• Mendel also investigated the inheritance
of two pairs of contrasting characters
in the pea plant.
• This is referred to as the dihybrid
cross.
Pure breeding
round seeds
yellow cotyledons
F1 progeny
X
pure breeding
wrinkled seeds
green cotyledons
all round
yellow peas
• The F1 progeny were selfed:• F2 progeny
Round
round
wrinkled
wrinkled
Yellow
315
green
108
yellow
101
green
32
The ratio between the numbers of
different seeds reduces to:
9.8 : 3.38 : 3.15 : 1.0
Or about
9 : 3 : 3 : 1
• Mendel also noticed that two new
combinations not represented in the
parents appeared in the progeny
• Round or wrinkled seeds can turn up
with either green or yellow cotyledons.
• The two pairs of factors were
inherited independently.
• Either one of the contrasting
characters could be passed on to the
offspring
• Therefore the heterozygote plant (F1 –
round and yellow) must produce four
types of gamete in equal numbers
F1 phenotype
RrYy
Produces 4 different gametes
RY
Ry
rY
ry
• Use this information to draw the initial
cross and the F1 cross on your
worksheet.
• How many different phenotypes are
there and what will they be?
Answers
• CROSS 1
• Parents Round yellow X
genotype RRYY
gametes
RY
wrinkled green
rryy
ry
RY
RY
ry
RrYy
RrYy
ry
RrYy
RrYy
• F1 phenotype – all round and yellow
• F1 genotype – all RrYy (heterozygous
for both characters)
F1 generation crossed
• F1 phenotype – round yellow X
round yellow
• Genotype
X
RrYy
rY
ry
• Gametes
- RrYy
RY
Ry
RY
Ry
rY
ry
RY
RRYY
RRYy
RrYY
RrYy
Ry
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
• F2 generation
Phenotypes –
Round and yellow – 9
Round and green – 3
Wrinkled and yellow - 3
Wrinkled and green - 1
The Dihybrid Test Cross
• As with monohybrids the appearance of
the heterozygous dominant offspring
will be identical to that of the
homozygous dominant organism.
• How could they be distinguished?
• If the organism in question is
heterozygous dominant and is crossed
with a homozygous recessive individual
then the resulting progeny would show
all possible combinations of characters
in the ratio
• 1:1:1:1
• This is described as a test cross in
which the genotype of the F1 offspring
can be determined.
• Perform a genetic cross to show the
possible genotypes resulting from a test
cross for a homozygous dominant
individual (RRYY) and a heterozygote
(RrYy)