Agriculture Reference
In-Depth Information
which, when converted to cM's is:
If the additive-dominance model does not
explain the variation found for plant height, give
three reasons for what might be the cause?
(3) Explain (using
A
as a dominant allele and
a
as
a recessive allele) the difference between geno-
types that are nulliplex, simplex, duplex, triplex
and quadruplex for a single dominant gene in a
tetraploid. Assuming no complications such as
double reduction, what would be the expected
ratio of nulliplex, simplex, duplex, triplex and
quadruplex resulting from a cross between two
auto-tetraploid lines that are duplex for a single
gene (i.e.
AAaa
A - - - 47.1 cM - - - B - - 1.5 cM - - QTL - - - 21.0 cM - - - C
In this simple case the QTL and the B-bands are
tightly linked and therefore selection based on the
B-band would be highly effective in selecting for the
QTL, and hence high seed yield. Actual examples in
plant breeding, however, are rarely this close. The close
proximity of the QTL to the B-band is also a reflection
of the high recombination frequencies (low linkage)
between the three bands in this simple example. If the
recombination frequencies between A-B and B-C were
halved (i.e. 15.7% and 9.0%) the QTL would have a
recombination of 3% with the B-band position. Sim-
ilarly, this example looked at only three bands on a
single chromosome, in real situations, many chromo-
somes will be involved and more loci examined on each
chromosome. However, the underlying theory is the
same.
AAaa
).
(4) Explain the meaning of a '
test cross
' as applied
to testing linkage disequilibrium in qualitative
genetics.
Two homozygous barley genotypes are chosen
for a linkage study. One of the parents had a long
awn and was short in stature. The other parent
had a short awn and was tall in stature. Long awn
is controlled by a single dominant gene (
AA
×
=
Long awn is dominant over
aa
) and plant height is
controlled by a single dominant gene (
TT
THINK QUESTIONS
=
Tall
is completely dominant over
tt
). The two lines are
crossed (i.e.
ttAA
(1) Explain what may cause a departure from a
9:3:3:1 expected frequency of phenotypes.
Describe an appropriate statistical test to
prove
your hypothesis.
(2) Past researchers have shown that an additive-
dominance model can explain the inheritance of
plant height in spring canola (
Brassica napus
L.).
Below are shown family means, the standard errors
(s.e.) of the mean and the number of plants that
these data are estimated from
×
TTaa
) and the resulting F
1
is
test crossed to a homozygous genotype with short
awn and short stature (
ttaa
). 4000 plants from the
test cross are grown and the following phenotypic
frequencies were observed:
Tall and Long awn
(
)
=
TA
396
Short and Long awn
(
tA
)
=
1610
P
1
,
P
2
,
F
1
and
F
2
Tall and Short awn
(
Ta
)
=
1590
families.
404
Determine the recombination percentage.
(5) Two homozygous squash plants were hybridized
and an F
1
family produced. One parent has long,
green fruit (
LLGG
), and the other had round, yel-
low fruit (
llgg
). 1600 F
2
progeny were examined
from selfing the F
1
s and the following numbers of
phenotypes observed:
Short and Short awn
(
ta
)
=
Family
Mean
s.e. of mean
Number
of plants
P
1
40.2
0.142
31
P
2
19.3
0.151
31
F
1
35.4
0.099
31
F
2
28.7
0.462
31
LLGG
L
-
gg
llG
-
llgg
Using an appropriate statistical test determine
if the interpretation of past researchers hold for
these phenotypes.
891
312
0
397
