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Experiment Summary Statements
1) The mutagen applied to the truebreeding seed induced
phenotype variations in the soybeans.
2)The mutagen induced mutations (changes) in the genetic
makeup of the plant which were passed on to offspring,
and thus the unique phenotypes were inherited.
3)The genetic makeup of the M2 plants was not always the
same. Some were homozygous, others heterozygous.
4) The mutations were random. Dr. Specht did not observe
the indeterminate mutant he was hoping for, but observed
many other random mutations instead.
What is a Monohybrid Cross?
A monohybrid cross is a cross between two plants that differ by
a single trait (mono). Monohybrid crosses are done to allow
plant breeders to follow the inheritance of the single trait more
easily.
Monohybrid crosses were used on the
dwarf and soybean lesion mutants to
learn about the inheritance of these
mutant traits.
Dwarf Mutant
Lesion Mutant
Soybean
Soybean
What is a reciprocal cross?
A cross is made between plants with
two different phenotypes using one
phenotype as the male and the other
phenotype as the female.
In a reciprocal cross, a second cross
is made switching the
phenotypes of the male and
female.
Reciprocal Cross
M3 plants homozygous for the dwarf mutant traits were crossed to
homozygous normal plants and the F1 seed harvested and bagged.
Pollen
donor
Pollen
donor
X
dd
X
DD
Cross 1:
Pollen donor = Mutant
DD
dd
Cross 2:
Pollen donor = Normal
Reciprocal Cross
Crosses were also made for soybean lesion mutants.
Pollen
donor
Pollen
donor
X
gg
X
GG
Cross 1:
Pollen donor = Mutant
GG
gg
Cross 2:
Pollen donor = Normal
Why do a reciprocal cross?
Sometimes traits are passed on only
maternally (through the female) or
paternally (throught the male).
If this is true for a particular trait,
the offspring of the two crosses in a
reciprocal cross will have different
phenotypes.
This allows plant breeders to determine if a
trait is passed on in this way.
Making a Cross
Stamen and Pollen =
male part
Stigma and style =
female part
Making a Cross
Soybeans are self-pollinating.
Therefore, cross pollination must be done manually.
View soybean crossing video
Making a Cross
The anthers are removed from female flower
to prevent self pollination.
Making a Cross
Pollen taken from anther of another flower and
dusted onto stigma of female flower.
Soybean flowers
Soybean anther
Soybean flowers
Cross pollinated
flowers are labeled.
Pods will form
containing seed
from the cross.
Dwarf Cross - F1 Seed
Plant
Seed
Plant
Seed
The F1 seed was planted
keeping the reciprocal cross
seed in a separate plot.
Pollen donor = Mutant
Pollen donor = Normal
Dwarf Cross - F1 Plants
Pollen donor = Mutant
Pollen donor = Normal
The F1 plant phenotypes were
observed, and the plants allowed to self-pollinate
to produce F2 seed.
Dwarf Cross - F1 Plants
Pollen donor = Mutant
Pollen donor = Normal
All of the F2 seed was harvested
with seeds from each individual plant
placed in a separate labeled envelope.
Dwarf Cross – F2 Seed
Pollen donor = Mutant
Pollen donor = Normal
The F2 seed was planted in progeny rows.
Again the reciprocal cross seed was
planted in a separate plot.
Dwarf Cross – F2 Plants
Pollen donor = Mutant
Pollen donor = Normal
The F2 plant phenotypes were observed
and recorded, and the plants allowed to self-pollinate
to produce F3 seed.
Dwarf Cross – F2 Plants
DWARF
DWARF
Pollen donor = Mutant
Normal
DWARF
Pollen donor = Normal
All of the F3 seed was harvested from each plant and
placed in separate envelopes labeled with the phenotype
of the parent plant (Dwarf or Normal).
Dwarf Cross – F3 Seed
DWARFNormal
Normal DWARF
NormalNormal
DWARF
DWARF
Normal Normal
NormalNormal
The F3 seed was planted in progeny rows.
The reciprocal cross seed was planted in the same plot.
Dwarf Cross – F3 Seed
DWARF
Normal
NormalDWARF
Normal
Normal
DWARFDWARF
NormalNormal
Normal
Normal
The F3 seed was planted in progeny rows.
Soybean Disease Lesion Mutant
At the same time, these steps were taken
with the soybean disease lesion mutants.
