1. Probability & Genetic Crosses

2. VOCABULARY TEST TOMORROW!!!
Learning Target:
How do you determine which trait is expressed from an organism’s genotype?
Science Starter:
Green body color (G) is dominant to yellow (g) body color. What would the genotype be for the phenotypes below?
______ ______ Green body color
______ Yellow body color
VOCABULARY TEST TOMORROW!!!

3. You will have 10 minutes to work on your create-a-face packet.
EVERYONE must finish the chart of the traits and answer analysis question 1.

4. Probability
If you roll one die, what’s the probability of rolling a 6?
1/6
(1 out of 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 landing on heads?
1/2
(1 out of 2)

5. 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

6. A Genetic Cross ♀ Bb x ♂ bb Bb, Bb, bb, bb Example: Brown hair (B) is
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

7. 2/4 (50%) chance of brown hair
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

8. Practice
In pea plants, smooth peas (S) are dominant to wrinkled peas (s).
A purebred smooth pea is crossed with a wrinkled pea.
♀ _____ x ♂ _____

9. Practice
In pea plants, smooth peas (S) are dominant to wrinkled peas (s).
A purebred 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

10. 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 ♂ _____

11. Practice In guinea pigs, dark fur (D) is dominant to light fur (d).
Cross a hybrid
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 fur
50% chance of light fur

12. 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

13. 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

14. 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

15. 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%

16. Punnett Squares Example: In dogs, short hair (S) is dominant to
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.
♂ SS
S S
25% S s SS SS
♀ Ss Ss Ss
100% chance of short hair

17. Why Use a Punnett Square?
Most scientists study MANY traits at one time
It gets VERY confusing without a Punnett Square!

18. 50% chance of straight toes
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
25% Ss Ss S s
♀ Ss ss ss
50% chance of straight toes
50% chance of curled toes

19. Practice In pea plants, yellow color (Y) is dominant to green (y).
Determine the possible
results for a cross of
2 hybrid
plants.
♂ Yy
Y y
25% YY Y y Yy
♀ Yy Yy yy
75% chance of yellow peas
25% chance of green peas

20. Special Cases ? Incomplete dominance Example: Carnations
Neither allele is dominant
Hybrid = in BETWEEN phenotypes
Example:
Carnations
CRCR = ?
CRCW =
CWCW =

21. Special Cases ? Codominance Example: Cows Neither allele is dominant
Hybrid = BOTH phenotypes
Example:
Cows
HWHW = ?
HWHB =
HBHB =

22. Let’s practice
Bikini Bottom Genetics #2
Exit Ticket—pg 41—Answer questions 3 & 4
(You must have the problems checked before leaving class!)