2. Punnett Squares
• A Punnett square is a grid that enables
one to predict the outcome of simple
genetic crosses
• Proposed by the English geneticist,
Reginald Punnett
3. Plant genes can have dominant and recessive alleles
… a red flower
r
The recessive
allele r produces …
… a white flower
R
The dominant allele
R produces …
Example:
4. A genetic cross in plants: Rr x Rr
There is only one way to make a homozygous plant with
two dominant genes…
R R RR
… and only one way to make a homozygous plant with
two recessive genes
rrr r
5. In the genetic cross Rr x Rr there are two ways of
making a heterozygous plant…
R r
Rr
Rr
Rr
OR
… so, heterozygous offspring are twice as likely as either
homozygous dominant or homozygous recessive offspring
6. We can show the likeliness or probability of different
genotypes in different ways:
1. As a percentage - Probability of RR is…
¼
½
25%
Probability of Rr is… 50%
Probability of rr is… 25%
2. As a fraction - Probability of RR is…
Probability of Rr is…
Probability of rr is… ¼
3. As a ratio - RR : Rr : rr = 1 : 2 : 1
7. There are three ways to make a plant with red flowers…
R R RR
R r
r R
Rr
Rr
… one produces a homozygous plant…
… and two produce heterozygous plants…
8. …but, there is only one way to produce a plant with white
flowers…
rrr r
So, in the genetic cross, Rr x Rr…
…red flowers are three
times more likely than
white flowers!!!
9. We can show the likeliness or probability of different
phenotypes in different ways:
1. As a percentage - Probability of red is…
¾
¼
75%
Probability of white is… 25%
2. As a fraction - Probability of red is…
Probability of white is…
3. As a ratio - red : white = 3 : 1
10. The Punnett Square
We can use the Punnett Square to work out the probability of
the different genotypes and phenotypes in a genetic cross
Step 1: Write down the genotypes of the parents.
= Rr x Rrx
Step 2: Write down the genotypes of the gametes that
each parent produces.
pollen Egg
cells
= R + r = R + r
11. Step 3: Draw your Punnett Square
Step 4: Write the genotype
of the different gametes
into your Punnett Square
Step 5: Write in the
different ways in which the
gametes can be combined…
Egg cells
Pollen
R r
R
r
R R R r
r R r r
…this shows you the
possible genotypes of the
offspring Genotypes of
offspring
12. Egg cells
Pollen
R r
R
r
R R R r
r R r r
Probability of Genotypes
There are four possible ways to
combine egg cells and pollen
Count up how many times
each genotype appears in
the Punnet Square – this
gives you its probability:
¼Probability of RR is…
Probability of Rr is…
Probability of rr is… ¼
= 25 %
= ½2
4
= 25 %
= 50 % The ratio of
RR : Rr : rr is…
1 : 2 : 1
13. Egg cells
Pollen
R r
R
r
R R R r
r R r r
Probability of Phenotypes
Look at the genotypes of the
offspring in your Punett Square…
…decide which phenotype
each one should produce.
Count up how many times
each phenotype appears in
the Punnet Square – this
gives you its probability:
Probability of red is… ¾ = 75 %
Probability of white is… ¼ = 25 %
The ratio of
red : white is…
3 : 1
14. Mendel’s Experiments
• Mendel also performed dihybrid crosses
– Crossing individual plants that differ in two
traits
• For example
– Trait 1 = Seed texture (round vs. wrinkled)
– Trait 2 = Seed color (yellow vs. green)
• There are two possible patterns of
inheritance for these traits
17. 315 round, yellow
seeds
101 wrinkled, yellow
seeds
108 round, green seeds
32 green, wrinkled
seeds
All round,
yellow
Round,
Yellow seeds
X wrinkled,
green seeds
F2 generationF1
generation
P Cross
DATA FROM ONE OF MENDEL’S DIHYBRID CROSSES
18. Interpreting the Data
• The F2 generation contains seeds with
novel combinations not found in the
parentals
– Round and Green
– Wrinkled and Yellow
• These are nonparentals
• Occurrence contradicts the linkage
model
19. • Independent assortment is also revealed by a
dihybrid test-cross
– TtYy X ttyy
• Thus, if the genes assort independently, the
expected phenotypic ratio among the offspring
is 1:1:1:1
21. Forked-line Method (fork
diagram)
• Calculate predicted ratios of offspring
by multiplying probabilities of
independent events
P yellow, round x green, wrinkled
