Punnett Squares – Dihybrid Crosses
Connections:
• Knowing how to set up a dihybrid cross.
• Determine the possible genotypes, phenotypes, and their ratios using a dihybrid
cross.
What is a dihybrid cross?
•
A dihybrid cross involves _________ traits.
•
Each trait has ________ alleles, so there are a total of __________ alleles.
Independent Assortment
•
During _______________ the alleles combine ________________.
•
This is independent assortment.
o Each allele of a particular trait has an ___________ chance of being in a gamete with each of
the alleles of the other trait.
•
Example: an organism is ________________________ for gene a and gene b.
o Its genotype is

•
___________
During _______________, A has an _____________ chance of ending up with ___ or ____. The
same applies to a.
Guinea Pig Example
•
Let’s look at two traits in guinea pigs.
o Hair color

_______________________

_______________________
o Hair length
•

_______________________

_______________________
If a male guinea pig is heterozygous for hair color and length what is its genotype?
o ___________
•
Review: What will its phenotype be?
o _______________________________
•
What are the possible gametes the guinea pig can produce?
o _________________________________
Let’s mate this male guinea pig with a female guinea pig whose genotype also heterozygous for hair color
and length.
•
What is the females genotype
o ______________
•
What are the possible gametes she can produce
o __________________________________
Setting Up A Dihybrid Cross
•
Just like a monohybrid cross, we must put the possible gametes of each parent on the sides.
•
Once our cross is complete we can determine the ratio of genotypes and phenotypes.
Ratios
Genotypic Ratios
Phenotypic Ratios
BBSS – ____________
________________________
BBSs – ____________
________________________
BBss – ____________
________________________
BbSS – ____________
________________________
BbSs – ____________
Bbss – ____________
bbSS – ____________
bbSs – ____________
bbss – ____________