Chapter 14 (Part 2)

DIHYBRIDSAP Biology

Ms. Day

DIHYBRID CROSSES:

Assuming genes follow Mendelian Genetics (complete dominance)

Dihybrid Crosses crosses involving crossing 2 DIFFERENT

traits at one timeExample: Mate 2 parents and look at the

probability of seeing 2 traits, such as: eye color AND hair colorfreckles AND dimples

How do You Do Dihybrid Crosses?

2 separate monohybrid crosses (in your head)

1 square for EACH traituse PROBABILITY RULES and MULTIPLY

The Multiplication and Addition Rules Applied to Genetic CrossesProbability Rule #1The multiplication rule (“and”)

States that the probability that 2 or more independent events will occur together is the product (x ing) product (x ing) of their individual probabilities

Probability Rule #2The 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

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 traitThen use PROBABILITY RULES and MULTIPLY

One BIG Punnett Square 1. Find the possible gametes using

F.O.I.L method AaBb x AABb

AB ABAb AbaB AB ab Ab

Place the gametes in the Punnett Square & fill in boxes

AB Ab aB ab

AB

Ab

AB

Ab

AABB AABb AaBB AaBb

AAbB AAbb AabB Aabb

AABB AABb AaBB AaBb

AAbB AAbb AabB Aabb

Genotypic RatioAAbB = 8/16 or 1/2AaBB = 2/16 or 1/8Aabb = 2/16 or 1/8AABB = 2/16 or 1/8AAbb = 2/16 or 1/8

Phenotypic RatioA= freckles, a= no frecklesB = Dimples, b= no dimples

AAbB = 8/16 (50%) freckles, dimplesAaBB = 2/16 (12.5%) freckles, dimples

Aabb = 2/16 (12.5%) freckles, no dimplesAABB= 2/16 (12.5%) freckles, dimples

AAbb = 2/16 (12.5%) freckles, no dimplesSo…12/16 (75%) freckles AND dimples

4/16 (25%) freckles AND no dimples

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

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?

½ Ff x Ff what is the probability of getting Ff?

½ So..the probability of getting HhFf is ½ x ½ = ¼

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?

AABbCcDDeeFfX

AaBbCcDdeeFf ½ x ¼ x ½ x ½ x 4/

4 x ½ = 4/256 = 1/64 or 1.5% chance

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

Would you like to know a few SHORT CUTS?

Short Cuts for MONOHYBRID CROSSES

Every parent “donates” only 1 allele to each offspringLaw of Segregation

When crossing 2 heterozygous individuals in complete dominance, you will ALWAYS get1:2:1 GENOTYPE ratio

1 homozygous dominant: 2 heterozygous: 1 recessive3:1 PHENOTYPE ratio

3 dominant phenotype: 1 recessive

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