Lesion Mutant Soybean
Soybean Disease Lesion Mutant Data
Normal X Mutant
F1 plants
F2 plants
F3 rows from normal F2s
F3 plants from mixed rows
Mutant X Normal
F1 plants
F2 plants
F3 rows from normal F2s
F3 plants from mixed rows
Individual Plants
Normal
Mutant
5
18
----172
0
6
----46
Individual Plants
Normal
Mutant
6
81
----178
0
27
----49
Row Type
all
mixed
normal
row
row
8
10
Row Type
all
mixed
normal
row
row
29
52
Summary of Results
1) All F1 plants had the normal phenotype
F1 Plants
Normal
Mutant
Ratio
11
0
1:0
2) In both the first cross and reciprocal cross,
- Mutant phenotype not apparent in F1’s
- Mutant phenotype did reappear in F2’s
- About ¼ of F2 plants had mutant
phenotype of dwarf or disease lesion.
F2 Plants
Normal
Mutant
Ratio
99
33
3:1
Summary of Results
3) Two types of plants had
the normal phenotype
in the F2. About 1/3 of
the F2 normals were
trubreeding while 2/3
had mixed progeny
rows.
4) Mutant F2 plants were
truebreeding and all of
their F3 progeny were
mutants.
F2
F3 rows = 1:2:1
Explaining the Results Using the
Principle of Segregation
1) The variation in the dwarf and lesion traits was controlled
by genes. Genes store the information that controls the
expression of traits. Alternative versions of genes called
alleles allow for the expression of different traits.
2) Genes are found in pairs in somatic cells that make up
the plant.
3) Paired genes separate during gamete formation and one
gene from each pair goes into a gamete.
4) Gametes fuse at random during sexual reproduction
producing the next generation seed.
Explaining the Results Using the
Principle of Segregation
Dr. Specht now had to use the data to figure out
what was happening on the gene level.
We will use the data to answer some
questions and see if we can arrive at the
same conclusions he did.
Explaining the Results Using the
Principle of Segregation
The Principle of Segregation explains what
Dr. Specht observed with his plants in the field.
He can test the validity of the principle
in explaining his results.
We will use the data to answer some
questions and see if we can arrive at the
same conclusions he did.
QUESTION?
Looking at the data for the soybean
lesion mutant, which trait was dominant?
Normal trait
Soybean lesion mutant trait
ANSWER
The normal trait is dominant.
Since there are only normal and no lesion
phenotypes in the F1, it can be assumed
normal is dominant and masked the
expression of the mutant trait (recessive).
F1 Plants
Normal
Mutant
Ratio
11
0
1:0
QUESTION?
What are the genotypes of the original
parents for the soybean lesion trait?
Use ‘A’ to represent the dominant allele and
‘a’ to represent the recessive allele.
AA X Aa
aa X Aa
AA X aa
ANSWER
The genotype of one parent is ‘AA’
and other parent is ‘aa’.
The experiment states that a cross was made
between a truebreeding normal plant and a
truebreeding soybean lesion mutant plant.
Truebreeding means both parents are homozygous
for the trait.
Therefore, neither parent could be genotype ‘Aa’.
We have determined that normal is dominant.
Therefore its genotype would be ‘AA’.
That means the lesion-type is recessive, genotype ‘aa’.
The original cross was: AA x aa
QUESTION?
What is(are) the genotype(s) of the
F1 generation for the soybean lesion trait?
Use ‘A’ to represent the dominant allele and
‘a’ to represent the recessive allele.
AA
Aa
aa
AA, Aa, & aa
ANSWER
The genotype of all the F1 offspring
is ‘Aa’
The ‘AA’ parent can
contribute only gametes
with ‘A’ alleles.
The ‘aa’ parent can
contribute only gametes
with ‘a’ alleles.
Therefore, the F1 progeny
will all have ‘Aa’ genotypes.
Normal parent
alleles
a
a
A
A
Aa
Aa
(Normal)
(Normal)
Aa
Aa
(Normal)
(Normal)
QUESTION?
What is(are) the genotype(s) of the
F2 generation for the soybean lesion trait
And in what ratios do they appear?
Use ‘A’ to represent the dominant allele and
‘a’ to represent the recessive allele.
