Dihybrid Crosses
A monohybrid cross involves only one
trait. This is what we have been looking
at so far. Ex. Only dimples, OR only
tongue rolling ability.
A dihybrid cross involves two traits. Also
known as two-factor cross.
Ex. dimples AND tongue rolling ability.
• Dihybrid crosses illustrate Mendel’s Principle
of Independent Assortment.
• The Principle of Independent Assortment
states that genes for different traits segregate
independently during the formation of
gametes.
• New combinations of gametes are formed,
creating genetic variety.
Meiosis creates genetic variety in
3 ways******
1. Crossing over
2. Independent assortment
3. creating gametes for fertilization –
ultimately combining the genes of 2
organisms
Example
Ex. Cross a male heterozygous for dimples
AND tongue rolling ability with a female
of the same genotype.
Presence of dimples:
D= dimples
d= no dimples
Tongue Rolling ability:
A =able to roll a= not able
STEP 1: WRITE EQUATION
father DdAa x mother DdAa
STEP 2: GENERATE GAMETES
STEP 2: GENERATE GAMETES using the FOIL
method for each parent. Make sure you get
one allele for each trait in your gamete (D
and A). You should always have 4 gametes,
even if they repeat.
FOIL = First, Outside, Inside, Last = remember
from math?
STEP 2: GENERATE GAMETES
FOIL = First, Outside, Inside, Last
moms gametes for DdAa
First: DA Outside: Da Inside: dA
Last: da
dad’s gametes for DdAa (the same as mom this
case)
DA
Da
dA
da
***STEP 2 Illustrates independent assortment, as D
allele has the same chance of being in a gamete
with A or a. (DA or Da equally likely).
STEP 3: 16 Box Punnett
STEP 3: DRAW a 16 box Punnett Square. COMPLETE the
Punnett Square as normal. Gametes on the outside,
offspring on the inside. Hint: Make sure you list the
dominant allele first, and the same trait first for every box.
STEP 4: DRAW A TABLE
• LIST the possible phenotypes – there will always be 4
• Assign them each a COLOR. Color the the box on the
genotypes that produce phenotypic
table. Possible Phenotypes
the phenotype
frequency
Dimples,
DDAA
9
Able to Roll
Dimples,
unable to roll
DDAa
DdAA
DdAa
DDaa
Ddaa
no dimples,
able to roll
ddAA
ddAa
3
no dimples,
unable to roll
ddaa
1
3
• LIST all the genotypes that would
produce that phenotype.
Possible Phenotypes
genotypes that produce
the phenotype
phenotypic
frequency
Dimples,
Able to Roll
9
Dimples,
unable to roll
DDAA
DDAa
DdAA
DdAa
DDaa
Ddaa
no dimples,
able to roll
ddAA
ddAa
3
no dimples,
unable to roll
ddaa
1
3
COLOR the genotypes the correct color
on the Punnett square.
DA
Da
dA
da
DA
DDAA
DDAa
DdAA
DdAa
Da
DDAa
DDaa
DdAa
Ddaa
dA
DdAA
DdAa
ddAA
ddAa
da
DdAa
Ddaa
ddAa
ddaa
• COUNT the number of boxes you have
colored in the Punnett.
• RECORD this in the phenotypic frequency
column.
Possible Phenotypes genotypes that
produce the
phenotype
Dimples,
Able to Roll
Dimples,
unable to roll
no dimples,
able to roll
no dimples,
unable to roll
DDAA
DDAa
DdAA
DdAa
DDaa
Ddaa
ddAA
ddAa
ddaa
phenotypic
frequency
9
3
3
1
STEP 5
• STEP 5: RECORD the phenotypic frequency as
a ratio. Make sure your numbers add up to
16!!!!
phenotypic ratio
9 dimples, able to roll :
3 dimples, unable to roll :
3 no dimples, able to roll :
1 no dimples, unable to roll
Independent Assortment occurs in meiosis I.
• The genes for different traits are on different
chromosomes.
• Homologous pair for tongue rolling lines up
independently of the homologous pair for
dimples.
• Two different alignments can occur, which
produce different combinations of gametes.
D
d
A
a
OR
D
d
a
A
D
A
D
A
d
D
a
d a
Gametes DA and da
a
d
A
D
a
D
a
d
A
D
a d
A d
Gametes Da and dA
A
• Exception: Genes on the same
chromosome do not sort independently.
They are linked genes. (Of course they do
not sort independently. They are
physically attached!)