Marfan Syndrome
Marfan syndrome is a disorder of the connective tissue. Connective tissue holds all parts of the body together and helps
control how the body grows. Because connective tissue is found throughout the body, Marfan syndrome features can
occur in many different parts of the body. Marfan syndrome symptoms are most often found in the heart, blood vessels,
bones, joints, and eyes. Sometimes the lungs and skin are also affected. Marfan syndrome does not affect intelligence.
About 1 in 5,000 people have Marfan syndrome. This includes men and women of all races and ethnic groups. Marfan
syndrome is caused by a defect (mutation) in the gene that tells the body how to make fibrillin-1 -- a protein that is an
important part of connective tissue. One of the most common symptoms of Marfan syndrome is enlarged or bulging aorta,
the main blood vessel that carries blood from the heart. Other symptoms include: long arms and legs, tall and thin body
type, curvature of the spine (scoliosis), chest sinks in or sticks out/pigeon breast, long and thin fingers, flexible joints, flat
feet, teeth that are too crowded, sudden collapse of the lung, or vision problems.
Mark has Marfan Syndrome. His wife Maria does not. She has normal connective tissue. They have had three
children. All of their children have Marfan Syndrome.
1. Which allele do you think is dominant? NORMAL CONNECTIVE TISSUE or MARFAN SYNDROME
M
m
M
MM
Mm
2. Look at the cross for Jackson and Calista to the left. They are both
homozygous
heterozygous
m
Mm
mm
3. They have a _______ chance of having offspring with Marfan
Syndrome.
4. Bryant has Marfan syndrome. What are his possible genotypes? ________
________
5. Kevin married Geneva. Their genotypes are not known. Kevin and Geneva had children. 50% of their
children have Marfan. 50% have normal connective tissue. Create several Punnett Squares that show all of the
possible combinations that produce offspring with Marfan Syndrome.
Genetic Cross 1:
Possible
Genotypes of
Offspring:
______________
Ratio: ________
Genetic Cross 2:
Genetic Cross 3:
Genetic Cross 4:
Possible
Genotypes of
Offspring:
______________
Possible
Genotypes of
Offspring:
______________
Possible
Genotypes of
Offspring:
______________
Ratio: ________
Ratio: ________
Ratio: ________
6. Study the Punnett Squares above. Which of your Punnett Squares above most likely represents a cross
between Kevin and Geneva? What are Kevin and Geneva’s genotypes in this Punnett Square? _______ and
_______
7. Matthew and Alexis have recently married. Neither of them have Marfan Syndrome, but Matthew’s
mother does. Alexis is concerned that her children might inherit this disorder. Create a Punnett Square that
shows a cross between Mathew and Alexis.
a. What is Matthew’s genotype? __________
b. What is Alexis’s genotype? __________
c. What is the chance that they will have offspring with Marfan
syndrome? ___________
d. If you were a genetic counselor, how would you explain this to Maria?
e. If Matthew does not have Marfan syndrome but his mother does, what
must his mother’s genotype be? ___________
8. If you cross a PURE HOMOZYGOUS DOMINANT (MM) organism with anything, _______% of the
offspring will show the dominant trait.
9. If you cross a hybrid (Mm) with a hybrid (Mm), ________% of the offspring will show the dominant
trait and _______% will show the recessive trait.
10. If you cross a homozygous recessive (mm) with another organism that is homozygous recessive (mm),
they have a _________% chance of having offspring with the dominant trait.
11. If you make a cross between 2 DOMINANT LOOKING organisms and the offspring show a 3:1
phenotype ratio (3 dominant:1 recessive) it should tell you that
A. One parent was heterozygous; the other parent was homozygous recessive.
B. One parent was homozygous dominant; the other was homozygous recessive
C. Both parents were heterozygous
D. Both parents were homozygous dominant
12. If you make a cross between 2 DOMINANT LOOKING organisms and the offspring show a 4:0
phenotype ratio (4 dominant:0 recessive) it should tell you that
A. One parent was heterozygous; the other parent was homozygous recessive.
B. One parent was homozygous dominant; the other was homozygous recessive
C. Both parents were heterozygous
D. Both parents were homozygous dominant
13. If you make a cross between 1 DOMINANT LOOKING organism and 1 RECESSIVE LOOKING
organism and all of the offspring have the dominant form of the trait, it should be a clue that
A. One parent was heterozygous; the other parent was homozygous recessive.
B. One parent was homozygous dominant; the other was homozygous recessive
C. Both parents were heterozygous
D. Both parents were homozygous dominant
14. If you cross two DIFFERENT LOOKING parents and only one of the traits shows up in the offspring,
but returns again in the next generation, it should be a clue that
A. One parent was heterozygous; the other parent was homozygous recessive.
B. One parent was homozygous dominant; the other was homozygous recessive
C. Both parents were heterozygous
D. Both parents were homozygous dominant
15. Circle the underlined statement that makes the sentence correct: With a dominant disorder like Marfan
Syndrome, a heterozygous individual will always or will never have the symptoms of the disease.
Using the information provided in each problem:
A. Give the genotypes of the parents
B. Complete the Punnett Square
C. Give the possible genotypic ratio of the offspring
D. Give the possible phenotypic ratio of the offspring
E. Answer the percentage question.
____________________________________________________________________
In humans, Marfan syndrome (M) is dominant over normal health (m).
1. Cross a homozygous dominant person with a person that is heterozygous.
A. Parent Genotypes ___________ x ___________
B.
C. Genotypic Ratio: __________________
D. Phenotypic Ratio: _________________
E. What percentage of offspring will have Marfan Syndrome? __________
2. Cross a person that is heterozygous with a person that is also heterozygous.
A. Parent Genotypes ___________ x ___________
B.
C. Genotypic Ratio: __________________
D. Phenotypic Ratio: _________________
E. What percentage of offspring will have Marfan Syndrome?____________
3. Cross a man that is homozygous recessive with a woman that is homozygous dominant.
A. Parent Genotypes ___________ x ___________
B.
C. Genotypic Ratio: __________________
D. Phenotypic Ratio: _________________
E. What percentage of offspring will have Marfan Syndrome? ____________
4. Cross a man that is homozygous recessive with a woman that is homozygous recessive.
A. Parent Genotypes ___________ x ___________
B.
C. Genotypic Ratio: __________________
D. Phenotypic Ratio: _________________
E. What percentage of offspring will have Marfan Syndrome?____________
5. Cross a man that is heterozygous with a woman that does not have Marfan syndrome.
A. Parent Genotypes ___________ x ___________
B.
C. Genotypic Ratio: __________________
D. Phenotypic Ratio: _________________
E. What percentage of offspring will have Marfan Syndrome? _____________