1 AA : 2 Aa : 1 aa
1 Aa
1 AA : 1 Aa : 1 aa
1 AA : 1 aa
ANSWER
The genotypes and ratios of the F2
offspring are 1 AA : 2 Aa : 1 aa
Since the F1 plant (Aa) is
“selfed”, each parent contributes
both an ‘A’ and ‘a’ allele.
The gametes for the
Punnett square would be
50% ‘A’ and 50% ‘a’.
F2 genotypic ratios are
1 AA : 2 Aa : 1 aa
Normal F1
parent alleles
A
a
A
AA
Aa
a
Aa
aa
QUESTION?
What are the phenotypes of the
F2 generation for the soybean lesion trait
And in what ratios do they appear?
2 Normal : 1 Lesion
1 Normal : 1 Lesion
3 Normal : 1 Lesion
ANSWER
The phenotypes and ratios of the F2
offspring are 3 Normal : 1 Lesion.
The ‘A’ allele, which codes for
the normal trait, is dominant.
Therefore, the only way to
get the lesion phenotype
is with the ‘aa’ genotype.
The ¼ ‘AA’ offspring and
the ½ ‘Aa’ offspring will
both have the normal
phenotype.
Normal F1
parent alleles
A
a
A
a
AA
Aa
(Normal)
(Normal)
Aa
aa
(Normal)
(Lesion)
QUESTION?
Of the F2 generation progeny with a
normal phenotype, what is the ratio of
homozygous to heterozygous plants
for the soybean lesion trait?
1 AA : 2 Aa
1 AA : 1 Aa
1 AA : 3 Aa
ANSWER
The ratio of homozygous to heterozygous F2 normal
plants is 1 Homozygous : 2 Heterozygous.
We do not even look at the
lesion offspring when figuring
this ratio.
Of the 3 normal offspring,
1 has the ‘AA’ genotype
(homozygous).
And 2 have the ‘Aa’
genotype (heterozygous).
Normal F1
parent alleles
A
a
A
a
AA
Aa
(Normal)
(Normal)
Aa
aa
(Normal)
(Lesion)
*BONUS QUESTION?*
If you were a plant breeder, what
experiment might you perform to determine
which of the F2 normal phenotype plants
were homozygous and which were
heterozygous?
Self pollinate the plants and plant them in
progeny rows to observe the offspring.
Cross pollinate the plants and plant them
in progeny rows to observe the offspring.
ANSWER
Self pollinate the plants and plant them in
progeny rows to observe the offspring.
Self pollinating the plants would give the results
below. If lesion plants were observed in the progeny
rows, you could determine that the selfed parent had
to pass on an ‘a’ allele and was heterozygous.
A
A
A
a
A
A
AA
AA
(Normal)
(Normal)
AA
(Normal)
AA
(Normal)
A
a
AA
Aa
(Normal)
(Normal)
Aa
aa
(Normal)
(Lesion)
Summary
A monohybrid cross is used to study the inheritance
of a single gene.
A reciprocal cross is used to determine if the
trait is sex-linked.
Soybeans must be cross-pollinated by hand.
The results of the soybean lesion cross indicate that
the trait is recessive.
The results observed in progeny of the experiment
follow the Principle of Segregation.
Summary of Results
1) All F1 plants had the normal phenotype
F1 Plants
Normal
Mutant
Ratio
11
0
1:0
2) In both the first cross and reciprocal cross,
- Mutant phenotype not apparent in F1’s
- Mutant phenotype did reappear in F2’s
- About ¼ of F2 plants had mutant
phenotype of dwarf or disease lesion.
F2 Plants
Normal
Mutant
Ratio
99
33
3:1
Summary of Results
3) Two types of plants had
the normal phenotype
in the F2. About 1/3 of
the F2 normals were
trubreeding while 2/3
had mixed progeny
rows.
4) Mutant F2 plants were
truebreeding and all of
their F3 progeny were
mutants.
F2
F3 rows = 1:2:1
Explaining the Results Using the
Principle of Segregation
1) The variation in the dwarf and lesion traits was controlled
by genes. Genes store the information that controls the
expression of traits. Alternative versions of genes called
alleles allow for the expression of different traits.
2) Genes are found in pairs in somatic cells that make up
the plant.
3) Paired genes separate during gamete formation and one
gene from each pair goes into a gamete.
4) Gametes fuse at random during sexual reproduction
producing the next generation seed.