Genetics: Punnett Squares
Date:______
On an island in the Pacific Ocean, there is a species of birds with different sized beaks, with one specific
gene determining their beak size. Birds who are homozygous dominant (BB) or heterozygous (Bb) for this
particular gene have small beaks, while birds who are homozygous recessive (bb) for this particular gene
have big beaks. For each combination of parents below, predict the frequency with which each
phenotype and genotype exists. Make sure to include gametes from each parent.
1) Homozygous Dominant ( BB ) x Homozygous Dominant ( bb )
Genotypes:
Phenotypes:
BB
______ %
Big Beaks
______ %
Bb
______ %
Small Beaks ______ %
Bb
______ %
Hint, do your percentages above add up to 100%?
2) Homozygous Recessive ( bb ) x Heterozygous ( Bb )
Genotypes:
Phenotypes:
BB
______ %
Big Beaks
______ %
Bb
______ %
Small Beaks ______ %
Bb
______ %
3) Heterozygous ( Bb ) x Heterozygous ( Bb )
Genotypes:
Phenotypes:
BB
______ %
Big Beaks
______ %
Bb
______ %
Small Beaks ______ %
Bb
______ %
Review:
What were all the alleles used above? ____________________________________________________
How many possible genotypes do you expect to find within this species, for beak size? _____________
ANSWER KEY
Name:___________________ Genetics: Punnett Squares
Date:______
On an island in the Pacific Ocean, there is a species of birds with different sized beaks, with one specific
gene determining their beak size. Birds who are homozygous dominant (BB) or heterozygous (Bb) for this
particular gene have small beaks, while birds who are homozygous recessive (bb) for this particular gene
have big beaks. For each combination of parents below, predict the frequency with which each
phenotype and genotype exists. Make sure to include gametes from each parent.
1) Homozygous Dominant ( BB ) x Homozygous Dominant ( bb )
B
Genotypes:
B
b
Bb
Bb
b
Bb
Bb
BB
______
0 %
0 %
______
Bb
______
100 %
100 %
Small Beaks ______
Bb
0 %
______
Phenotypes:
Big Beaks
Hint, do your percentages above add up to 100%?
2) Homozygous Recessive ( bb ) x Heterozygous ( Bb )
b
b
B
Bb
Bb
b
bb
bb
Genotypes:
BB
0 %
______
50 %
______
Bb
50 %
______
50 %
Small Beaks ______
Bb
50 %
______
Phenotypes:
Big Beaks
3) Heterozygous ( Bb ) x Heterozygous ( Bb )
B
Genotypes:
b
B
BB
Bb
b
Bb
bb
BB
25 %
______
25 %
______
Bb
50 %
______
75 %
Small Beaks ______
Bb
25 %
______
Phenotypes:
Big Beaks
Review:
B and b
What were all the alleles used above? ____________________________________________________
3
How many possible genotypes do you expect to find within this species, for beak size? _____________