6 Basic Steps to Completing Single Cross
Punnett Squares
Problem : A harmless chemical known as PTC can be
added to strips of paper to test for the presence of
dominant & recessive alleles. Being able to taste the
chemical (T) is dominant to being a non-taster (t).
If two heterozygous tasters are crossed, what are the
possible genotypes and phenotypes of their children?
Step #1 – Create a key showing traits as
dominant/recessive.
T = PTC taster
t = non-taster
6 Basic Steps to Completing Single Cross
Punnett Squares
Step # 2 – Make a list of possible allele combinations
& the phenotypes that each would
produce.
TT= PTC-taster
Tt = PTC-taster
tt = non-taster
Step # 3 – Identify the parents that are being used in
the cross.
Tt x Tt
6 Basic Steps to Completing Single Cross Punnett Squares
Step # 4 – Draw a Punnett square to determine
possible offspring.
T
t
T
TT
Tt
t
Tt
tt
Step # 5 – Identify the Genotypic ratio.
1 : 2 : 1
TT Tt tt
Step # 6 – Identify the Phenotypic ratio.
3
PTC-tasters
:
1
non-taster