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Scenarios for Building a Pedigree
1. Nick and Nora have come to a genetic counselor to determine their chances of
having a child with Huntington’s disease, a degenerative neurological disorder
that appears at about 50 years of age and is inherited with the autosomal
dominant allele H. The autosomal recessive allele h does not contribute to the
development of the disease. Both Nick and Nora are in their thirties and neither
shows any signs of the disease at present. Nick’s grandfather (his father’s father)
died of Huntington’s disease. Nick’s grandfather’s mother (Nick’s greatgrandmother) did not have the disease. There is no history of Huntington’s
Disease in Nora’s family or in Nick’s mother’s or paternal grandmother’s family.
a. Draw the pedigree that shows the familial relationships described in the
paragraph above. Use a circle to denote a woman, a square to denote a
man and a filled symbol to show individuals who have Huntington’s
disease. In this pedigree, you are attempting to trace the inheritance of
the H allele. Beneath each symbol, show the genotype of each individual
as far as it is known. You may use blanks to indicate that part of the
genotype is unknown, if applicable.
b. What is the probability that Nick is a carrier of the H allele?
c. What is the probability that Nick and Nora will have a child with
Huntington’s disease?
d. Out of 4 children born to Nick and Nora, what is the probability that 2 will
have Huntington’s disease?
e. Out of 3 children born to Nick and Nora, what is the probability that at
least one will have Huntington’s disease?
2. 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.
a. Draw the pedigree that shows the familial relationships described in the
paragraph above. Use a circle to denote a woman, a square to denote a
man and a filled symbol to show individuals who have myotonic
dystrophy. In this pedigree, you are attempting to trace the inheritance of
the D allele. Beneath each symbol, show the genotype of each individual
as far as it is known. You may use blanks to indicate that part of the
genotype is unknown, if applicable.
b. What is the probability that Jenny is a carrier of the D allele?
c. What is the probability that Joseph and Jenny will have a child with
myotonic dystrophy?
d. Out of 3 children born to Joseph and Jenny, what is the probability that 1
will have myotonic dystrophy?
e. Out of 5 children born to Joseph and Jenny, what is the probability that at
least one will have myotonic dystrophy?
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3. Galactosemia is a metabolic disorder that results from the autosomal recessive
allele g. Individuals who do not have the disease have the dominant allele G.
Betty, who does not have galactosemia, marries Bill, who also does not have
galactosemia. Betty’s grandfather (her mother’s father) and Bill’s uncle (his
father’s brother) have galactosemia. Betty’s grandmother (her mother’s mother)
does not have the disease and is not a carrier of the galactosemia allele. Neither
of Bill’s paternal grandparents (his father’s mother and father) have
galactosemia. There is no history of galactosemia in Betty’s father’s family or in
Bill’s mother’s family.
a. Draw the pedigree that shows the familial relationships described in the
paragraph above. Use a circle to denote a woman, a square to denote a
man and a filled symbol to show individuals who have the disease
galactosemia. In this pedigree, you are attempting to trace the inheritance
of the g allele. Beneath each symbol, show the genotype of each
individual as far as it is known. You may use blanks to indicate that part of
the genotype is unknown, if applicable.
b. What is the probability that Betty is a carrier of the g allele?
c. What is the probability that Bill is a carrier of the g allele?
d. What is the probability that Betty and Bill’s first child will have
galactosemia?
e. Out of 4 children born to Betty and Bill, what is the probability that 2 will
have galactosemia?
f. Out of 3 children born to Betty and Bill, what is the probability that at least
one will have galactosemia?
4. 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.
a. Draw the pedigree that shows the familial relationships described in the
paragraph above. Use a circle to denote a woman, a square to denote a
man and a filled symbol to show individuals who have the disease
alkaptonuria. In this pedigree, you are attempting to trace the inheritance
of the a allele. Beneath each symbol, show the genotype of each
individual as far as it is known. You may use blanks to indicate that part of
the genotype is unknown, if applicable.
b. What is the probability that Nancy is a carrier of the a allele?
c. What is the probability that Norman is a carrier of the a allele?
d. What is the probability that Nancy and Norman’s first child will have
alkaptonuria?
e. Out of 5 children born to Nancy and Norman, what is the probability that 3
will have alkaptonuria?
f. Out of 4 children born to Nancy and Norman, what is the probability that
at least one will have alkaptonuria?
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5. Ocular albinism is inherited with the X-linked recessive allele a. The X-linked
dominant allele A codes for full ocular color. George has ocular albinism. His wife
Ginny has full ocular color. Both of Ginny’s parents and both maternal
grandparents (Ginny’s mother’s parents) have full ocular color but her great
grandmother (the mother of Ginny’s maternal grandmother) has ocular albinism.
There is no history of ocular albinism in Ginny’s father’s family.
a. Draw the pedigree that shows the familial relationships described in the
paragraph above. Use a circle to denote a woman, a square to denote a
man and a filled symbol to show individuals who have ocular albinism. In
this pedigree, you are attempting to trace the inheritance of the a allele.
Beneath each symbol, show the genotype of each individual as far as it is
known. You may use blanks to indicate that part of the genotype is
unknown, if applicable.
b. What is the probability that Ginny is a carrier of the a allele?
c. Out of 4 children born to Ginny and George, what is the probability that 2
will be daughters with ocular albinism?
d. Out of 3 children born to Ginny and George, what is the probability that at
least one will be a son with ocular albinism?
6. 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.
a. Draw the pedigree that shows the familial relationships described in the
paragraph above. Use a circle to denote a woman, a square to denote a
man and a filled symbol to show individuals who are colorblind. In this
pedigree, you are attempting to trace the inheritance of the c allele.
Beneath each symbol, show the genotype of each individual as far as it is
known. You may use blanks to indicate that part of the genotype is
unknown, if applicable.
b. What is the probability that Jean is a carrier of the c allele?
c. Out of 3 children born to Jean and John, what is the probability that 2 will
be colorblind sons?
d. Out of 4 children born to Jean and John, what is the probability that at
least one will be a colorblind daughter?