1. Objectives
1. Define Dihybrid Inheritance and Law of
Independent Assortment.
2. How did Gregor Mendel conduct his
experiments which led to the formulation of
the 2nd law?
3. How does one solve a problem on dihybrid
inheritance using the punnet square?

2. Law of Independent Assortment
• States those alleles of different genes assort
independently of another during gamete
formation.
• One gene determines flower color, a second
gene determines the length of stem, a third
gene determines shape of pods, and so on.

3. Dihybrid Inheritance
• Refers to the simultaneous inheritance of two
characters
• For example, Mendel crossed plants that
varied in both seed shape and seed color

4. Mendel’s Experiment
• Mendel investigated the inheritance of seed
shape (round or wrinkled) and seed color (green
or yellow) at the same time.
• From his monohybrid crosses he knew that round
seeds were dominant to wrinkled seeds and
yellow seeds were dominant to green seeds.
• He chose to cross plant that were pure breeding
for both dominant features (round and yellow
seeds) with plants that were pure breeding for
both recessive features (wrinkled and green
seeds).

5. Problem Solving for Dihybrid Cross
1. Predicting the genotype of offspring
• Determine all possible combinations of alleles
in the gametes for each parent.
• Half of the gametes get a dominant S and a
dominant Y allele; the other half of the
gametes gets a recessive s and a recessive y
allele.
• Both parents produce 25% each of SY, Sy, sY,
and sy.

6. 2. Punnett Square
• 4x4 square panel
• Since each Parent produces 4 different
combinations of alleles in the gametes, draw a
4 square by 4 square punnett square.

7. 3. Gametes from Parent 1
• List the gametes for Parent 1along one edge of
the punnett square.

8. 4. Gametes from Parent 2
• List the gametes for Parent 2along one edge of
the punnett square.

9. 5. Alleles from Parent 1
• Fill out the squares with the alleles of Parent
1.

10. 6. Alleles from Parent 2
• Fill out the squares with the alleles
from Parent 2.
• The result is the prediction of all possible
combinations of genotypes for the offspring
of the dihybrid cross, SsYy x SsYy.

11. Predicting the phenotype of offspring
• Spherical, yellow phenotype
- There are 9 genotypes for spherical, yellow
seeded plants. They are:
- SSYY (1/16)
SSYy (2/16)
SsYY (2/16)
SsYy (4/16)

12. •Spherical, green phenotype
- Two recessive alleles result in green seeded
plants.
- There are 2 genotypes for spherical, green
seeded plants. They are:
- SSyy (1/16)
Ssyy (2/16)

13. Dented, yellow phenotype
- Two recessive s alleles result in dented
seeded plants.
- There are 2 genotypes for dented, yellow
seeded plants. They are:
- ssYY (1/16)
ssYy (2/16)

14. Dented, green phenotype
- A ssyy plant would be recessive for both
traits.
- There is only 1 genotypes for dented, green
seeded plants. It is:
ssyy (1/16)