Name: __________________________
Period: _____
PUNNETT SQUARES – December 12, 2014
Punnett Square – Diagram showing the gene combinations that might result from a
genetic cross
Used to calculate the probability of inheriting a particular trait
Probability – The chance that a given event will occur
When 2 heterozygous genotypes are crossed (Aa x Aa), a parent can give contribute
either a recessive or a dominant allele to the offspring.
Y-Yellow
y-white
Genotype:
1:2:1
(YY: Yy: yy)
Phenotype:
¾ (75%) Yellow
¼ (25%) White
How to Make a Punnett Square:
Example: Give the genotype and phenotype for the following cross: TT x tt (T = Tall and t = Short)
Step 1: Set Up Punnett Square
(put one parent on the top and
the other along the side)
Step 2: Complete the Punnett Square
by crossing each male and female
gamete (to form a possible zygote).
Step 3: Calculate the genotype
and phenotype (remember that
each box is 25%).
Genotype:
4 Tt (100%)
Phenotype:
100% Tall
1) Give the genotype and phenotype for the following cross: Tt x tt
What are the probabilities for each genotype? ____________________________________________
What are the probabilities for each phenotype? ___________________________________________
Some Terminology:
 P1 – Original parents
 F1 – First generation
 F2 – Second generation
 P1 X P1 = F1
 F1 X F1 = F2
2) Complete the following Punnett Square showing a cross between Bb and Bb. B is the dominant allele for Black
hair. b is the recessive allele for blonde hair.
Genotypes and the probabilities for each: ___________________________
Phenotypes and the probabilities for each: __________________________