|Introduction to Cell & Molecular Biology (BIOL121) - Dr. S.G. Saupe (firstname.lastname@example.org); Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321|
Studies in Genetics
that you are a genetic counselor and that your services have been sought
to answer a variety of questions. What
is response to the following?
Remember, you must educate your clients and you must be unbiased.
baby is born with cystic fibrosis. What
must be true of the genotypes of the biological parents, neither of whom has
cystic fibrosis? What is the baby's genotype?
40-year old math teacher is told she has Huntington's Disease (HD).
What can be said about the genotypes of the her biological parents?
What is the genotype of the teacher?
non-hemophiliac parents of a 2-year-old boy are told that the boy has
hemophilia. What would the
child's genotype be? What can
we tell about
the genotypes of his biological parents?
cancer develops in many smokers.
publications have suggested
a strong family tendency to this type of cancer.
What might be
an explanation for the fact that not all smokers develop lung cancer?
couple's first child is diagnosed as having sickle cell disease. Neither
parent has the disease. What is
the probability that their second child
will also have this disease? What
is the probability that their third child
will have it?
couple has learned that the husband has a severe form of heart disease
that is inherited in an autosomal dominant fashion.
They have had
two children, neither of whom inherited this disorder. What is the chance
that each of these children would inherit heart disease from their father?
What about any future children of this couple?
child is born to parents who never had any symptoms of PKU. Indeed,
there have never been any symptoms of PKU as far back as anyone can remember
in either family. Why would the
biological parents of
the child with this disorder not have any symptoms themselves?
Is there any guarantee that a baby will be born healthy if there is no history of inherited disease on either side of the family, if the parents are not exposed to any agent know to cause birth defects and if the results of tests performed before the baby's birth are normal?
1. Mrs. Smith is 25 years old and
nine weeks pregnant. She has two
healthy, active children ages four and six years.
However, she is presently
taking a psychotherapeutic drug, Sinequon (doxepin), for anxiety.
Several months earlier she had a fifth-month miscarriage.
concern is her exposure to radiation while living for 18 years at Love Canal.
What do you tell her? (Hint:
Physician's Desk Reference will
describe effect of prenatal exposure to medications. What about the effects
2. Jill and Kevin are seriously
considering an immediate termination
of a 16 week pregnancy. Jill, 25
years old, has had two sonograms;
both indicated a live fetus, developing at the proper rate but with a growing
cyst on the neck. Such cysts are
often associated with chromosomal
abnormalities such as Turner's or Down's syndrome; others are found with neural
tube defects. The radiologists has
told them they are
at a high risk of having a child with a sever birth defect but that the cyst may
only represent a benign tumor, removable at birth. During the initial
discussion you discover this is an unplanned pregnancy; Jilll was taking Ortho-Novum
birth control pills the first 45 days of gestation. (hints: study prenatal
detection of neural tube defects, such as spina bifida and its postnatal
treatment and prognosis; also effect of hormones on fetal development)
3. Ruth is 25 years old and
pregnant. Her father, age 50, was recently
diagnosed with the genetic disorder olivopontocerebellar atrophy (OPCA).
Typically, this disorder is dominantly inherited.
The age of onset
is between 30 and 40 years of age. The
prognosis is progressive loss
of muscle coordination. (Hint:
a parallel with Huntington's Disease)
4. Sarah, age 14 months, was born
with microcephaly, small head (cranium)
size and is severely retarded. Chromosome
analysis indicates that
she lacks a piece of one of her number 13 chromosomes. Her parents
come in to have their chromosomes analyzed to see whether one of them may have
been a carrier for a translocation that could result in children affected like
Sarah or children with other chromosomal, anomalies.
