Definition
• Crosses looking at two traits simultaneously
• Mendels law of segregation states that
• Mendels law of independent assortment
states that
An example: Parents
• Wrinkled peas = r
• Round peas = R
• Green peas = y
• Yellow peas = Y
Cross 2 pure breeding parents. One is
homozygous dominant for both traits, the
other homozygous recessive for both. The
parental genotypes are:
RRYY (round yellow) x rryy (wrinkled green)
For parent RRYY what are all the possible
combinations of alleles in the gametes?
For parent rryy what are all the possible
combinations of alleles in the gametes?
RRYY (round yellow) x rryy (wrinkled green)
gametes
RY
RY
RY
RY
ry
ry
ry
ry
• The Punnett grid needs to be 4x 4 since
each parent produces 4 combinations of
gametes
• All offspring are:
RrYy is called a dihybrid
• What are all the possible combinations of
alleles in the gametes of a dihybrid?
RrYy
Create a Punnett grid
To show a cross between two dihybrids, RrYr
(round yellow) and RrYr (round yellow):
Another example
The phenotypes of two independent
traits show a 9:3:3:1 ratio in the F2
generation. In this example, coat color
is indicated by B (brown, dominant) or
b (white) while tail length is indicated
by S (short, dominant) or s (long).
When parents are homozygous for
each trait ('SSbb and ssBB), their
children in the F1 generation are
heterozygous at both loci and only
show the dominant phenotypes. If the
children mate with each other, in the F2
generation all combination of coat
color and tail length occur: 9 are
brown/short (purple boxes), 3 are
white/short (pink boxes), 3 are
brown/long (blue boxes) and 1 is
white/long (green box)
THE DIHYBRID CROSS
Studying the inheritance of two
characters simultaneously
Mendel’s peas
Character
Seed shape
Cotyledon colour
Trait
Allele
Round
R
Wrinkled
r
Yellow
Y
Green
y
Combinations
Genotype
RRYY
RRYy
RrYY
RrYy
RRyy
Rryy
rrYY
rrYy
rryy
Phenotype
Round Yellow
Round Yellow
Round Yellow
Round Yellow
Round Green
Round Green
Wrinkled Yellow
Wrinkled Yellow
Wrinkled Green
Is the inheritance of one character affected by the
inheritance of another?
P Phenotypes
Round
Yellow
F1 Phenotypes
x
Wrinkled
Green
All Round
Yellow
(Pure Bred)
(Selfed)
F2 Phenotypes
Seed colour
Yellow
Green
TOTAL
Seed
Round
315
108
423
shape
Wrinkled
101
32
133
TOTAL
416
140
556
RATIO
RATIO
Is the inheritance of one character
affected by the inheritance of another?
P Phenotypes
Round
Yellow
F1 Phenotypes
x
Wrinkled
Green
All Round
Yellow
(Pure Bred)
(Selfed)
F2 Phenotypes
Seed colour
Yellow
Green
TOTAL
RATIO
Seed
Round
315
108
423
3.18
shape
Wrinkled
101
32
133
1
TOTAL
416
140
556
RATIO
2.97
1
© 2007 Paul Billiet ODWS
A dihybrid cross can be treated as two separate
monohybrid crosses
The expected probability of each type of
seed can be calculated:
•
•
•
•
Probability of an F2 seed being round = 75% or ¾
Probability of an F2 seed being wrinkled =
Probability of an F2 seed being yellow =
Probability of an F2 seed being green =
A dihybrid cross can be treated as two
separate monohybrid crosses
The expected probability of each type
of seed can be calculated:




Probability of an F2 seed being round = 75% or ¾
Probability of an F2 seed being wrinkled = 25% or ¼
Probability of an F2 seed being yellow = 75% or ¾
Probability of an F2 seed being green = 25% or ¼
Therefore

Probability of an F2 seed being round and yellow
= ¾ x ¾ = 9/16 =
56.25%

Probability of an F2 seed being round and green
=

Probability of an F2 seed being wrinkled and yellow
=

Probability of an F2 seed being wrinkled and green
=
Therefore
• Probability of an F2 seed being round and yellow
= ¾ x ¾ = 9/16 =
56.25%
• Probability of an F2 seed being round and green
= ¾ x ¼ = 3/16 =
18.75%
• Probability of an F2 seed being wrinkled and yellow
= ¼ x ¾ = 3/16 =
18.75%
• Probability of an F2 seed being wrinkled and green
= ¼ x ¼ = 1/16 =
6.25%
Predicting how many seeds we could expect to
get in a sample
556 x 9/16 round yellow
We could
expect
What Mendel
observed
313
315
556 x 3/16 round green
108
556 x 3/16 wrinkled yellow
101
556 x 1/16 wrinkled green
32
Predicting how many seeds we could expect to get
in a sample
We could
expect
What Mendel
observed
556 x 9/16 round yellow
313
315
556 x 3/16 round green
104
108
556 x 3/16 wrinkled yellow
104
101
556 x 1/16 wrinkled green
35
32
THE LAW OF INDEPENDENT ASSORTMENT
• It appears that the inheritance of seed shape
has no influence over the inheritance of seed
colour
• The two characters are inherited
INDEPENDENTLY
• The pairs of alleles that control these two
characters assort themselves independently

The pairs of chromosomes could orientate
in different ways at metaphase 1 –
random assortment
Gamete production is able to generate
a wide variety due to:
• Crossing over during prophase I
• Random orientation during metaphase I
Dihybrid cross genetic diagram
P
Phenotypes Round Yellow
seed
Genotypes
Gametes
X
Wrinkled
Green seed
RRYY
rryy
meiosis
meiosis
RY
ry
(Pure bred)
fertilisation
F1 Phenotypes
RrYy
Genotypes
Round Yellow
Proportions
100%
(Selfed)
Dihybrid cross genetic diagram
F1 Phenotypes
RrYy
Genotypes
Round Yellow
Proportions
100%
(Selfed)
meiosis
Gametes
Y
y
R
RY
Ry
r
rY
ry
Dihybrid cross genetic diagram
Gametes
Y
y
R
RY
Ry
r
rY
ry
fertilisation
F2
Genotypes
RY
Ry
rY
ry
RY
RRYY
RRYy
RrYY
RrYy
Ry
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
Dihybrid cross proportions
Phenotypes
Round Yellow
Round Green
Wrinkled Yellow
Wrinkled Green
Proportions
9/16 or 56.25%
Dihybrid cross proportions
Phenotypes
Round Yellow
Proportions
9/16 or 56.25%
Round Green
3/16 or 18.75%
Wrinkled Yellow
3/16 or 18.75%
Wrinkled Green
1/16 or 6.25%
Dihybrid test cross
• In monohybrid crosses, to know if a
dominant trait is homozygous (RR) or
heterozygous (Rr) it is necessary to carry out
a test cross
• This is done with a homozygous recessive
(rr) individual
• The same is true for a dihybrid cross where
the test cross is made with an individual
which is homozygous recessive for both
characters (rryy)
Dihybrid test cross
Phenotypes
Genotypes
Round Yellow
RrYy
Gametes
RY, Ry, rY, ry
Genotypes
ry
Phenotypes
X
Wrinkled Green
rryy
ry
RY
Ry
rY
ry
RrYy
Rryy
rrYy
rryy
Round
Round
Wrinkled
Wrinkled
Yellow
Green
Yellow
Green
Proportions 1/4 or 25% 1/4 or 25% 1/4 or 25% 1/4 or 25%