Download Probability & Genetics

Probability & Genetics
Punnet Squares
Probability

Probability
The likelihood that a particular event will occur.
 Used to predict the results of an event.
 Ex. What is the likelihood that when you flip a
coin you’ll get heads?

How Does Probability Relate to Genetics?


Remember that Mendel counted the
offspring from every cross that he carried
out.
When Mendel crossed two plants that were
hybrid for stem height (Tt) he found that
¾ of the F1 plants had tall stems.
 ¼ of the plants had short stems.
 He got these numbers over and over.
 The probability was ¾ tall and ¼ short.
 Mendel used probabilities to predict the results
of a genetic cross.

Punnett Square
Shows all the
possible outcomes or
combinations of
alleles for a genetic
cross
Helps to determine
the probability that
an offspring will
have a certain trait
Reminder of Terms

Alleles


Two forms of a gene (dominant & recessive)
Dominant Allele
Stronger of two genes expressed in the hybrid
 Represented by a capital letter (R)


Recessive Allele
Gene that is masked and shows up less often in
a cross
 Represented by a lowercase letter (r)

Phenotypes vs. Genotypes

Genotype
Actual gene combination for a trait
 Ex. TT, Tt, tt


Phenotype
Physical appearance, or its visible traits that
result from a genotype
 Ex. Short or Tall

Genotype & Phenotype in
Flowers
Genotype of alleles:
R = red flower
r = yellow flower
All genes occur in pairs, so 2
alleles affect a characteristic
Possible combinations are:
Genotypes
RR
Phenotypes
RED
Rr
rr
RED YELLOW
Types of Genotypes
 Homozygous Genotype





An organism that has 2 identical alleles
Either 2 dominant or 2 recessive genes
AKA: Purebred
Ex. RR Homozygous dominant
Ex. rr Homozygous recessive
 Heterozygous Genotype
 An organism that has 1 dominant allele
and one recessive allele
 AKA: Hybrid
 Ex. Rr
Setting up a Punnett Square
1. Draw a large square, and then divide it into
4 equal sections (also squares).
Using a Punnett Square
2. Find two parents to mate that you know
their genotype
•
Ex. A red flower (genotype Rr) and a white
flower (genotype rr). Rr x rr
3. Place one of the parents on top, and one
on the left.
Using a Punnett Square
4. Take each letter in each column and
combine it with each letter from each row
in the corresponding square.
Using a Punnett Square
5. Use the possible genotypes to determine the
probability (chance) that you will have a
particular phenotype or genotype.

Ex.
Red phenotype (2/4 or 50%)
White phenotype (2/4 or 50%)
Phenotype Ratio: 1:1
Genotype Ratio: 1Rr:1rr
Punnett Square Work

Construct a Punnett Square for Black Mice
(BB) X Black Mice (Bb). Note: bb=white

Black Mice (BB) X Black Mice (Bb).
B
B
B
BB
BB
b
Bb
Bb

Black Mice (BB) X Black Mice (Bb).

B
B
B
BB
BB
b
Bb
What were the genotypes
produced by the cross?


What were the phenotypes
produced by the cross?


Bb
All black
What were the two
homozygous genotypes?
Were there two?


BB and Bb
BB
What was the heterozygous
genotype?

Bb

Black Mice (BB) X Black Mice (Bb).

B
B
B
BB
BB
b
Bb
Bb
What was the
genotypic ratio?
 1BB:

1Bb
What was the
phenotypic ratio?
 All
black
What do we mean by a Punnett Square?
Genetic Practice
Problems
Breed the P1 generation
tall
(TT) x dwarf (tt)
pea plants
T
t
t
T
Solution:
tall (TT) vs. dwarf (tt) pea plants
T
T
t
Tt
Tt
produces the
F1 generation
t
Tt
Tt
All Tt = tall
(heterozygous tall)
Breed the F1 generation
tall (Tt) vs. tall (Tt) pea
plants
T
T
t
t
Solution:
tall (Tt) x tall (Tt) pea plants
T
t
T
TT
Tt
t
Tt
tt
produces the
F2 generation
1/4 (25%) = TT
1/2 (50%) = Tt
1/4 (25%) = tt
1:2:1 genotype
3:1 phenotype
Back to
Mendel’s Peas
Crossing Mendel’s Parental
Trait: Seed Shape
Alleles: R – Round
r – Wrinkled
Cross: Round seeds
x Wrinkled seeds
RR
x
rr
r
r
R
Rr
Rr
R
Rr
Rr
Genotype: Rr
Phenotype: Round
Genotypic
Ratio: All alike
Phenotypic
Ratio: All alike
Crossing Mendel’s P Generation
Homozygous dominant x Homozygous
recessive
Offspring all Heterozygous (hybrids)
Offspring called F1 generation
Genotypic & Phenotypic ratio is ALL
ALIKE
F1 X F1 Cross
Trait: Seed Shape
Alleles: R – Round
r – Wrinkled
Cross: Round seeds
x Round seeds
Rr
x
Rr
R
r
R
RR
Rr
r
Rr
rr
Genotype: RR, Rr, rr
Phenotype: Round &
wrinkled
G.Ratio: 1:2:1
P.Ratio: 3:1
F1 X F1 Cross Review
Heterozygous x heterozygous
Offspring:
25% Homozygous dominant RR
50% Heterozygous Rr
25% Homozygous Recessive rr
Offspring called F2 generation
Genotypic ratio is 1:2:1
Phenotypic Ratio is 3:1
What Do the Peas Look Like?
How Do We Find Out if Something is
Homozygous Dominant or Heterozygous?
Perform a Test Cross
Take a hybrid from the F2
generation and cross with a pure
There are two possible
testcrosses:
Homozygous dominant x Hybrid
Homozygous recessive x Hybrid
Test Cross
st
(1 )
Trait: Seed Shape
Alleles: R – Round
r – Wrinkled
Cross: Round seeds
x Round seeds
RR
x
Rr
R
r
R
RR
Rr
R
RR
Rr
Genotype: RR, Rr
Phenotype: Round
Genotypic
Ratio: 1:1
Phenotypic
Ratio: All alike
Test Cross (2nd)
Trait: Seed Shape
Alleles: R – Round
r – Wrinkled
Cross: Wrinkled seeds x Round seeds
rr
x
Rr
R
r
r
Rr
Rr
r
rr
rr
Genotype: Rr, rr
Phenotype: Round &
Wrinkled
G. Ratio: 1:1
P.Ratio: 1:1
Testcross Review
Homozygous x heterozygous(hybrid)
Offspring:
50% Homozygous RR or rr
50% Heterozygous Rr
Phenotypic Ratio is 1:1
Called Test Cross because the
offspring have SAME genotype as
parents
Summary Questions



What is meant by probability? How is
probability related to genetics?
How are Punnett squares useful to
geneticists?
What is the difference between a phenotype
and a genotype? Give an example of each.
Applying What We’ve Learned



In pea plants, the allele for round seeds (R)
is dominant over the allele for wrinkled
seeds (r).
Construct a Punnett square that shows a
cross between a heterozygous plant with
round seeds (Rr) and a homozygous plant
with wrinkled seeds (rr).
What is the probability that an offspring will
have wrinkled seeds?