The Monohybrid Cross
…
Mendel’s Work, applied

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These problems will focus on one
trait (like plant height for example)
We will use Punnett’s Squares to
determine what happens to this trait
over the course of a few generations
This method can be used on almost
all genetics problems
An Example…

We will replicate one of Mendel’s examples.

Cross a pure bred tall plant with a
pure bred dwarf plant. Use Punnett’s
squares to show the F1 and F2
generations
P1
X
P2
Pheno
Tall
X
Short
geno
TT
X
tt
T
X
t
Possible
Gametes
In this case, each parent can only give one
possible gamete
Now to make the square…
The number of possible gametes per
parent decides the dimensions of the
square.
Our example will use a 1 x 1 square…
T
t
Tt
Parent’s Gametes
Offspring’s genotype
Parent’s Gametes
Now the analsis…
F1 generation
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T
t
Tt
This means that all of the offspring in
the F1 generation will have the genotype
T t. The Genotypic ratio is all Tt
Knowing that T is the dominant allele,
all of the offspring will have the
phenotype “Tall”. The Phenotypic Ratio
is All Tall.

Our Statement must state what we
see!
All of the F1 pea plants are Tall

Next, we will cross two plants from the
F1 generation.
F1
X
F1
Pheno
Tall
X
Tall
geno
Tt
X
Tt
T or t
X
T or t
Possible
Gamete
s
Each parent can give either one of their gametes
Now, to make the square:
One parent’s possible gametes
T
T
t
TT
Tt
t
Tt
tt
The other’s possible gametes
Offspring’s Genotypes
Now, more
analysis…
T
F2 generation
1 of the 4 has the genotype TT,
t
T
TT
Tt
The genotypic ratio is 1 TT : 2
t
Tt : 1 tt
Tt
tt
2 have the genotype Tt, and
1 has the genotype tt.
3 of the 4 offspring will be
Tall (either Tt or TT), and 1
will be short (tt). The
Phenotypic ratio is going to
be 3 Tall : 1 Short
Remember! The Statement!

State what we SEE
The F2 phenotypic ratio for pea plants
is
3:1 Tall:Short.
The Guinea Pig problem…
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Suppose that Guinea Pig colour is
determined by one gene. B (brown
coat) is dominant to b (grey coat)
Show the F1 generation for a cross
of
B b x B b using Punnet’s
squares
Give the Genotypic and phenotypic
ratios for each generation
The Guinea Pig Problem:
P1
Pheno
geno
Possible
Gametes
Brown
X
X
Bb
X
B or b
X
P2
Brown
Bb
B or b
Both Parents can give either gamete
The Guinea Pig Problem
B
Genotypic Ratio:
1BB : 2Bb : 1 bb
b
B
BB
Bb
b
Bb
bb
Phenotypic Ratio:
3 Brown : 1 Grey
Statement: The F1 phenotypic ratio for
Guinea Pig’s coat colour is 3 Brown : 1
Grey
The Guinea Pig problem…
Tips for monohybrids
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Do each generation one step at a time
What gametes can each parent contribute?
What are the dimensions of each square?
Count how many of each type of offspring
you will have in terms of BOTH genotype
and phenotype.
REMEMBER YOUR STATEMENTS
The Test Cross
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What if you had a female brown Guinea Pig
at home, and wanted to know her
GENOTYPE?
How would you be able to find out? We’d
use a Test Cross
A Test Cross is crossing the unknown with a
homozygous recessive.
In this case, you would need a male
homozygous recessive… he would have to
be grey
Baby Guinea Pig…
Lets say we do this cross…and get this result?
x
=
This was the cross: BB x bb

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We know the grey Guinea Pigs is bb.
But the Brown Guinea Pig could be
BB or Bb.
All Brown offspring means that our
brown Guinea Pig was genotype BB,
because 100% of the offspring had
the dominant trait.
Baby Guinea Pigs…
How about this result?
x
=
This was the cross: Bb x bb
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Half brown and half grey offspring
means that our brown Guinea Pig
was genotype Bb, because Half got
the ‘B’ gamete and half got the ‘b’
gamete.
Half express the recessive trait, and
half express the dominant trait.