5. Mr. Right requests paternity
testing for himself. His wife has left
him for their mutual best friend. She
is asking for a divorce, claiming she
has loved the other man for a long time and that he is in fact the father of
their four-month old son. Mr. Right
wants visitation rights and parental
responsibilities if he is the biological father. The testing requires blood
samples the infant, the mother and the husband. The results, using
six genetic markers, blood groups such as ABO, Rh and HLA (human
leukocyte-associated antigens) types, indicate the husband cannot be the
Case 6. A husband and his pregnant wife seek genetic counseling. Each carries one flawed copy of the gene responsible for achondroplasia; thus both are dwarfs. Recently, a California research team described the mutation in a gene on chromosome 4 that causes achondroplasia. The counselor explains that genetic testing can determine whether the fetus has inherited the achondroplasia mutation. In the discussion, the couple inform the you that they will abort any fetus that carries two mutant genes, That's not surprising, since children born with two such genes rarely survive beyond infancy. Indeed, the couple has had such a child. This time around, they say, they want a baby who is heterozygous for the achondroplasia trait. Such a child inherits a flawed gene from one parent and a healthy gene from the other parent. That genetic combination means the child will be a dwarf just like the parents. At the same time, the parents say they will abort any fetus that does not inherit one copy of the mutant gene. Should the counseling center perform the test, knowing that couple plant to abort a healthy fetus. (Consider - achondroplasia is a serious disorder, often with abnormal bones; and problems of dwarfs raising normal child)
7. A husband and wife have a child
who suffers from cystic fibrosis,
an incurable, fatal hereditary disease that result in frequent infections and
difficulty breathing. The couple
want to determine their risk
of having another child with this disorder.
Because CF is a recessive
disorder, a child usually must inherit the C gene from both parents to get the
disease. A child with just one C
gene is a carrier; such
a person doesn't have the disorder by can pass the trait on the next generation.
The DNA test revealed that the mother of the child carried the C
trait. However, her husband did
not. Indeed, the DNA tests showed that
he was not the biological father of the child.
That fact significantly decreased
this couples chance of having another child with C. Should the
counselor tell the couple about the nonpaternity findings?
Should the mother
be told privately? Is the center
colluding with the mother to withhold
information from the husband? Should
the center contact the biological
father and him know that he is probably a carrier for the mutant gene for C.
Case 8. A 37 year old pregnant woman worries about her family's history of mental retardation. Genetic testing shows this woman is a carrier of the mutant gene for fragile X syndrome, an inherited form of mental retardation. US and Dutch researchers have demonstrated that carriers and people with the condition show repetitive DNA segments in a gene called FMR-1. The woman is offered prenatal testing and says she will abort an affected fetus. She also indicates that she will abort a female fetus that is a carrier of the flawed gene. She states that she wants her legacy of this inherited disorder to end. Does this woman have the right to abort a healthy fetus that happens to be a carrier of a genetic mutation?
Case 9. A 30-year old woman has been diagnosed with familial adenomatous polyposis, an inherited condition that puts her at extremely high risk of colon cancer. Indeed, most people with this condition, if untreated, will develop the cancer by age 40. The patient's own mother died of colon cancer at age 32. Despite this grim history, the woman refuses to share the diagnosis with her family, including her four siblings and her husband. In addition, she does not want her two children to be tested for the genetic flaw. Researchers know that the mutant gene responsible for this disorder lies on the long arm of chromosome 5. This flaw results in hundreds of thousand of polyps, small wart-like growths, carpeting the lining of the colon. Some of the polyps will become malignant. The woman's refusal to tell her family puts the genetic counseling center in a quandary. To honor this patient's request might harm her family. Without appropriate medical intervention, family members with the mutant gene remain at extremely high risk of dying of colon cancer. People undergo frequent screening for suspicious growths in the colon. An aggressive approach slashed their risk of dying. Should you disclose the results of the tests to the rest of the family despite her objections? Who owns genetic information?
Persis. 1989. Case Studies as
Teaching Tools in Human Genetics
American Biology Teacher 51:418.
Fackelmann, Kathy. 1994. Beyond the genome. The ethics of DNA testing. Science News146:298.
Questions from American Biology Teacher (1988) 50 (8) :496.
Last updated: July 14, 2009 � Copyright by SG Saupe