What is a
Punnett Square?
• Punnett square is a diagram
to understand Mendel's
conclusions
• It starts with genes and alleles
• Every organism has two sets of
instructions (alleles) for each
characteristic.
• A sexual reproduction organism
receives one set of instructions (genes)
from each parent.
• In using the Punnett square, dominant
alleles (traits) are symbolized with
capital letters and recessive alleles
(traits) are symbolized with lowercase
letters
• The letters on the outside of the box
represent the parent’s alleles.
• The letters inside the box represent
the genotype of the new organism.
Phenotype is the combination that is
created.
• The letters on top move down and
the letters on the side move to the
right
Punnett Square R=round pea – dominant trait
r = wrinkled pea – recessive trait
The 4 squares show all the possible combinations-genotype
r
R
R
R
R
r
r
r
R
r
R r
RR plant grows a round pea & rr grows a
wrinkled pea Both are true-breeding
R
R
r
R
R
r
R
r
r
r
Rr would be a cross-breeding plant. What
type of pea would be produced?
round
RR grows a round seed
Rr grows a round seed
What does rr grow?
Wrinkled seed
• Quiz on these notes – tomorrow
• Next lesson: How is Meiosis the
key to the great amount of
diversity of life on Earth?
• Out the door
–On a Punnett square, how are
dominant alleles (traits) and
recessive alleles (traits)
symbolized?