1. DIHYBRIDS AP Biology Ms. Day
Chapter 14 (Part 2)
DIHYBRIDS
AP Biology
Ms. Day

2. Assuming genes follow Mendelian Genetics
DIHYBRID CROSSES:
Assuming genes follow Mendelian Genetics
(complete dominance)

3. What is “Probability”? Why did Mendel get his results?
What is the probability that Mendel would get his results EACH time?
PROBABILITY = likelihood (chance) that an event will happen

4. Dihybrid Crosses
crosses involving crossing 2 DIFFERENT traits at one time
Example: Mate 2 parents and look at the probability of seeing 2 traits, such as:
eye color AND hair color
freckles AND dimples

5. The Multiplication and Addition Rules Applied to Genetic Crosses
Probability Rule #1
The multiplication rule (“and”)
States that the probability that 2 or more independent events will occur together is the product (x ing) of their individual probabilities

6. Probability Rule #2 The rule of addition (“or”)
States that the probability that any 1 of 2 or more exclusive events will occur is calculated by adding (+ ing) together their individual probabilities

7. How do You Do Dihybrid Crosses?
3 ways
1. one BIG Punnett Square (16 boxes instead of 4)
2. Make 2 separate Punnett Squares
1 square for EACH trait
Then use PROBABILITY RULES and MULTIPLY
3. MULTUPLY FRACTIONS IN YOUR HEAD!!!

8. How can you find possible gametes combinations?
Find the possible gametes using F.O.I.L method
AaBb (male) x AABb (female)
AB AB
Ab Ab
aB AB repeat
ab Ab repeat

9. Place the gametes in the Punnett Square & fill in boxes
AB Ab aB ab AB Ab
AABB
AABb
AaBB
AaBb
AAbB
AAbb
AabB
Aabb

10. Genotypic Ratio AAbB = 8/16 or 1/2 AaBB = 2/16 or 1/8

11. Phenotypic Ratio A= freckles, a= no freckles
B = Dimples, b= no dimples
AAbB = 8/16 (50%) freckles, dimples
AaBB = 2/16 (12.5%) freckles, dimples
Aabb = 2/16 (12.5%) freckles, no dimples
AABB= 2/16 (12.5%) freckles, dimples
AAbb = 2/16 (12.5%) freckles, no dimples
So…12/16 (75%) freckles AND dimples
4/16 (25%) freckles AND no dimples

12. Solving a Dihybrid Cross
Write out a key for EACH trait
AA, Aa= Freckles, aa= No freckles
BB, Bb= Dimples, bb= No dimples
Figure out the genotypes of EACH parent and put them together
Look at the alleles for EACH gene SPERATELY & figure out the FRACTION ratio of possible genotype and phenotypes
trait #1  genotype/phenotype possibilities
trait #2 genotype/phenotype possibilities
PUTTING IT ALL TOGETHER…Figure out genotype/phenotype possibilities of offspring by multiplying fractions

13. Probability Bellringer
Ms. Day has 4 sisters. What is the probability that this will happen (her mother having 5 girls in a row)?
½ x ½ x ½ x ½ x ½ =??
1/32 chance
or % chance

14. But…You STILL do NOT need to use a Punnett Square 
Every organism ALWAYS receives 1 allele for EACH gene from MOM and DAD
For Example: H= tall & h= short, F=purple & f = white
In a cross HHFf x hhff, what will is the phenotype possibilities?
4/4 Tall (HH) x ½ Purple (Ff)=
PR= 50% Tall & Purple
In a cross HhFf x hhFf, what will is the possibility that the offspring will have the 1st genotype?
HhFf?
Hh x hh  what is the probability of getting Hh?
Ff x Ff  what is the probability of getting Ff?
So..the probability of getting HhFf is ½ x ½ = ¼

15. Now…Let’s do Multicharter Problems 
What is the probability of producing an offspring with the phenotype AaBBCcDDeeFf in a cross between 2 parents with the following genotypes?
AABbCcDDeeFf X
AaBbCcDdeeFf
½ x ¼ x ½ x ½ x 4/4 x ½
= 4/256 = 1/64 or 1.5% chance

16. Would you like to know a few SHORT CUTS?

17. Short Cuts for MONOHYBRID CROSSES
Every parent “donates” only 1 allele to each offspring
Law of Segregation
When crossing 2 heterozygous individuals in complete dominance, you will ALWAYS get
1:2:1 GENOTYPE ratio
1 homozygous dominant: 2 heterozygous: 1 recessive
3:1 PHENOTYPE ratio
3 dominant phenotype: 1 recessive

18. Short Cuts for DIHYBRID CROSSES
When crossing 2 heterozygous individuals in complete dominance, you will ALWAYS get
9:3:3:1 PHENOTYPE ratio
9 dominant, dominant phenotype
3 dominant, recessive phenotype
3 recessive, dominant phenotype
1 recessive, recessive phenotype
NOTE: The genotypes have to be ALL heterozygous
Ex: HhFf x HhFf