PUNNETT
SQUARES
~pg. 92
PUNNETT SQUARES
Punnett Square - a diagram used to predict
the possible offsprings of a cross
Created by Reginald Punnett
The alleles of one parent are written across
the top of the square (one allele per box)
and those of the other parent are written on
the side
Maternal
Paternal
B
b
B
BB
Bb
b
Bb
bb
Genotype Frequency: 25% BB, 50% Bb, 25% bb
Phenotype Frequency: 75% Black Fur, 25% White Fur
EXAMPLE: GUINEA PIG DIAGRAM
 In guinea pigs, black fur (B)
is dominate to white fur
(b). A heterozygous
female mates with a white
male. What are the
genotype and phenotype
ratios of the offspring?
(Make a punnett square)
B
b
b
Bb
bb
b
Bb
bb
Genotype: 50% Bb, 50% bb
Phenotype: 50% Black, 50% White
EXAMPLE: FLOWER PETAL DIAGRAM
 In a certain flower type,
red colored petals (R) is
dominate to yellow
colored petals (r). A
homozygous red and a
homozygous yellow flower
are crossed. What are the
genotype and phenotype
ratios of the offspring?
(Make a punnett square)
R
R
r
Rr
Rr
r
Rr
Rr
Genotype: 100% Rr
Phenotype: 100% Red colored petals
MAKE UP A PROBLEM THAT
REQUIRES A PUNNETT
SQUARE TO SOLVE. WRITE IT
OUT AND THEN SWITCH
WITH A NEIGHBOR. SOLVE
EACH OTHERS PROBLEM. 
WHY DOES THE PUNNETT SQUARE WORK?
 It all goes back to meiosis…each side represents a sperm
or egg. The boxes simply give you the statistical chance that
a certain sperm will fertilize a certain egg.
 Consider a pea plant that is both round and tall = RrTt
 When this plant’s cells go through meiosis, the alleles
segregate – each sperm/egg receives a random
combination…
 These possible combinations include: RT, Rt, rT, rt