Punnett Squares

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

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:

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

In the genetic cross Rr x Rr there are two ways of making a heterozygous plant…

R r

Rr

Rr

RrOR

… so, heterozygous offspring are twice as likely as either homozygous dominant or homozygous recessive offspring

We can show the likeliness or probability of different genotypes in different ways:

1. As a percentage -

Probability of RR is…

¼½

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

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…

…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!!!

We can show the likeliness or probability of different phenotypes in different ways:

1. As a percentage -

Probability of red is…

¾¼

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

The Punnett Square

We can use the Punnett Square to work out the probability of the different genotypes and phenotypes in a

genetic crossStep 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

Step 3: Draw your Punnett Square

Step 4: Write the genotype of the different gametes into your Punnett SquareStep 5: Write in the different ways in which the gametes can be combined…

Egg cells

Polle

n

R r

R

r

RR R r

rR r r

…this shows you the possible genotypes of the offspring Genotypes of

offspring

Egg cells

Polle

n

R r

R

r

RR R r

rR 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 %=

½

24

= 25 %

= 50 %

The ratio of RR : Rr : rr is…

1 : 2 : 1

Egg cells

Polle

n

R r

R

r

RR R r

rR 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

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

Figure 2.7

Figure 2.8

P Cross F1 generation

F2 generation

Round,Yellow seeds X wrinkled, green seeds

All round, yellow

315 round, yellow seeds101 wrinkled, yellow seeds108 round, green seeds 32 green, wrinkled seeds

DATA FROM ONE OF MENDEL’S DIHYBRID CROSSES

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

• 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

Dihybrid Punnett Square

Forked-line Method (fork diagram)

• Calculate predicted ratios of offspring by multiplying probabilities of independent events

P yellow, round x green, wrinkled

Fork Diagram for Trihybrid Cross