Two Trait Crosses:
The Dihybrid Cross
Essential Questions:
How are dihybrid crosses different from monohybrid crosses?
What Mendelian Conclusion explains the possibility of Dihybrid crosses?
How do you set up a Dihybrid cross?
What is a Dihybrid Cross?

Determines the
probability of genetic
outcomes for TWO
traits instead of ONE.

Single Trait cross are
called MONOHYBRID
crosses!

What 2 traits are being
considered here?

Coat Colour

Tail Length
How are Dihybrid Crosses
Possible?

Mendel Concluded that due
to the Law of Independent
Assortment, different traits
are not linked and have
equal probability of showing
up in offspring.
Example: Tall Purple plants,
Tall White plant, short purple
plants and short white plants.
Keep in mind, dominance will
skew the ratios, but all of these
outcomes are POSSIBLE!

Mendel’s Laws of Heredity
 Law of independent assortment:

each pair of alleles segregates into
gametes independently
 4 sets of gametes are produced in
equal probability
 YR, Yr, yR, yr
 Only true for genes on separate
chromosomes!
 How come?
How to’s of Dihybrid Crosses
•
1. Figure out the alleles:
–
Identify what trait/letter is Dominant (B – Black fur)
–
Identify what trait/letter is Recessive (b – Brown fur)
•
2. Draw your box (16 squares for dihybrids!)
•
3. Determine the Possible gametes (sex cells) that could be made from the parents.
•
•
–
You should have 4 combinations (For AaBb: AB, Ab, aB, & ab)
–
The letters should be all different for each combination! (Yr or Ab)
4. Label each side of Box, Plug & Chug!
–
Put the same letters together again (AABb)
–
Make sure to put dominant alleles First! (AaBb)
5. Determine your possible Genotypes! (1/16 bbrr, etc)
–
•
Double check your work, all the possible genotypes should add up to 16!
6. Determine your possible Phenotypes! (1/16 brown wrinkled, etc)
–
Double check your work, all the possible phenotypes should add up to 16!
Dihybrid cross
P
true-breeding
yellow, round peas
Y = yellow
R = round
true-breeding
green, wrinkled peas
x
YYRR
yyrr
y = green
r = wrinkled
All HYBRIDS!
yellow, round peas
F1
100%
generation
(hybrids)
YyRr
self-pollinate
F2
generation
Dominant/
Dominant
9/16
yellow
round
peas
Recessive
/Dominant
Dominant/
Recessive
Recessive/
Recessive
3/16
green
round
peas
3/16
yellow
wrinkled
peas
1/16
green
wrinkled
peas
Mendel’s
Magic
Ratio of
Hybrids!
9:3:3:1
Your Turn!
Dihybrid cross of color and seed shape
YyRr x YyRr
YR
Yr
yR
yr
YR
YYRR
YYRr
YyRR
YyRr
Yr
YYRr
YYrr
YyRr
Yyrr
yR
YyRR
YyRr
yyRR
yyRr
yr
YyRr
Yyrr
yyRr
yyrr
9/16
yellow
round
3/16
green
round
3/16
yellow
wrinkled
1/16
green
wrinkled