probability & genetic crosses. probability if you roll one die, what’s the probability of...
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Probability & Genetic Crosses
ProbabilityIf you roll one die, If you roll one die, what’s the what’s the probability of probability of rolling a 6?rolling a 6?
If you pull one card If you pull one card from the deck, what’s from the deck, what’s the probability of the probability of getting the ace of getting the ace of hearts?hearts? If you flip a coin, If you flip a coin,
what’s the what’s the probability of probability of landing on heads?landing on heads?
1/6
(1 out of 6)
1/52
(1 out of 52)
1/2
(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
Example: Brown hair (B) is
dominant to blond hair (b). What are the possible
genotypes for their offspring?
bb Bb
♀ Bb x ♂ bb
Bb, Bb, bb, bb
A Genetic Cross If you know parent genotypes, you can
predict offspring genotypes
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
♀ 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 homozygous smooth pea is
crossed with a wrinkled pea.
♀ _____ x ♂ _____
Practice In pea plants, smooth peas (S) are
dominant to wrinkled peas (s). A homozygous smooth
pea is crossed with a wrinkled pea.
Determine the possible offspring genotypes & phenotypes.
♀ SS x ♂ ss
Ss, Ss, Ss, Ss
100% chance of smooth peas
Practice In guinea pigs, dark fur (D) is
dominant to light fur (d). Cross a heterozygous dark guinea
pig with a light guinea pig.
♀ _____ x ♂ _____
Practice In guinea pigs, dark fur (D) is
dominant to light fur (d). Cross a heterozygous
dark guinea pig with a light guinea pig.
Determine the possible offspring genotypes & phenotypes.
♀ Dd x ♂ dd
Dd, Dd, dd, dd
50% chance of dark fur50% chance of light fur
A Genetic Cross If you know offspring genotypes,
you can predict parent genotypes
Example: Brown eyes (B) are
dominant to blue eyes (b). If Dash’s genotype is bb,
what are his parents’ genotypes?
bb
? ? bb 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
? ? Dd 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
Example: In dogs, short
hair (S) is
dominant to
long hair (s). Determine the possible
offspring phenotypes.
25% 25%
25% 25%
♀
Ss
♂ SS
S
s
S S
100% chance of short hair
SS
Ss
SS
Ss
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 & heterozygous female.
25% 25%
25% 25%
♀
Ss
♂ ss
50% chance of straight toes
50% chance of curled toes
S
s
s s
Ss Ss
ss ss
Practice In pea plants, yellow color (Y) is
dominant to green (y).
Determine the possible
results for a cross of
2 heterozygous
plants.
25% 25%
25% 25%
♀ S
s
♂ Ss
S s
S
s
SS Ss
Ss ss
75% chance of yellow peas
25% chance of green peas
Special Cases
Incomplete dominance Neither allele is dominant Heterozygous = in BETWEEN phenotypes
Example:
CarnationsCRCR =
CWCW =
CRCW = ?
Special Cases
Codominance Neither allele is dominant Heterozygous = BOTH phenotypes
Example:
CowsHWHW =
HBHB =
HWHB = ?