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 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
Rr
Rr
r
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
Rr
Rr
r
Rr
Rr
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?
Let’s Practice
 Homozygous (HO) or Heterozygous (HE)
AA ____
HO
Bb ____
HE
HE
Cc ____
HE
Dd ____
Ee ____
HE
ff ____
HO
HO
GG ____
HO
HH ____
Determine Phenotype
Purple flowers are
dominant to white flowers
Brown eyes are dominant to
blue eyes
Purple
PP _____________
Purple
Pp _____________
White
pp _____________
Brown
BB ____________
Brown
Bb ____________
Blue
bb ____________
For each phenotype list the
genotype
Straight hair is dominant to
curly.
Pointed heads are dominant
to round heads.
SS
PP
____________
straight ____________
pointed
____________
straight ____________
pointed
Ss
Pp
ss
pp
____________
curly
____________
round
Problem #1
Step 1
Draw punnett square
 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 #1
Determine the genotype of parents, and
place alleles on punnett square
 In pea plants, spherical
 ________
Ss x ________
Ss
Step 2
s
S
S
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?
s
Problem #1
Multiply sides to get possible offspring
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
s
 In pea plants, spherical
Ss
ss
S
Step 3
SS
Ss
Problem #1
Step
12
punnett
square
Interpret
theParent
possible
offspring
Step4
3Draw
Multiply
Determine
genotypes
 SpongeBob SquarePants recently
r
r
met SpongeSusie Roundpants at a ________
rr
Rr x ________
dance. SpongeBob is heterozygous
for his square shape, but
R
r
SpongeSusie is round (recessive).
What is the ratio of round to
Rr
rr
square pants?
2:2
Rr
rr
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 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
Step 3
 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
Gremlin
YyRr
Step 4
 Write all of the possible
YR
combinations of alleles
on the sides and top of YR
the punnett square.
 YR
Yr
 Yr
 yR
yR
 yr
yr
Yr
yR
yr
Step 5
 Do
the cross
 Remember
to keep
all of one letter
together and all of
the 2nd letter
together.
YR
Yr
yR
yr
YR
YYRR
YYRr
YyRR
YyRr
Yr
YYRr
YYrr
YyRr
Yyrr
yR
YyRR
YyRr
yyRR
yyRr
yr
YyRr
Yyrr
yyRr
yyrr
Step 6
 Go through the results
and determine what
the phenotype of each
offspring would be
Yr
yR
yr
YYRR
YYRr
YyRR
YyRr
Yellow
round
Yellow,
round
Yellow,
round
Yellow,
round
YYRr
YYrr
YyRr
Yyrr
Yellow,
round
Yellow,w
rinkled
Yellow,
round
Yellow,
wrinkled
YyRR
YyRr
yyRR
yyRr
Yellow,
round
Yellow,
round
Green,
round
Green,
round
YyRr
Yyrr
yyRr
Yyrr
Yellow,
round
Yellow,
wrinkled
Green,
round
Green,
wrinkled
YR
YR
Yr
yR
yr
Step 7
 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
YYRR
YYRr
YyRR
YyRr
Yellow
round
Yellow,
round
Yellow,
round
Yellow,
round
YYRr
YYrr
YyRr
Yyrr
Yellow,
round
Yellow,w
rinkled
Yellow,
round
Yellow,
wrinkled
YyRR
YyRr
yyRR
yyRr
Yellow,
round
Yellow,
round
Green,
round
Green,
round
YyRr
Yyrr
yyRr
Yyrr
Yellow,
round
Yellow,
wrinkled
Green,
round
Green,
wrinkled
YR
YR
Yr
yR
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.L
TtYy
First-TY
Outside- Ty
Inside- tY
Last- ty
Gremlin
TtYy
Problem #4
5
6
Fill
Analyze
inpunnett
results
square
Step
square
Step137
and
4the
Determine
gametes and place on square
2Draw
Determine
Answer
question(s)
genotypes
 In horses, black is dependent upon a
bt
bt
bt
bt
dominant gene (B), and chestnut  ________ x ________
BBTT
bbtt
color upon its recessive allele (b).
