Dihybrid Crosses
Biology 30
Mrs. S. Pipke-Painchaud
Dihybrid Crosses
 the natural progression for Mendel was to
study 2 characteristics at the same time.
 thus, the study of 2 pairs of contrasting
traits at the same time = a dihybrid cross
 ex.. round yellow seeds X wrinkled green seeds
Mendel’s Fourth Law
 The Law of Independent Assortment
 During gamete formation, segregating pairs of
unit factors assort independently of each other.
 the two traits are inherited totally independently of
each other.
 i.e. color is inherited independently of seed shape.
 Example:
 Homozygous round green X wrinkled homozygous yellow
Solving Dihybrid Crosses
 Follow the steps
 If you were to cross a Homozygous
round green X wrinkled homozygous
yellow plant, what would your phenotypic
and genotypic ratios be?
Step 1: Make a Key….




G= yellow
g = green
W = round
w = wrinkled
Step 2: Give the genotypes
of the parents. . .
 Homozygous round green X wrinkled
homozygous yellow plant
 P1 = WWgg X wwGG
Step 3:
Determine the Gametes . . .

WWgg X wwGG
WWgg = Wg, Wg, Wg, Wg
wwGG = wG, wG, wG, wG
HINT: Foil to get the gametes
Step 4:
Fill in the Punnett square . . .
Wg
Wg
Wg
Wg
wG
WwGg
WwGg
WwGg
WwGg
wG
WwGg
WwGg
WwGg
WwGg
wG
WwGg
WwGg
WwGg
WwGg
wG
WwGg
WwGg
WwGg
WwGg
Step 5:
Answer any questions . . .
 Genotypic ratio
 = all WwGg
 Phenotypic ratio
 = all round yellow
G= yellow
g = green
W = round
w = wrinkled
F1 X F1
= WwGg X WwGg
 Remember to determine the gametes for your
punnett square.
Fill in the Punnett Square
WG
Wg
wG
wg
WG
WWGG
WWGg
WwGG
WwGg
Wg
WWGg
WWgg
WwGg
Wwgg
wG
WwGG
WwGg
wwGG
wwGg
wg
WwGg
Wwgg
wwGg
wwgg
The PATTERN . . .
WG
Wg
wG
wg
WG
WWGG
WWGg
WwGG
WwGg
Wg
WWGg
WWgg
WwGg
Wwgg
wG
WwGG
WwGg
wwGG
wwGg
Wg
WwGg
Wwgg
wwGg
wwgg
What are the ratios?
 Genotypic Ratio:









1 WWGG
2 WWGg
1 WWgg
2 WwGG
4 WwGg
2 Wwgg
1 wwGG
2 wwGg
1 wwgg
WG
Wg
wG
wg
WG
WWGG
WWGg
WwGG
WwGg
Wg
WWGg
WWgg
WwGg
Wwgg
wG
WwGG
WwGg
wwGG
wwGg
Wg
WwGg
Wwgg
wwGg
wwgg
Phenotypic Ratio:
 Genotypic Ratio:









1 WWGG
2 WWGg
1 WWgg
2 WwGG
4 WwGg
2 Wwgg
1 wwGG
2 wwGg
1 wwgg
** Explain the patterns
G= yellow
g = green
W = round
w = wrinkled
 Phenotypic Ratio:
 9 Yellow Round
 Dominant Dominant
 3 Yellow Wrinkled
 Dominant Recessive
 3 Green Round
 Recessive Dominant
 1 Green Wrinkled
 Recessive Recessive
The Short Cut:
 can only be used if you have
 (Big little Big little X Big little Big little)
 BbTt x BbTt
 Do these fit?
 BBTt X BbTt
 BbTt x Bb tt
Try this:
C= Inflated Pod
c = constricted pod
D = Tall
d = dwarf
 The cross: a plant heterozygous for inflated
pod and heterozygous tall is crossed with a
another heterozygous inflated pod and
heterozygous tall plant. What are the
genotypic and phenotypic ratios?
 The parents are:
 CcDd x CcDd
C= Inflated Pod
c = constricted pod
D = Tall
d = dwarf
Example:
 CcDd x CcDd
 Does this fit the
short cut?
 Yes
Genotypic Ratio:
Phenotypic Ratio:
1 CCDD
2 CCDd
(D)
9 Inflated Tall
1 CCdd
2 CcDD
(D)
®
1 ccdd
(D)
3 Constricted Tall
1 ccDD
2 ccDd
®
3 Inflated Dwarf
4 CcDd
2 Ccdd
(D)
®
1
®
Constricted Dwarf
Now try . . .
 Refer to the assigned questions in your
duotang.