PUNNET

SQUARES

“The law of Independent Assortment”

Alleles of different genes separate completely from one another during

gamete formation

XY

X Y

Which means in Mendel’s flower experiments, he took a purebred purple

flower and crossed with a purebred white white flower.flower.

P = Purple color p = white color

=

=

PP Homozygous dominant

pp Homozygous recessive

The purple flower would separate into -

PP

P P

The white flower would separate into -

p p

pp

A Punnett square - is used to predict the outcome of a genetic cross by considering all the possible combinations of alleles in the cross.

We are just going to concentrate on 1 trait for now.

So our cross would be PP x ppPurple White

A FEW RULES ABOUT PUNNET SQUARES

1. Alleles are listed alphabetically.

2. Dominant traits always are written before recessive for that allele. Rr rR

So let’s go back to our cross PP x ppPurple White

Draw the punnet square on your notes.

When using a punnett square, you take the first set of alleles and put them on top.

PP X ppPP

Then take the 2nd set of alleles and put them on the left side.

pp

Now add each letter into the corresponding box.

P P

p

p

Pp Pp

Pp Pp

Purple (P) is dominant over white (p)

So what is the phenotype of all the offspring?

P P

p

p

Pp Pp

Pp Pp

All Purple

P P

p

p

Purple (P) is dominant over white (p)

So what is the genotype of all the offspring?

Pp Pp

Pp Pp

All

Pp

When looking at Mendel’s flower experiment, he crossed the P1 purple with the P1 white flower and got 100% purple

flowers.

X

P1 F1

When doing the punnett square, we predicted they would all be 100%

purple. We were right!

P P

p

p

Pp Pp

Pp Pp

Then Mendel decided to cross a F1 purple flower with another F1 purple

flower.

So let’s try this with a punnett square. P P

p

p

Pp Pp

Pp Pp

Pp X Pp

P

P

p

p

PP Pp

Pp pp

P – dominant

P - recessive

P

P

p

p

PP Pp

Pp pp

What is the Phenotype for this cross?

3 Purple to 1 white1 white

What is the genotype for this cross in %?

P

P

p

p

PP Pp

Pp pp

25 % = PP (homozygous dominant) 1 out of 450% = Pp (heterozygous) 2 out of 425 % = pp (homozygous recessive) 1 out of 4

Mendel got a 3 Purple : 1 white from his experiments

X

F1

3 Purple : 1 White

Using our punnett square, we proved Mendel was correct.

P

P

p

p

PP Pp

Pp pp

Now let’s determine the sex of a baby!

XY = male XX = female

X

X

X

Y

XX XY

XX XY

What is the Phenotype of the baby?

50% male 50% female

What is the Genotype of the baby?

50% XY 50% XX

Question: If the first baby is a boy, what is the chance the second child will be a boy?

X

X

X

Y

XX XY

XX XY

Answer: 50% - every time you have a baby , you start all over

Let’s practice a few Punnett squares to make sure you got it.

Do the following crosses on your paper and give me the phenotype and

genotype for the possible outcomes.

Tall plants (T) are dominant over short plants (t). Cross a purebred Tall with a

hybrid Tall. TT x Tt

T T

T

t

TT TT

Tt Tt

Phenotype 100% Tall

Genotype 50% TT

50% Tt

Green Peas (G) are dominant over Yellow Peas (g)

Cross a Hybrid Green Pea with a Hybrid Green Pea

G g

G

g

GG

Gg

Gg

gg

Phenotype

25% - GG 50% - Gg

25% - gg

Gg x Gg

3 Green: 1 yellow

Genotype

Black hair (B) is dominant to blonde hair (b) Cross a homozygous recessive blonde with a

heterozygous black haired person. bb x Bb

b b

B

b

Bb

bb

Bb

bb

50% black haired 50% blonde

50% - Bb 50% - bb

Phenotype

Genotype

VOCABULARY !Law of Independent AssortmentPunnett Square

OK, let’s try a Spongebob Squarepants worksheet