Dihybrid Crosses

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Slide 1

Dihybrid Crosses

The Law of Independent Assortment

Slide 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

Slide 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.

Slide 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

Slide 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

Homozygous tall, red = TTRR

tR

tr

tR

tr

TR

TR

TR

TR

Slide 6

Step #3: Punnett Square

TR

TR

TR

TR

Gametes from TALL red parent

tR

tr

tR

tr

Gametes from dwarf pink parent

TtRR

TtRr

TtRr

TtRR

TtRr

TtRR

TtRR

TtRr

TtRr

TtRr

TtRR

TtRR

TtRr

TtRr

TtRR

TtRR

The Punnett square determines the genotypes of the offspring

Slide 7

Step #4: Identify Phenotype Ratios in Offspring

TtRR

TtRr

TtRr

TtRR

TtRr

TtRR

TtRR

TtRr

TtRr

TtRr

TtRR

TtRR

TtRr

TtRr

TtRR

TtRR

Tall, red = 8/16

Tall, pink = 8/16

Tall, white = 0/16

Short, red = 0/16

Short, pink = 0/16

Short, white = 0/16