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How to make a punnett square. 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. - PowerPoint PPT Presentation
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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 #1Step 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
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
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 6
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 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
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
What percentage of offspring could be chestnut and pacing?
Problem #4Step 1 Draw punnett square
________ x ________
In 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.
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
NNBB: NnBb: nnbb:
Problem #5Step 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 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
________ x ________
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?
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
________ x ________
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?
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
Fresh/shallow: fresh/deep: rotted/shallow: rotted/deep