A 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 crossesThese are crosses that only look at a

single traitEx: hair color, eye color, albinismEx: What are the chances that my

baby will have brown eyes?

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

These squares represent the possible offspring

Step 2Determine 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 3Now 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 4Now 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?

PhenotypicAsk yourself:

How many dominant phenotypes? How many recessive phenotypes?

Let’s PracticeHomozygous (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 #1Draw punnett squareStep 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?

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

Step 2

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?

________ x ________ Ss Ss

S s

S

s

Problem #1Multiply sides to get possible offspring Step 3

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?

________ x ________ Ss Ss

S s

S

s Ss

SsSS

ss

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

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?

________ x ________ Rr rr

R r

r

r Rr

rrRr

rr

2:2

Step 3 MultiplyStep 2 Determine Parent genotypes

Dihybrid crossesDihybrid 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 1Draw 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- YROutside- YrInside- yRLast- yr

YyRr

Gremlin

Step 4Write all of the

possible combinations of alleles on the sides and top of the punnett square.YRYryRyr

YR

YR

Yr

Yr

yR

yR

yr

yr

Step 5

YR

YR

Yr

Yr

yR

yR

yr

yr

Do the cross Remember to keep

all of one letter together and all of the 2nd letter together.

YYRR YYRr YyRR YyRr

YYRr YYrr YyRr Yyrr

YyRR YyRr yyRR yyRr

YyRr Yyrr yyRr yyrr

Step 6Go 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 7Figure 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 PracticeDetermine 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

Problem #4

What percentage of offspring could be chestnut and pacing?

Step 1 Draw punnett squareIn horses, black is dependent

upon a dominant gene (B), and chestnut color upon its recessive allele (b).

The trotting gait is due to a dominant allele, (T), the pacing gait to its recessive allele, (t).

Cross a homozygous black, homozygous trotting horse with a chestnut, pacing horse.

________ x ________ BBTT bbtt

BT

bt

Bb

BbBb

Bb

0%

Step 3 and 4 Determine gametes and place on squareStep 2 Determine genotypesStep 5 Fill in the squareStep 6 Analyze resultsStep 7 Answer question(s)

BT BT BT

btbt

bt

BbTt chestnut, trotting

BbTt chestnut, trotting

BbTt chestnut, trotting

BbTt chestnut, trotting

BbTt chestnut, trotting

BbTt chestnut, trotting

BbTt chestnut, trotting

BbTt chestnut, trotting

BbTt chestnut, trotting

BbTt chestnut, trotting

BbTt chestnut, trotting

BbTt chestnut, trotting

BbTt chestnut, trotting

BbTt chestnut, trotting

BbTt chestnut, trotting

BbTt chestnut, trotting

Problem #5

NNBB: NnBb: nnbb:

Step 1 Draw punnett squareSuppose 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?

________ x ________ nnbb NnBb

nb

nb

Bb

BbBb

Bb

0/16

Step 3 and 4 Determine gametes and place on squareStep 2 Determine genotypesStep 5 Fill in the squareStep 6 Analyze resultsStep 7 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

Problem #6Step 1 Draw punnett square

Carrion beetles lay their eggs in dead animals and then bury them in the ground until they hatch. Assume that the preference for fresh meat (F) is dominant to the preference for rotted meat and that the tendency to bury the meat shallow (S) is dominant to the tendency to bury the meat deep. Suppose a female and male carrion beetle heterozygous both traits mate. What is the phenotypic ratio?

________ x ________ FfSs FfSs

FS

fs

Bb

BbBb

Bb

Step 3 and 4 Determine gametes and place on squareStep 2 Determine genotypesStep 5 Fill in the square

Fs fS fs

fSF

sF

S

FFSS fresh,

shallow

FFSs fresh,

shallow

FfSs fresh,

shallow

FfSS fresh,

shallow

FFSs fresh,

shallow

FFss fresh, deep

FfSS fresh,

shallow

Problem #6

Fresh/shallow: fresh/deep: rotted/shallow: rotted/deep

Carrion beetles lay their eggs in dead animals and then bury them in the ground until they hatch. Assume that the preference for fresh meat (F) is dominant to the preference for rotted meat and that the tendency to bury the meat shallow (S) is dominant to the tendency to bury the meat deep. Suppose a female and male carrion beetle heterozygous both traits mate. What is the phenotypic ratio?

________ x ________ FfSs FfSs

FS

fs

Bb

BbBb

Bb

9:3:3:1

Step 5 Fill in the squareStep 6 Analyze resultsStep 7 Answer question(s)

Fs fS fs

fSF

sF

S

FFSS fresh,

shallow

FFSs fresh,

shallow

FfSS fresh,

shallow

FfSs fresh,

shallow

FfSS fresh,

shallow

FFSs fresh,

shallow

FFss fresh, deep

Ffss fresh, deep

ffSS rotted,

shallow

ffSs rotted,

shallow

Ffss fresh, deep

ffSs rotted,

shallow

ffss rotted, deep

FfSS fresh,

shallow

FfSS fresh,

shallow

FfSS fresh,

shallow