1. Dihybrid Crosses

2. Mendel’s Second Experiment
following 2 traits at once found that the inheritance of one trait (on one set of homologous chromosomes) has no influence on the inheritance of a second trait (on another set of homologous chromosomes)
eg. hair colour has nothing to do with widow's peak if these traits are on different chromosomes

3. Mendel’s Law of Independent Assortment
Mendel’s Second Law:
The Law of Independent Assortment‐ if genes are located on separate chromosomes, they will be inherited independently of one another

4. Example - Cat’s Fur
Traits: length of fur‐‐short or long (angora)
colour: 'wild grey' or all others
*note these are both single genes, each with 2 alleles
F: (dominant allele) for short fur
f: (recessive allele) for long fur
G: (dominant allele) for wild grey coloured fur
g: (recessive allele) for orange coloured fur
Sample Problem:
Cross a purebred short haired wild grey cat with a long‐haired orange cat

5. Example Cont.
Cross a purebred short haired wild grey cat with a long‐haired orange cat P1 P2
Phenotype
Genotype
Possible Gametes F1

6. Example - dihybrid cross
Cross 2 F1 cats and determine the phenotypic ratio of a dihybrid cross.
Therefore... F1
Phenotype
Genotype
Possible Gametes F2

7. How to read the results F2 FG Fg fG fg
FFGG
FFGg
FfGG
FfGg
FFgg
Ffgg
ffGG
ffGg
ffgg
The ratio for a dyhibrid cross between two double heterozygous traits is as follows:
9 F_G_
Short haired, wild grey fur
3 F_gg
Short haired, orange fur
3 ffG_
Long haired, wild grey fur
1 ffgg
Long haired, orange fur

8. Sample Problem 2:
A dihybrid cross is used to determine the offspring based on two traits.
e.g. In Drosophila melanogaster, wings are dominant to vestigial wings, and grey body colour is dominant to black. Determine the expected offspring for the F1and F2 generations of a purebred recessive male and purebred dominant female. Use punnett squares to show your work.
Let A represent wings, and a represent vestigial wings.
Let G represent grey, and g represent black

9. Brad: _________________ Angelina: _________________
Sample Problem 3:
In humans, brown hair (B) is dominant over blond hair (b) and normal lips (L)are dominant over "bee‐sting" lips (l). Brangelina are planning on having yet another baby. Since Shiloh's hair is blond and she has "the lips", you can determine Brad and Angelina's genotypes.
Brad: _________________ Angelina: _________________
Determine the potential genotypes of Brangelina's 7th child using a punnett square.
What is the probability they will have another blond‐haired bee‐stung lipped child?
What is the probability they will have a brown‐haired normal?