Human Pedigree Problems (PDF)

Sex linked recessive genes
Human Pedigree Genetics Problem Set
Genes carried on the X chromosome have a distinctive pattern of inheritance. Because males are
hemizygous, i.e. they have only one copy of the X chromosome, and because the Y chromosome
carries very few genes (though those which it carries are often homologous to X linked genes)
then recessive mutations manifest themselves in the phenotype of males. If the mutant gene is
lethal (such as Duchenne Muscular Dystrophy) then it takes an unusual event to produce an
affected female.
An X linked pedigree
Problem 1: A human geneticist determined the pedigree shown in the diagram with filled
symbols showing the affected individuals. How is this pattern of inheritance described?
Example: A hypothetical pedigree for this family, showing heterozygous carriers in both the
parental and grandparental generations, is shown in the diagram. This demonstrates the use of
markers for identifying heterozygotes.
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Problem 2: The accompanying pedigree is for a rare, but relatively mild, hereditary disorder of
the skin.
a. Is the disorder inherited as a recessive or a dominant phenotype? State reasons for your
answer.
b. Give genotypes for as many individuals in the pedigree as possible. (Invent your own defined
allele symbols.) Consider the four unaffected children of parents III-4 and III-5. In all four-child
progenies from parents of these genotypes, what proportion is expected to contain all unaffected
children?
Problem 3: Consider the accompanying pedigree of a rare autosomal recessive disease, PKU.
a. List genotypes of as many of the family members as possible.
b. If individuals A and B marry, what is the probability that their first child will have PKU?
c. If their first child is normal, what is the probability that their second child will have PKU?
d. If their first child has the disease, what is the probability that their second child will be
unaffected?
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Problem 4: Here are four human pedigrees. The black symbols represent an abnormal
phenotype inherited in a simple Mendelian manner.
a. For each pedigree, state whether the abnormal condition is dominant or recessive. Try to state
the logic behind your answer.
b. For each pedigree, describe the genotypes of as many persons as possible.
Problem 5: The following is a complex pedigree for two traits, red hair and body hair.
What are the genetic basis of these traits. Are they related to each other.
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. Do you think that the red-hair allele is common or rare in the population as a whole?
Problem 6: The accompanying pedigree was obtained for a rare kidney disease.
a. Deduce the inheritance of this condition, stating your reasons. b. If individuals 1 and 2 marry,
what is the probability that their first child will have the kidney disease?
Problem 7: Consider the accompanying pedigree of a rare autosomal recessive disease, PKU.
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a. List genotypes of as many of the family members as possible.
b. If individuals A and B marry, what is the probability that their first child will have PKU? If
their first child is normal, what is the probability that their second child will have PKU? If their
first child has the disease, what is the probability that their second child will be unaffected?
Problem 8: Which best describes the genetics of the afflicting allele in the following pedigree
(it is a pedigree of taste blindness)?
Problem 9: Albinism is inherited as an autosomal recessive. In the figure below, assuming that
persons from the general population are not heterozygous for albinism (Aa), what are the
genotypes of all persons whose genotypes are known? (i.e., indicate the genotypes on the figure
for all known AA, Aa, and aa individuals)
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Tutorial: Using RFLP-analysis to hunt for human genes
RFLPs, or "restriction fragment length polymorphisms", is a term to describe different alleles of
a genetic locus that can be distinguished by Southern hybridization. Consider a simple example
of two different alleles for human ß-globin. The normal allele has three restriction enzyme
cleavage sites, but a mutation that causes sickle-cell anemia also results in an allele that has only
two cleavage sites. As shown in the figure below, the two different alleles display different
patterns following gel electrophoresis and Southern hybridization.
Sickle-cell anemia is an autosomal, recessive human disease. A hypothetical pedigree for parents
each heterozygous for the sickle-cell allele is shown in the figure below. The inheritance of the
normal and sickle-cell allele in this family can be followed by RFLP-analysis. Each of the
different genotypes, homozygous normal (AA), heterozygous carrier (Aa), and homozygous
recessive with sickle-cell anemia (aa) can be identified by RFLP analysis.
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