1. The Law of Independent Assortment
Dihybrid Crosses
The Law of Independent Assortment

2. Independent Assortment
The inheritance pattern of one trait will not affect the inheritance pattern of another
Independent assortment occurs during meiosis
For a single human gamete, the possible ways chromosomes may assort is astounding:
223 = 8,388,608 possible combinations

3. Dihybrid Cross
In snapdragons, tallness (T) is dominant to dwarfness(t), while red color is due to gene (R) and white to its corresponding allele (r). The heterozygous condition results in pink (Rr) flower color. A dwarf pink snapdragon is crossed with a plant homozygous for tallness and red flowers. Give the possible genotypes and corresponding phenotypes for all of the possible F1 generation.

4. Step #1: Determine Genotypes of Parents
“A dwarf pink snapdragon is crossed with a plant homozygous for tallness and red flowers. “
Dwarf pink = ttRr
Homozygous tall, red = TTRR

5. Step #2: Determine Genotypes of Gametes
“A dwarf pink snapdragon is crossed with a plant homozygous for tallness and red flowers. “
Dwarf pink = ttRr tR tr tR tr
Homozygous tall, red = TTRR TR TR TR TR

6. Gametes from TALL red parent
Step #3: Punnett Square
The Punnett square determines the genotypes of the offspring
Gametes from TALL red parent TR TR TR TR tR
TtRR
TtRR
TtRR
TtRR tr
TtRr
TtRr
TtRr
TtRr
Gametes from dwarf pink parent tR
TtRR
TtRR
TtRR
TtRR tr
TtRr
TtRr
TtRr
TtRr

7. Step #4: Identify Phenotype Ratios in Offspring
Tall, red = 8/16
Tall, pink = 8/16
Tall, white = 0/16
Short, red = 0/16
Short, pink = 0/16
Short, white = 0/16
TtRR
TtRR
TtRR
TtRR
TtRr
TtRr
TtRr
TtRr
TtRR
TtRR
TtRR
TtRR
TtRr
TtRr
TtRr
TtRr