Punnet Squares – Monohybrid Cross
A punnet square is a grid for organizing genetic information. They are used to predict probability of genetic
phenotypes & genotypes. Remember the outcomes are theoretical probabilities & not actual fact.
You can use punnett squares for monohybrid (single traits eg flower colour) or dihybrid crosses (two traits, exflower color & seed texture.)
Example 1. Use a Punnett square to determine the offspring of the F1 & F2 generations. Purple color is
dominant over white.
P generation:
Trait:
Alleles:
Purple Flower(purebred)
x
x
White Flower (pure bred)
P Cross
P
P
x
x
_____________
p
p
F1 Generation
Trait:
Alleles:
___________
(a) What is the phenotypic ratio?
(b) What is the genotypic ratio?
F1 Cross
F2 Generation
(a) What is the phenotypic ratio now?
(b) What is the genotypic ratio now?
(c) What is the ratio of homozygous to heterozygous offspring?
(1) Complete the following activities using your knowledge of Punnett Squares.
P Generation
(homozygous)
Trait
Widows peak (dominant)
Alleles
P Cross
F1 Generation
Trait:
Alleles:
(a) What is the phenotypic ratio?
(b) What is the genotypic ratio?
F1 Cross
F2 Generation
(d) What is the phenotypic ratio now?
(e) What is the genotypic ratio now?
(f) What is the ratio of homozygous to heterozygous offspring?
No widows peak (recessive)
P Generation
(homozygous)
Trait:
Red eyes (dominant)
Alleles:
P Cross
F1 Generation
Trait:
Alleles:
(a) What is the phenotypic ratio?
(b) What is the genotypic ratio?
F1 Cross
F2 Generation
(g) What is the phenotypic ratio now?
(h) What is the genotypic ratio now?
(i) What is the ratio of homozygous to heterozygous offspring?
White eyes (recessive)
Punnett Squares - Dihybrid Cross
The only difference between a Dihybrid cross & a
Monohybrid cross is that instead of focusing on one trait, we
will focus on two.
x
Homozygous Tall & Green
Genotype:
a.
b.
c.
d.
e.
TTGG
What is the combination of plants that we now have?
What is the ratio of plants that we now have?
What is the ratio of phenotypes?
What is the ratio of genotypes?
How many Tall trees with Green pods are there?
Dwarf & yellow
ttgg
Punnett Squares – Dihybrid cross
P Generation
(homozygous)
Trait:
Homozygous purple flower
&heterozygous round seeds
White flower & wrinkled seeds
Genotype:
P Cross
F1 Generation
Traits:
Alleles:
(a) What is the phenotypic ratio?
(b) What is the genotypic ratio?
(c) What is the ratio of homozygous to heterozygous offspring?
(d) What is the probability of getting a purple flower wrinkle seed offspring?
P Generation
(homozygous)
Trait:
Heterozygous purple flower &
Heterozygous round seeds
Genotype:
P Cross
F1 Generation
Traits:
Alleles:
(a) What is the phenotypic ratio?
(b) What is the genotypic ratio?
(c) What is the ratio of homozygous to heterozygous offspring?
Heterozygous purple flower &
Heterozygous round seeds