Analyzing Pedigrees

Analyzing Pedigrees
Activity
Student Handout
INTRODUCTION
You may have heard people say a health condition like asthma or allergies “runs in the family.” That expression
means that more than one individual in the family has the condition. That pattern is typically a clue that the
condition may be caused, at least in part, by genetics.
So, can you predict who in a family is most likely to inherit a particular health condition? Pedigrees are a tool
that can help health professionals do just that. In this activity, you will practice interpreting actual pedigrees and
consider their benefits and limitations.
What Are Pedigrees?

Pedigrees are charts that trace the inheritance of a trait or health condition in several biologically related
individuals spanning more than one generation.
Most health conditions are determined by the functions of many genes and the environment. However, some
are determined largely by a single gene — more precisely, by changes in the DNA (or genetic variants) that affect
a single gene. For such conditions, it is possible to infer their pattern of inheritance (for example, autosomal
dominant or recessive) by examining pedigrees.
To create a pedigree, health professionals such as genetic counselors — who have specialized training in
medical genetics and will typically advise individuals about the chances of inheriting or passing on various
genetic diseases — combine family history information with, when available, information from medical and
genetic tests. They then use the pedigree to identify people who may develop the health condition and/or who
may carry genetic variants associated with that condition.
PROCEDURE
Examine the following pedigrees and answer the questions associated with each. Your instructor may only
assign certain pedigrees and questions.
For all the pedigrees in this activity:

• Shaded symbols indicate individuals affected with the health condition of interest.
• Unshaded symbols indicate unaffected individuals.
• Squares indicate men.
• Circles indicate women.
• Diamonds indicate individuals whose gender is unknown or who do not identify as a particular gender.
• If included, Roman numerals (e.g., I, II, III) on the left-hand side of a pedigree indicate generations. Arabic
numerals (e.g., 1, 2, 3) under the pedigree symbols note the number of individuals in that generation.
PART 1: Practice Interpreting Pedigrees

Examine Pedigree 1, then answer the questions that follow.
www.BioInteractive.org Published August 2023

Page 1 of 7
Activity
Analyzing Pedigrees Student Handout
Pedigree 1. Example of a pedigree.
1. Which of the following can be inferred about Individual 4 in the second generation (Individual II-4)?
a. Individual II-4 is a woman.
b. Individual II-4 has two known biological children.
c. Individual II-4 has one deceased biological sibling who was unaffected.
d. Individual II-4 is unaffected.
Explain your reasoning for the answer you selected:
2. Individual II-3 had a child (Individual III-5) using a sperm donor. Which of the following can be inferred about
Individual III-5?
a. She is not genetically related to other individuals in Generation III.
b. She has inherited 50% of her DNA from Individual II-3.
c. She has not inherited any DNA from Individual II-3.
d. She is affected with the health condition of interest.
PART 2: Examining Patterns of Autosomal Inheritance

Now examine Pedigree 2, a larger pedigree published in a scientific journal.

Page 2 of 7
Activity
Pedigree 2. A large pedigree spanning seven generations. (Adapted from Zoghbi et al. 1988.)
3. Pedigree 2 most likely shows the inheritance of an autosomal dominant condition because:
a. Most affected individuals are men.
b. The family is very large.
c. The condition occurs in every generation.
d. Filled symbols always show an autosomal dominant trait.
4. The following figure represents a small section of Pedigree 2.
Which of the following does this figure show about the affected individual?
a. She has several affected biological children.
b. She had four biological children with three different partners.
c. She had three biological children with two different partners.
d. She has three biological siblings who are all unaffected.

Page 3 of 7
Activity
Pedigree 2 is of a large family with several members affected by spinocerebellar ataxia 1 (SCA1), a condition
that involves the nervous system. Most individuals with SCA1 don’t have any symptoms as children. However,
over time, people with SCA1 start to lose control over their balance and movement. The symptoms gradually
worsen, and people typically die 10 to 30 years after symptoms first appear. There is currently no cure for SCA1.
SCA1 is caused by genetic variants in a single gene. Individuals with one copy of a SCA1-associated variant are
almost certain to develop SCA1. Almost all individuals with SCA1 are heterozygous, since individuals with two
copies of the variant have very severe symptoms and are unlikely to survive into adulthood.
Pedigree 2. This large pedigree shows the inheritance of SCA1. (Adapted from Zoghbi et al. 1988.)
5. If an individual affected with SCA1 has a biological child with an individual without the condition, what are
the chances that their child has SCA1?
a. 0%
b. 25%
c. 50%
d. 100%
6. If one of your genetic relatives had SCA1, do you think you would want to get tested for a genetic variant
associated with the condition? (There are no right or wrong answers.)
a) Yes
b) No
Beta thalassemia major is an inherited condition in which the body produces low levels of functioning
hemoglobin. Hemoglobin is the protein in your red blood cells that binds oxygen. People with beta thalassemia
major do not get enough oxygen delivered to the cells of their body, which may cause them to feel tired, weak,
or short of breath. Doctors can usually diagnose beta thalassemia major in children during their first two years of
life. With proper medical care, most individuals with beta thalassemia major can have minimal symptoms.
Pedigree 3 shows a large extended family with several individuals with beta thalassemia major.
Page 4 of 7
Activity
Pedigree 3. A pedigree that shows the inheritance of beta thalassemia major. The half-shaded symbols
represent individuals who are heterozygous carriers, which means that they “carry” one copy of the variant
associated with beta thalassemia major, but they don’t typically have disease symptoms. NA means no available
data regarding the condition. (Adapted from Baig et al. 2008.)
7. Pedigree 3 is typical of autosomal recessive inheritance because:

a. Most affected individuals are women.
b. Many people have died, as shown by the crossed-out symbols.
c. Affected individuals inherited DNA from individuals who are also affected.
d. Affected individuals inherited DNA from individuals who are carriers.
8. If a carrier has a biological child with an individual with beta thalassemia major, what are the chances the
child has beta thalassemia major?
a. 0%
b. 25%
c. 50%
d. 100%
9. If one of your relatives had beta thalassemia major, do you think you would want to get tested for the
disease-associated variant? (There are no right or wrong answers.)
a. Yes
b. No

Page 5 of 7

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What Are Pedigrees?

For all the pedigrees in this activity:

PART 1: Practice Interpreting Pedigrees

www.BioInteractive.org Published August 2023

Pedigree 1. Example of a pedigree.

PART 2: Examining Patterns of Autosomal Inheritance

www.BioInteractive.org Published August 2023

4. The following figure represents a small section of Pedigree 2.

www.BioInteractive.org Published August 2023

7. Pedigree 3 is typical of autosomal recessive inheritance because:

www.BioInteractive.org Published August 2023

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