1. 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?

2. 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

3. 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!

4. 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?

5. 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!

6. Dihybrid cross P YYRR yyrr 100% F1 YyRr 9:3:3:1 F2 x true-breeding
yellow, round peas
true-breeding
green, wrinkled peas x
YYRR
yyrr
Y = yellow
R = round
y = green
r = wrinkled
All HYBRIDS!
100% F1
generation
(hybrids)‏
yellow, round peas
YyRr
Wrinkled seeds in pea plants with two copies of the recessive allele are due to the accumulation of monosaccharides and excess water in seeds because of the lack of a key enzyme. The seeds wrinkle when they dry.
Both homozygous dominants and heterozygotes produce enough enzyme to convert all the monosaccharides into starch and form smooth seeds when they dry.
Mendel’s Magic Ratio of Hybrids!
Dominant/Dominant
Dominant/Recessive
Recessive/Recessive
self-pollinate
Recessive/Dominant
9/16
yellow
round
peas
9:3:3:1
3/16
green
wrinkled
1/16 F2
generation

7. Your Turn! Dihybrid cross of color and seed shape
YyRr x
YyRr
9/16
yellow
round YR Yr yR yr YR Yr yR yr
3/16
green
round
YYRR
YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
3/16
yellow
wrinkled
YyRR
YyRr
yyRR
yyRr
1/16
green
wrinkled
YyRr
Yyrr
yyRr
yyrr