Hands On
5 Steps to Solving
Monohybrid Crosses
First, analyze the
problem and
highlight important
information
PROBLEM: Dimples are dominant
over no dimples. A father is
homozygous dominant for dimples and
a mother is homozygous recessive for
dimples. What are the chances that
their child will have dimples?
Step 1: Choose a letter to
represent a trait.
(hint: Dominant trait named dimples starts with “D”,
recessive gets the same but small letter)
Dimples = D
No dimples = d
Step 2: Write down the genotype
of the parents. Remember that
each parent has 2 alleles.
Mom
Dad
d d
DD
Find coins to represent each parents’ alleles.
Step 3: Write the genetic cross.
dd
x
DD
Step 4: Set up the Punnett Square.
Write in the parent alleles on the Punnett Square.
Like a multiplication table, it does not matter BUT
we find it helpful to place mom’s alleles across the
top and dad’s down the side of the Punnett Square.
What is a Punnett Square?
A Punnett Square is a tool like a
multiplication table which scientists use to
show all the possible combinations of alleles
that can result from a genetic cross.
The Punnett Square shows all the possible
outcomes that any 1 child may inherit.
Each time mom and dad have another child
it is a new roll of the same genetic dice.
Place coins to represent each parents’ alleles directly on top of
the written allele. For each fertilization move mom’s “allele”
and dad’s “allele” to join in the Punnett Square box.
d
d
D
Dd
Dd
D
Dd
Dd
Step 5: Determine the phenotypes
and genotypes of the offspring.
Genotype(s): Dd
Phenotype(s): Dimples
Now let’s talk about probability numbers...
The Punnett Square has 4 boxes in it.
Each box represents ¼ or 25% probability to occur.
For this mating, the 4 boxes representing possible offspring
get the genotype of Dd. So there is 100% chance (4 x 25%)
that offspring phenotype will have dimples.
Do you like broccoli or brussel sprouts?
In humans, being a taster of the chemical
PTC, a bitter component found in broccoli
and brussel sprouts, is dominant over being
a non-taster (recessive).
Can you taste PTC? Let’s find out…
PTC = phenylthiocarbamide
Problem 2
First, analyze the
problem and highlight
important information
A man heterozygous for this trait of
tasting PTC marries a female who is
homozygous recessive (non-taster) for
this trait. Use a Punnett square to
predict the genotypes and phenotypes
of any offspring.
Step 1: Choose a letter to
represent a trait.
Taster = T
Non Taster = t
Step 2: Write down the genotype
of the parents. Remember that
each parent has 2 alleles.
Mom
Dad
tt
Tt
Find coins to represent each parents’ alleles.
Step 3: Write the genetic cross.
Tt
x
tt
Step 4: Set up the Punnett Square.
Write in the parent alleles on the Punnett Square.
Like a multiplication table, it does not matter BUT
we find it helpful to place mom’s alleles across the
top and dad’s down the side of the Punnett Square.
Place coins to represent each parents’ alleles directly on top of
the written allele. For each fertilization move mom’s “allele”
and dad’s “allele” to join in the Punnett Square box.
t
t
T
Tt
Tt
t
tt
tt
Step 5: Determine the phenotypes
and genotypes of the offspring.
Genotype(s): Tt and tt
Phenotype(s): Taster and non-tasters
Now let’s talk about probability numbers…
The Punnett Square has 4 boxes in it. Each box represents ¼
or 25% probability to occur.
For this mating, 2 offspring boxes receive the genotype of Tt
There is 50% chance (2 x 25%) that phenotype will be tasters.
2 boxes receive the genotype of tt so there is 50% chance (2
x 25%) that offspring phenotype will be non-tasters.