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Probability & Genetic Crosses
Probability
1/6
(1 out of 6)
If you roll one die,
what’s the probability of
rolling a 6?
If you pull one card from the
deck, what’s the probability
of getting the ace of hearts?
1/52
(1 out of 52)
If you flip a coin, what’s
the probability of
1/2
landing on heads? (1 out of 2)
A Genetic Cross

Gregor Mendel (1865) discovered that
probability could be used to predict the
results of a genetic cross (mating)

If you know parent genotypes, you can
predict offspring genotypes
A Genetic Cross

If you know parent genotypes, you can predict
offspring genotypes
Bb
Example:
 Brown hair (B) is
dominant to blond hair (b).
 What are the possible
genotypes for their
offspring?
bb
♀ Bb x ♂ bb
Bb, Bb, bb, bb
A Genetic Cross

If you know parent genotypes, you can predict
offspring genotypes
Bb
Example:
 Brown hair (B) is
dominant to blond hair (b).
 What is the probability that
the Incredibles’ next child
will have brown hair?
bb
♀ Bb x ♂ bb
Bb, Bb, bb, bb
2/4 (50%) chance of
brown hair
Practice


In pea plants, smooth peas (S) are
dominant to wrinkled peas (s).
A purebred smooth pea is crossed with a
wrinkled pea.
♀ _____ x ♂ _____
Practice



In pea plants, smooth peas (S) are
dominant to wrinkled peas (s).
A purebred smooth
♀ SS x ♂ ss
pea is crossed with a
wrinkled pea.
Determine the possible
Ss, Ss, Ss, Ss
offspring genotypes &
100% chance of
smooth peas
phenotypes.
Practice


In guinea pigs, dark fur (D) is dominant to
light fur (d).
Cross a hybrid dark guinea pig with a light
guinea pig.
♀ _____ x ♂ _____
Practice



In guinea pigs, dark fur (D) is dominant to
light fur (d).
Cross a hybrid
♀ Dd x ♂ dd
dark guinea pig with a
light guinea pig.
Determine the possible
Dd, Dd, dd, dd
offspring genotypes &
50% chance of dark fur
50% chance of light fur
phenotypes.
A Genetic Cross

If you know offspring genotypes, you
can predict parent genotypes
Example:


Bb
?
b
?b
Brown eyes (B) are
dominant to blue eyes (b).
If Dash’s genotype is bb,
what are his parents’
genotypes?
bb
Practice

In guinea pigs, dark fur (D) is dominant to
light fur (d).

Two dark-fur guinea
pigs produce a light-fur
offspring.
What is the genotype
of the offspring?

dd
Practice



In guinea pigs, dark fur (D) is dominant to
light fur (d).
Two dark-fur guinea
pigs produce a light-fur
offspring.
What must the parent
genotypes be?
Dd
?
D
?d
dd
Punnett Squares

Created by Reginald Punnett (1900) to
make predicting the outcome of genetic
crosses easier

show all the possible
outcomes of a genetic
cross
show the probability
of each outcome

25%
25%
25%
25%
Punnett Squares


show all the possible outcomes of a genetic cross
show the probability of each outcome
♂ SS
♀ Ss
Example:
 In dogs, short
hair (S) is
dominant to
long hair (s).
 Determine the possible
offspring phenotypes.
S
S
S
SS
SS
25%
s
Ss
25%
25%
Ss
25%
100% chance of
short hair
Why Use a Punnett Square?
Most scientists
study MANY
traits at one time
It gets VERY
confusing
without a
Punnett Square!
Practice

In humans,
straight toes (S)
is dominant to curled toes (s).
Determine the possible
results for a cross
between a recessive
male & hybrid
female.
♀ Ss

S
s
♂ ss
s
s
Ss
Ss
25%
25%
ss
ss
25%
25%
50% chance of straight toes
50% chance of curled toes
Practice

In pea plants, yellow color (Y) is
dominant to green (y).
♂ Yy
Y
Determine the possible
results for a cross of
2 hybrid
plants.
♀ Yy

Y
YY
25%
y
Yy
25%
y
Yy
25%
yy
25%
75% chance of yellow peas
25% chance of green peas
Special Cases

Incomplete dominance


Neither allele is dominant
Hybrid = in BETWEEN phenotypes
Example:
Carnations
CRCR =
CRCW =
CWCW =
?
Special Cases

Codominance


Neither allele is dominant
Hybrid = BOTH phenotypes
Example:
Cows
HWHW =
HWHB =
HBHB =
?
Easter Egg Genetics
Trait: Color
Dominant Color
Recessive Color
Phenotype
Allele
Phenotype
Allele
Blue
B
Green
b
Yellow
Y
Orange
y
Purple
P
pink
p