Section 6-5
Part 2
Dihybrid
Crosses
Write everything
that is Yellow
Recall …
• Phenotype: The way an
organism looks
•Black, white, tall short
• Genotype: The gene
combination of an
organism
•Homozygous Dominant,
Heterozygous, homozygous
recessive
A.
Crosses
1. Monohybrid Crosses:
• Parents differ by just one trait
• (flower color)
2. Dihybrid Crosses:
• A cross that involves
2 different traits
• (flower color AND plant height)
B. Naming Dihybrid
Crosses
1.Traits are written together
in pairs
2. All dominant traits are
written first
• So, if a plant’s genotype is
heterozygous tall (Tt) and
heterozygous round (Rr), it
would be written TtRr
Dihybrid Punnet Square
• In dihybrid crosses there
are 4 possible allele
combinations from each
parent
• Each combination gets its
own column or row
Setting up a Dihybrid Punnet Square
Write all of this …
• Example Cross: TtRr x TTRR
TR
TR
TR
TR
TR
Tr
tR
tr
Filling in the Square
TR
Tr
tR
tr
TR TTRR TTRr
TtRR
TtRr
TR TTRR TTRr
TtRR
TtRr
TR TTRR TTRr
TtRR
TtRr
TtRR
TtRr
TR
TTRR
TTRr
Results:
TTRR TTRr
TtRR
TtRr
TTRR TTRr
TtRR
TtRr
TTRR TTRr
TtRR
TtRr
TTRR TTRr
TtRR
TtRr
Genotypes:
TTRR = 4
TTRr = 4
TtRR = 4
TtRr = 4
Phenotypes: All offspring are
tall and round
Remember the The Law of
Independent Assortment:
• Genes for different traits
are inherited
independently of each
other
• When you have 2 or more
different traits they are
not “stuck” together