Download Punnett Squares

Punnett Square Notes
Reginald Punnett
What is probability?
A number that describes how
likely it is that a certain
event will occur
If you toss a coin, what is the
probability that it will land on
heads?
1 in 2 = ½ = 50%
Punnett Squares
A chart used to visualize
all the possible
combinations of alleles
from a genetic cross
In a genetic cross, the allele
that each parent will pass on
to its offspring is based on
probability. We use Punnett
squares to help us visualize
the results.
Alleles
Dominant alleles are
symbolized with capital
letters (B, X, C)
Recessive alleles are
symbolized with lowercase
letters (b, x c)
Key Terms
 Phenotype: physical characterisitic/appearance
 Genotype: genetic makeup (allele combinations,
letters)
 Homozygous:


Homo = same
Same alleles- TT, bb, DD
 Heterozygous:
 Hetero = different
 Different alleles- Tt, Bb, Dd
 Hybrid
Step 1: Read the
problem and write out
the parental cross.
Problem: A cross between a
hybrid tall plant (Tt)
and a short plant (tt)
Parental Cross:
Tt x tt
Step 2. Draw a box and
divide it into 4 squares.
Step 3. Write one parent’s
alleles along the top, and the
other parent’s along the left side
Step 4. Copy the alleles along
the left side into the boxes on
their right. And copy the alleles
on the top into the boxes
underneath.
Tt
Tt
tt
tt
Step 5. The completed
Punnett square shows all the
possible allele combinations
for the offspring.
Tt
Tt
tt
tt
Step 6. List the possible
genotypes and phenotypes of
the offspring.
Genotype
(allele combination)
2 Tt
2 tt
Phenotype
=
=
(visible trait)
2 tall
2 short
Step 7. Find the genotype
and phenotype frequencies.
The frequency is the % probability (likelihood) of the
offspring having that particular genotype or phenotype.
Genotypes:
Tt: # of Tt X 100 = 2 X 100 = 50 % chance
Total
4
tt: # of tt X 100 = 2 X 100 = 50 % chance
Total
4
Phenotypes:
Tall: # Tall X 100 = 2 X 100 = 50 % chance
Total
4
short: # short X 100 = 2 X 100 = 50 % chance
Total
4