be the expected frequency of genotypes and
phenotypes be in the F
3
family?
(b) How many F
3
genotypes would need to be
evaluated to be 99% sure of having at least
one plant which is homozygous YSR resistant
and dwarf?
From cross (2) the following results were
obtained from the test cross (res.
×
TTRR
) and plants
assessed for dwarfism and disease resistance. The
number of plants which were observed in the four
possible phenotype classes is shown below from
three different trials:
T
_
R
_
T
_
rr
ttR
_
ttrr
(1)
8960
3002
3050
988
(2)
9041
3009
0
3950
=
resistant
(3)
0
12 001
2997
1002
and susc.
=
susceptible):
It is known that these two genes are located on
different chromosomes. Explain what could have
caused the segregation pattern in each example
(i.e. (1), (2) and (3)). Using a suitable chi-
square test, determine if the segregation pattern
for example (1) in part (a), are correct.
(12) One parent (P
1
), the F
1
and B
1
families were
grown in a replicated yield trial. From each pop-
ulation, plant height (mm) was recorded on 10
plants 35 days after planting.
=
YSR res. and shatter susc.
5000
=
YSR res. and shatter res.
1290
=
YSR susc. and shatter susc.
1310
YSR susc. and shatter res.
=
5400
(c) Given that the F
1
family was selfed to pro-
duce an F
2
(without selection) what would
be the expected frequency of genotypes and
phenotypes in the F
2
family?
The mean and
