1. Interpreting Pedigrees
Analyzing family trees for patterns of inheritance

2. Interpreting Pedigrees
What is a pedigree?
How can we look for patterns to determine inheritance of traits?

3. Pedigrees Pedigrees are genetic family trees.
A Pedigree
Pedigrees are genetic family trees.
Pedigree symbols represent individuals.

4. Indications Within Pedigrees for Recessive Traits
Most important indicator a trait is recessive:
Two affected parents cannot have unaffected offspring.

5. Indications Within Pedigrees for Recessive Traits
Other possible indicators of a recessive trait:
Traits typically skip generations.
Affected individuals typically have two unaffected parents.
~1/4 of siblings of an affected individual are affected.
Consanguineous matings (between relatives) increase the risk of a recessive trait.
Relatives may both be carriers of the same recessive allele, inherited from a common ancestor.

6. Indications Within Pedigrees for Dominant Traits
There are no “carriers” of a dominant trait, since the dominant allele is never masked or hidden.
Anyone who inherits even a single dominant allele must exhibit the dominant trait.

7. Indications Within Pedigrees for Dominant Traits
Most important indicators a trait is dominant:
Every affected individual has at least one affected parent.
Two unaffected parents must have all unaffected children.

8. Indications Within Pedigrees for Dominant Traits
Other possible indicators of a dominant trait:
Traits typically occur every generation.
~1/2 of affected parent’s progeny are affected.
Two affected parents can produce unaffected offspring.

9. Professor Hershberger’s Rules for Interpreting Pedigrees
Hershberger Rule 1: Match a genotype to each phenotype.
Genetics Principle: “We are what our genes make us.”
If the individual exhibits the recessive phenotype, he/she is aa (XaXa or XaY for an X-linked trait)
If the individual exhibits the dominant phenotype, he/she is A_ (XA_ or XAY for an X-linked trait).

10. Professor Hershberger’s Rules for Interpreting Pedigrees
Hershberger Rule 2: Where possible, track alleles (genes) UP the pedigree, from child to parent.
Genetics Principle: “We inherit one copy (allele) of each gene from each of our parents.”
A recessive child must have parents who are each either a carrier or affected.
A dominant child must have one dominant parent.

11. Professor Hershberger’s Rules for Interpreting Pedigrees
Hershberger Rule 3: Where possible, track alleles (genes) DOWN the pedigree, from parent to child.
Genetics Principle: “Each parent passes one of his/her alleles on to each child.”
A recessive parent cannot have non-carrier children.

12. Professor Hershberger’s Rules for Interpreting Pedigrees
Hershberger Rule 4: Calculate probabilities.
Genetics Principle: “Inheritance follows Mendel’s Law of Segregation.”
“When applying Hershberger Rules 1 through 3 does not reveal a genotype, draw the Punnett square and calculate genotype probabilities.”