Download Interpreting Pedigrees

INTERPRETING
PEDIGREES
Analyzing family trees for patterns of
inheritance
Interpreting Pedigrees
• What is a pedigree?
• How can we look for patterns to determine inheritance of
traits?
Pedigrees
• Pedigrees are
genetic family trees.
• Pedigree symbols
represent individuals.
A Pedigree
Indications Within Pedigrees for
Recessive Traits
• Most important
indicator a trait is
recessive:
• Two affected parents
cannot have
unaffected offspring.
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.
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.
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.
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.
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).
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.
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.
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.”