BT BT
BT BT
 The trotting gait is due to a
BbTt
BbTt
BbTt
BbTt
dominant allele, (T), the pacing gait
chestnut, chestnut, chestnut, chestnut,
to its recessive allele, (t).
trotting Bb
trotting
trottingBbtrotting
BbTt
BbTt
BbTt
BbTt
 Cross a homozygous black,
chestnut, chestnut,
chestnut, chestnut,
homozygous trotting horse with a
trotting
trotting
trotting
trotting
chestnut, pacing horse.
BbTt
BbTt
BbTt
BbTt
chestnut,
trotting
Bb
chestnut,
Bbchestnut,
trotting
chestnut,
trotting
trotting
BbTt
BbTt
BbTt
BbTt
chestnut,
trotting
chestnut,
trotting
chestnut,
trotting
chestnut,
trotting
What percentage of offspring could be chestnut
and pacing?
0%
Problem #5
5
6
Fill
Analyze
inpunnett
results
square
Step
square
Step137
and
4the
Determine
gametes and place on square
2Draw
Determine
Answer
question(s)
genotypes
 Suppose the alpha male and alpha
nb
nB
Nb
NB
female of a wolf pack are black with  ________ x ________
nnbb
NnBb
blue eyes and normal colored with
brown eyes, respectively. Assume
nb
nb
nb nb
further that normal coat color (N) is
NnBb NnBb
NnBb
NnBb
normal,
normal,
normal,
normal,
dominant to black (n) and brown
brown
brown
brown Bb brown
Bb
eyes (B) are dominant to blue (b).
Nnbb
Nnbb
Nnbb
Nnbb
normal,
normal,
normal,
normal,
The female is also heterozygous for
blue
blue
blue
blue
both traits.
nnBb
nnBb
nnBb
nnBb
Bb
Bb
black,
black,
black,
black,
 What fraction of the offspring
brown
brown
brown
brown
could have the following
nnbb
nnbb
nnbb
nnbb
genotypes?
black,
black,
black,
black,
blue
NNBB: 0/16 NnBb: 4/16 nnbb:
4/16
blue
blue
blue
Problem #6
fs
fS
Fs
FS
Fill
inpunnett
square
Step
square
Step135
and
4the
Determine
gametes and place on square
2Draw
Determine
genotypes
 Carrion beetles lay their eggs in
dead animals and then bury them  ________ x ________
FfSs
FfSs
in the ground until they hatch.
FS
Fs
fS
Assume that the preference for
fs
FFSS
FFSs
FfSS
FfSs
fresh meat (F) is dominant to the
fresh,
fresh,
fresh,
fresh,
shallow
shallowBbshallow
preference for rotted meat and that shallow Bb
the tendency to bury the meat
FFSs
FFss
FfSS
fresh,
fresh,
fresh,
shallow (S) is dominant to the
shallow
deep
shallow
tendency to bury the meat deep.
Bb
Bb
Suppose a female and male carrion
beetle heterozygous both traits
mate. What is the phenotypic
ratio?
Problem #6
Step 7
5 Answer
6
Fill in thequestion(s)
Analyze
results
square
 Carrion beetles lay their eggs in dead
fs
What is the phenotypic ratio?
fS
Fs
FS
animals and then bury them in the
 ________
FfSs x ________
FfSs
ground until they hatch. Assume that
the preference for fresh meat (F) is
FS
Fs
fS
fs
dominant to the preference for rotted
FFSS
FFSs
FfSS
FfSs
meat and that the tendency to bury the
fresh,
fresh,
fresh,
fresh,
shallow Bb
shallow
shallowBbshallow
meat shallow (S) is dominant to the
tendency to bury the meat deep.
FFSs
FFss
FfSS
Ffss
fresh,
fresh,
fresh,
fresh,
Suppose a female and male carrion
shallow
deep
shallow
deep
beetle heterozygous both traits mate.
FfSS
FfSS
ffSS
ffSs
fresh,
shallow
Bb
fresh,
Bbrotted,
shallow
rotted,
shallow
shallow
FfSS
Ffss
ffSs
ffss
fresh,
shallow
fresh,
deep
rotted,
shallow
rotted,
deep
Fresh/shallow: fresh/deep: rotted/shallow: rotted/deep
9:3:3:1