How to make a punnett square

1
How to make a punnett square

• A step by step method

2
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.

3
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?

4
Step 1

• Draw a box and divide it into 4 squares. These
  squares represent the possible offspring

5
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
6
Step 3

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

R
R


Rr
r
Rr
Rr
Rr
r
7
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


Rr
r
Rr
Rr
Rr
r
8
Ratios

• Genotypic
• 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?

9
Lets Practice

• Homozygous (HO) or Heterozygous (HE)

AA ____Bb ____Cc ____Dd ____ Ee ____ff ____GG ____ HH ____
HO
HE
HE
HO
HE
HO
HE
HO
10
Determine Phenotype
Purple flowers are dominant to white flowersPP _____________Pp _____________pp _____________ Brown eyes are dominant to blue eyesBB ____________Bb ____________bb ____________
Brown
Purple
Purple
Brown
White
Blue
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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
PP
Ss
Pp
ss
pp
12
Problem 1

• Draw punnett square

• Step 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?

13
Problem 1

• Determine the genotype of parents, and place
  alleles on punnett square

• Step 2

Ss

• ________ 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
S s


S s
14
Problem 1

• Step 3

• Multiply sides to get possible offspring

Ss

• ________ 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
S s


Ss
ss
S s
Ss
SS
15
Problem 1

• Interpret the possible offspring

• Step 4

• 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

• ________ x ________

Ss
S s


Ss
ss
S s
Ss
SS
3/4
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Problem 2

• Step 4 Interpret the possible offspring

• Step 1 Draw punnett square

Step 3 Multiply
Step 2 Determine Parent genotypes

• 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

• ________ x ________

Rr
R r


Rr
rr
r r
rr
Rr
22
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Problem 3

• Step 4 Interpret the possible offspring

• Step 1 Draw punnett square

Step 3 Multiply
Step 2 Determine Parent genotypes

• 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 Squidwards offspring could be
  green?

BB
bb

• ________ x ________

B B


Bb
Bb
b b
Bb
Bb
0
18
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.

19
Step 1

• Draw a box and divide it into 16 squares.

20
Step 2

• Determine 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

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Step 3
Gremlin

• Determine 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




22
Step 4

• Write 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
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Step 5
yr

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

YR
Yr
yR
YYRR Yellow round
YYRr Yellow, round
YyRR Yellow, round
YyRr Yellow, round




YR
YYRr Yellow, round
YYrr Yellow,wrinkled
YyRr Yellow, round
Yyrr Yellow, wrinkled
Yr
YyRR Yellow, round
YyRr Yellow, round
yyRR Green, round
yyRr Green, round
yR
YyRr Yellow, round
Yyrr Yellow, wrinkled
yyRr Green, round
Yyrr Green, wrinkled
yr
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Step 6
yr

• Figure out the phenotypic ratio or answer
  whatever question they ask you.
• 9331
• Example what are the chances of having a yellow
  and wrinkled pea?
• 3/16

YR
Yr
yR
YYRR Yellow round
YYRr Yellow, round
YyRR Yellow, round
YyRr Yellow, round




YR
YYRr Yellow, round
YYrr Yellow,wrinkled
YyRr Yellow, round
Yyrr Yellow, wrinkled
Yr
YyRR Yellow, round
YyRr Yellow, round
yyRR Green, round
yyRr Green, round
yR
YyRr Yellow, round
Yyrr Yellow, wrinkled
yyRr Green, round
Yyrr Green, wrinkled
yr
25
Lets 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

Gremlin TtYy
F.O.I.L TtYy First- Outside- Inside- Last-
TY
Ty
tY
ty
26
Problem 4

• Step 1 Draw punnett square

Step 2 Determine genotypes
Step 5 Analyze results
Step 3 and 4 Determine gametes and place on square
Step 6 Answer question(s)

• 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

• ________ x ________

nb
nb
nb
nb
NnBb normal, brown
NnBb normal, brown
NnBb normal, brown
NnBb normal, brown




NB
Bb
Bb
Nnbb normal, blue
Nnbb normal, blue
Nnbb normal, blue
Nnbb normal, blue
Nb
nnBb black, brown
nnBb black, brown
nnBb black, brown
nnBb black, brown
Bb
Bb
nB
nnbb black, blue
nnbb black, blue
nnbb black, blue
nnbb black, blue
nb

• NNBB NnBb nnbb

0/16
4/16
4/16