Download - How to make a punnett square
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How to make a punnett squareA step by step method
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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.
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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?
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Step 1 Draw a box and divide it into 4 squares.
These squares represent the possible offspring
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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
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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
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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
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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?
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Let’s Practice Homozygous (HO) or Heterozygous (HE)
AA ____Bb ____Cc ____Dd ____
Ee ____ff ____GG ____ HH ____
HO
HE
HE
HE HO
HO
HO
HE
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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
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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
Ss
ss
PP
Pp
pp
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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
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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
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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
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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
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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
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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
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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.
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Step 1
Draw a box and divide it into 16 squares.
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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
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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
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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
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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
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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
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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
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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