Pedigrees
First Decision
Is it autosomal or
X-linked inheritance?
Autosomal: both genders
are equally affected;
male-to-male transmission
observed
If not, propose X-linkage
Second Decision
Is it dominant or recessive?
Dominant: trait does not
usually skip generations;
each person with at least
one copy of the dominant
allele will show the trait
If not, propose recessive
inheritance
Autosomal Recessive
• Males and females are
equally likely to have
the trait
Autosomal Dominant
• Males and females are
equally likely to have the
trait
• Traits do not skip
generations
• The trait is present
whenever the
corresponding gene is
present
• Male to male
transmission
• Traits often skip
generations
• Often, both parents of
the offspring who have
the trait are
heterozygotes
• Only homozygous
individuals have the trait
• Traits may appear in
siblings without
appearing in their
parents
• If a parent has the trait,
those offspring who do
not have it are
heterozygous carriers
X-linked Recessive
• Trait is more common in
males than females
X-linked Dominant
• All daughters of a male
who has the trait will
also have the trait
• No male to male
transmission
• A female who has the
trait may or may not
pass the gene to her son
or daughter
• All daughters of a male
who has the trait are
heterozygous carriers
• The son of a female
carrier has a 50 percent
chance of having the
trait
• No male-to-male
transmission
• Mothers of male who
have the trait are either
heterozygous carriers or
homozygous and
express the trait
• Daughters of female
carriers have a 50
percent chance of being
carriers
Scenario 2, Page 1-29 of Lab Manual
a.
b.
c.
d.
e.
Joseph and Jenny have come to a genetic counselor
to determine their chances of having a child with
myotonic dystrophy, a degenerative muscular
disorder that appears at about 50 years of age and
is inherited with the autosomal dominant allele D.
The autosomal recessive allele d does not
contribute to the development of the disease. Both
Joseph and Jenny are in their thirties and neither
shows any signs of the disease at present. Jenny’s
grandmother (her father’s mother) has the disease.
Jenny’s grandmother’s father (Jenny’s greatgrandfather) did not have the disease. There is no
history of myotonic dystrophy in Joseph’s family or
in Jenny’s mother’s or paternal grandfather’s family.
Draw the pedigree that shows the familial
relationships described in the paragraph above.
What is the probability that Jenny is a carrier of the
D allele?
What is the probability that Joseph and Jenny will
have a child with myotonic dystrophy?
Out of 3 children born to Joseph and Jenny, what is
the probability that 1 will have myotonic dystrophy?
Out of 5 children born to Joseph and Jenny, what is
the probability that at least one will have myotonic
dystrophy?
Scenario 4, Page 1-30 of Lab Manual
a.
b.
c.
d.
e.
f.
Alkaptonuria is a metabolic disorder that results from
the autosomal recessive allele a. Individuals who do
not have the disease have the dominant allele A.
Nancy, who does not have alkaptonuria, marries
Norman, who also does not have alkaptonuria.
Norman’s grandmother (his father’s mother) and
Nancy’s uncle (her mother’s brother) have
alkaptonuria. Norman’s grandfather (his father’s
father) does not have the disease and is not a
carrier of the alkaptonuria allele. Neither of Nancy’s
maternal grandparents (her mother’s mother and
father) have alkaptonuria. There is no history of
alkaptonuria in Norman’s mother’s family or in
Nancy’s father’s family.
Draw the pedigree that shows the familial
relationships described in the paragraph above.
What is the probability that Nancy is a carrier of the
a allele?
What is the probability that Norman is a carrier of
the a allele?
What is the probability that Nancy and Norman’s first
child will have alkaptonuria?
Out of 5 children born to Nancy and Norman, what is
the probability that 3 will have alkaptonuria?
Out of 4 children born to Nancy and Norman, what is
the probability that at least one will have
alkaptonuria?
Scenario 6, Page 1-31 of Lab Manual
a.
b.
c.
d.
Red-green colorblindness is inherited with
the X-linked recessive allele c. The Xlinked dominant allele C codes for full color
vision. John is colorblind. His wife Jean has
full color vision. Both of Jean’s parents and
both maternal grandparents (Jean’s
mother’s parents) have full color vision but
her great grandfather (the father of Jean’s
maternal grandmother) is colorblind. There
is no history of colorblindness in Jean’s
father’s family.
Draw the pedigree that shows the familial
relationships described in the paragraph
above.
What is the probability that Jean is a carrier
of the c allele?
Out of 3 children born to Jean and John,
what is the probability that 2 will be
colorblind sons?
Out of 4 children born to Jean and John,
what is the probability that at least one will
be a colorblind daughter?