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

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

3. 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

4. How do You Do Dihybrid Crosses?  2 separate monohybrid crosses (in your head)  1 square for EACH trait  use PROBABILITY RULES and MULTIPLY

5. The Multiplication and Addition Rules Applied to Genetic Crosses Probability Rule #1 The multiplication rule (“ and ”) product (x ing) 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? 2 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

8. One BIG Punnett Square 1. Find the possible gametes using F.O.I.L method AaBb x AABbABAb aBAB abAb

9. Place the gametes in the Punnett Square & fill in boxes AB Ab aB ab AB Ab AB Ab AABBAABbAaBBAaBb AAbBAAbbAabBAabb AABBAABbAaBBAaBb AAbBAAbbAabBAabb

10. Genotypic Ratio AAbB = 8/16 or 1/2 AaBB = 2/16 or 1/8 Aabb = 2/16 or 1/8 AABB = 2/16 or 1/8 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 Rules and Dihybrid Crosses  A dihybrid or other multicharacter cross Is equivalent to two or more independent monohybrid crosses occurring simultaneously  In calculating the chances for various genotypes from such crosses Each character first is considered separately and then the individual probabilities are multiplied together

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 1 st genotype?  HhFf? Hh x hh  what is the probability of getting Hh? ½½ Ff x Ff  what is the probability of getting Ff? ½½  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. 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

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

18. 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

19. 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