Biology 12
• These are crosses that involve genes for two different
characteristics
• The natural progression for Mendel was to study two
characteristics at the same time.
• Thus, the study of 2 pairs of contrasting traits at the same time
= a dihybrid cross
• ex.. round yellow seeds X wrinkled green seeds
• Principle of Independent Assortment
• When more then one trait is studied in the same cross, the genes for each
trait sort into gametes independently of the genes of the other traits
Let’s cross a homozygous round/yellow (AABB) with a
homozygous wrinkled/green (aabb) plant….
Character
Seed shape
Seed colour
Trait
Allele
Round
A
Wrinkled
a
Yellow
B
Green
b
1. Determine Genotypes of Parents
2. Determine Genotypes of Gametes (remember
gametes only have 1 copy of each gene) and think
FOIL (from math class!)
3. Punnett Square: need 16 squares now, 4 gametes
from each parent
4. Identify Phenotype Ratios in Offspring
o In snapdragons…
o Tallness (T) is dominant to dwarfness(t)
o Red color is due to gene (R) and white to its
corresponding allele (r).
o The heterozygous condition results in pink (Rr)
flower color.
o 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.
“A dwarf pink snapdragon is crossed with
a plant homozygous for tallness and red
flowers. “
Parent 1:Dwarf pink = ttRr
Parent 2:Homozygous tall, red
= TTRR
Dwarf pink = ttRr
tR
tr
tR
tr
Homozygous tall, red = TTRR
TR TR
TR TR
The Punnett square
determines the
genotypes of the
offspring
tR
Gametes from
dwarf pink parent
tr
tR
tr
Gametes from
TALL red parent
TR
TR
TR
TR
TtRR
TtRR
TtRR
TtRR
Tall, red = 8/16
Tall, pink = 8/16
TtRr
TtRr
TtRr
TtRr
TtRR
TtRR
TtRR
TtRR
TtRr
TtRr
TtRr
TtRr
Tall, white = 0/16
Short, red = 0/16
Short, pink = 0/16
Short, white = 0/16
• Page 154 questions 1, 3 (and then try #2)