A Punnett square is a grid that predicts the possible genotypes of offspring from a genetic cross. It shows all the possible combinations of alleles from the parents. For a genetic cross between plants that are heterozygous (Rr) for flower color, a Punnett square predicts: - A 25% chance of homozygous dominant (RR) offspring with red flowers - A 50% chance of heterozygous (Rr) offspring with red flowers - A 25% chance of homozygous recessive (rr) offspring with white flowers Since there are twice as many ways to produce heterozygous offspring as either homozygous type, heterozygous offspring are twice as likely. Looking

1. Punnett Squares

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
Example:
The dominant allele
R R produces …
… a red flower
r
The recessive
allele r produces …
… a white flower

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
r r rr

5. In the genetic cross Rr x Rr there are two ways of
making a heterozygous plant…
R r Rr
OR
r R Rr
… 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… 25%
Probability of Rr is… 50%
Probability of rr is… 25%
2. As a fraction - Probability of RR is… ¼
Probability of Rr is… ½
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…
… one produces a homozygous plant…
R R RR
… and two produce heterozygous plants…
R r Rr
r R Rr

8. …but, there is only one way to produce a plant with white
flowers…
r r rr
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… 75%
Probability of white is… 25%
2. As a fraction - Probability of red is… ¾
Probability of white is… ¼
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.
x = Rr x Rr
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 Egg cells
Step 4: Write the genotype R r
of the different gametes
into your Punnett Square
R RR R r
Pollen
Step 5: Write in the
different ways in which the r rR rr
gametes can be combined…
…this shows you the
possible genotypes of the
Genotypes of
offspring offspring

12. Probability of Genotypes
Egg cells
There are four possible ways to
combine egg cells and pollen R r
Count up how many times
each genotype appears in R RR R r
Pollen
the Punnet Square – this
gives you its probability: r rR rr
Probability of RR is… ¼ = 25 %
Probability of Rr is…
2
4 = ½ = 50 % The ratio of
Probability of rr is… ¼ = 25 % RR : Rr : rr is…
1:2:1

13. Probability of Phenotypes
Egg cells
Look at the genotypes of the
offspring in your Punett Square… R r
…decide which phenotype
each one should produce. R RR R r
Pollen
Count up how many times
each phenotype appears in r rR rr
the Punnet Square – this
gives you its probability:
Probability of red is… ¾ = 75 % The ratio of
red : white is…
Probability of white is… ¼ = 25 % 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

15. Figure 2.7

16. Figure 2.8

17. DATA FROM ONE OF MENDEL’S DIHYBRID CROSSES
P Cross F1 F2 generation
generation
Round, All round, 315 round, yellow
Yellow seeds yellow seeds
X wrinkled, 101 wrinkled, yellow
green seeds seeds
108 round, green seeds
32 green, wrinkled
seeds

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

20. Dihybrid
Punnett
Square

21. Forked-line Method (fork
diagram)
• Calculate predicted ratios of offspring
by multiplying probabilities of
independent events
P yellow, round x green, wrinkled

22. Fork Diagram for Trihybrid
Cross