How to make a punnett squareA step by step method

What is a punnett square? A punnett square is a diagram used to

try to predict the outcome of a cross between 2 parents.

It does not tell us what will happen, but it does tell us the likelihood that something will happen.

Monohybrid crosses These are crosses that only look at a

single trait Ex: hair color, eye color, albinism Ex: What are the chances that my baby

will have brown eyes?

Step 1 Draw a box and divide it into 4 squares.

These squares represent the possible offspring

Step 2 Determine the

genotypes of the parents. ____x_____

Place the alleles for the genotypes of the parents on the sides and top of the square.

As a general rule, we put the dad on the top and the mom on the side.

R R

r

r

Step 3

Now you multiply the sides with the top to get the genotype of the 4 offspring boxes.

r

r

R R

Rr Rr

Rr Rr

Step 4

Now you can see what the genotypes of the offspring would be.

In this case there is a 100% chance of the offspring being heterozygous for the trait because all 4 out of 4 boxes are capital, lower case.

r

r

R R

Rr Rr

Rr Rr

RatiosGenotypic

Ask your self: How many homozygous dominants? How many heterozygous? How many homozygous recessives?

Phenotypic Ask yourself:

How many dominant phenotypes? How many recessive phenotypes?

Let’s Practice Homozygous (HO) or Heterozygous (HE)

AA ____Bb ____Cc ____Dd ____

Ee ____ff ____GG ____ HH ____

HO

HE

HE

HE HO

HO

HO

HE

Determine Phenotype

Purple flowers are dominant to white flowers

PP _____________Pp _____________pp _____________

Brown eyes are dominant to blue eyes

BB ____________Bb ____________bb ____________

Purple

Purple

White

Brown

Blue

Brown

For each phenotype list the genotype

Straight hair is dominant to curly.

____________ straight____________ straight____________ curly

Pointed heads are dominant to round heads.

____________ pointed____________ pointed____________ round

SS

Ss

ss

PP

Pp

pp

Problem #1

In pea plants, spherical seeds (S) are dominant to dented seeds (s). In a genetic cross of two plants that are heterozygous for the seed shape trait, what fraction of the offspring should have spherical seeds?

Step 1Draw punnett square

Problem #1Determine the genotype of parents, and place alleles on punnett square

Step 2

________ x ________ In pea plants, spherical seeds (S) are dominant to dented seeds (s). In a genetic cross of two plants that are heterozygous for the seed shape trait, what fraction of the offspring should have spherical seeds?

Ss Ss

S s

S

s

Problem #1

Multiply sides to get possible offspring Step 3

________ x ________ In pea plants, spherical seeds (S) are dominant to dented seeds (s). In a genetic cross of two plants that are heterozygous for the seed shape trait, what fraction of the offspring should have spherical seeds?

Ss Ss

S s

S

s Ss

SsSS

ss

Problem #1Interpret the possible offspringStep 4

________ x ________ In pea plants, spherical

seeds (S) are dominant to dented seeds (s). In a genetic cross of two plants that are heterozygous for the seed shape trait, what fraction of the offspring should have spherical seeds?

Ss Ss

S s

S

s Ss

SsSS

ss

3/4

Problem #2Step 4 Interpret the possible offspringStep 1 Draw punnett square

________ x ________ SpongeBob SquarePants

recently met SpongeSusie Roundpants at a dance. SpongeBob is heterozygous for his square shape, but SpongeSusie is round (recessive). What is the ratio of round to square pants?

Rr rr

R r

r

r Rr

rrRr

rr

2:2

Step 3 MultiplyStep 2 Determine Parent genotypes

Problem #3Step 4 Interpret the possible offspringStep 1 Draw punnett square

________ x ________ Squidward is purebred for

light blue skin, which is the dominant trait for body color. He recently married a nice girl who has light green skin, which is a recessive trait. Create a Punnett square to show the possibilities that would result if Squidward and his new bride had children.

What percentage of Squidward’s offspring could be green?

BB bb

B B

b

b Bb

BbBb

Bb

0%

Step 3 MultiplyStep 2 Determine Parent genotypes

Dihybrid crosses Dihybrid crosses are punnett squares

used to predict the probability of an offspring having 2 traits at the same time.

Ex: What are the chances of my baby having brown eyes AND blond hair.

Step 1

Draw a box and divide it into 16 squares.

Step 2Determine the

genotypes of the parents

Ex: cross two pea plants that are heterozygous for yellow peas and round peas.

Yellow is dominant to green and round is dominant to wrinkled.

YyRr x YyRr

Step 3Determine all of the

possible gamete combinations for each parent by using the F.O.I.L. or Gremlin method

FOIL= first, outside, inside last

Ex: YyRr First- YR Outside- Yr Inside- yR Last- yr

YyRr

Gremlin

Step 4Write all of the

possible combinations of alleles on the sides and top of the punnett square. YR Yr yR yr

YR

YR

Yr

Yr

yR

yR

yr

yr

Step 5

Go through the results and determine what the phenotype of each offspring would be

YR

YR

Yr

Yr yR

yr

yrYYRR

Yellow round

YYRr

Yellow, round

YYRr

Yellow, round

YYrr

Yellow,wrinkled

YyRR

Yellow, round

YyRr

Yellow, round

Yyrr

Yellow, wrinkled

YyRr

Yellow, round

YyRR

Yellow, round

YyRr

Yellow, round

yyRR

Green, round

yyRr

Green, round

Yyrr

Green, wrinkled

yyRr

Green, round

Yyrr

Yellow, wrinkled

YyRr

Yellow, round

yR

Step 6

Figure out the phenotypic ratio or answer whatever question they ask you. 9:3:3:1

Example: what are the chances of having a yellow and wrinkled pea? 3/16

YR

YR

Yr

Yr yR

yr

yrYYRR

Yellow round

YYRr

Yellow, round

YYRr

Yellow, round

YYrr

Yellow,wrinkled

YyRR

Yellow, round

YyRr

Yellow, round

Yyrr

Yellow, wrinkled

YyRr

Yellow, round

YyRR

Yellow, round

YyRr

Yellow, round

yyRR

Green, round

yyRr

Green, round

Yyrr

Green, wrinkled

yyRr

Green, round

Yyrr

Yellow, wrinkled

YyRr

Yellow, round

yR

Let’s Practice Determine the gametes for the following

cross: heterozygous tall plant with heterozygous yellow seeds with another of the same genes. Tall and yellow are dominant

F.O.I.LTtYy

First- Outside- Inside- Last-

Gremlin

TtYyTY

TytY

ty

NNBB: NnBb: nnbb:

Problem #4Step 1 Draw punnett square

________ x ________

Suppose the alpha male and alpha female of a wolf pack are black with blue eyes and normal colored with brown eyes, respectively. Assume further that normal coat color (N) is dominant to black (n) and brown eyes (B) are dominant to blue (b). The female is also heterozygous for both traits.

What fraction of the offspring could have the following genotypes?

nnbb NnBb

nb

nb

Bb

BbBb

Bb

0/16

Step 3 and 4 Determine gametes and place on squareStep 2 Determine genotypesStep 5 Analyze resultsStep 6 Answer question(s)

nb nb nb

nBN

bN

B

NnBb normal, brown

Nnbb normal,

blue

nnBb black, brown

NnBb normal, brown

NnBb normal, brown

NnBb normal, brown

Nnbb normal,

blue

Nnbb normal,

blue

Nnbb normal,

blue

nnBb black, brown

nnBb black, brown

nnBb black, brown

nnbb

black, blue

nnbb

black, blue

nnbb

black, blue

nnbb

black, blue

4/16 4/16