1. Two Trait Crosses: The Dihybrid Cross

2. What is a Dihybrid Cross?
Determines the probability of genetic outcomes for TWO traits instead of ONE.

3. How are Dihybrid Crosses Possible?
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.

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

5. How to’s of Dihybrid Crosses
1. Figure out the alleles:
2. Draw your box (16 squares for dihybrids!)
3. Determine the Possible gametes (sex cells) that could be made from the parents.
4. Label each side of Box,
5. Determine your possible Genotypes!
6. Determine your possible Phenotypes!

6. Practice
What are the chances of getting a Tall Green Plant when a Tall Green and a Tall Yellow Pea pant are crossed.
(TtGG x Ttgg)

7. 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

8. 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