Pedigrees
Objectives:
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

to determine the genotypes of individuals in a pedigree
to recognize and determine the meaning of symbols used in a pedigree
to use a pedigree to determine how a trait is inherited
Introduction:
A pedigree is a chart that shows the relationships within a family and can be used to determine if a trait is
inherited and how it is inherited. A circle represents a female and a square represents a male. Dark circles or
squares indicate individuals with a disease/trait. A marriage is indicated by a horizontal line connecting a circle
to a square. A line perpendicular to a marriage line indicates the offspring. Each horizontal line indicates a
generation.
Procedure:
1.
Shown below is a pedigree chart for the inheritance of achondroplasia (ay-kon-druh-play-zhuh), a form
of dwarfism. Examine the pedigree chart, and answer the following questions.
a.
Is the gene that causes this form of dwarfism a recessive or dominant trait? ___________________
How do you know?_________________________________________________________________
________________________________________________________________________________
b.
Using (D) to represent the dominant allele and (d) to represent the recessive allele, write the
genotypes of the numbered individuals.
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3
2
4
5
6
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c. Based on the frequency of dwarves among the people you have seen in your lifetime, do you think that
the allele for achondroplasia is common or rare in the population?
_____________________________________
2.
The next pedigree depicts a family with the inherited disease hemophilia. What is hemophilia?
______________________________________________________________________________
How is hemophilia inherited? _________________________________________________________
Fill in the genotypes of each individual in the pedigree and then answer the questions below. Females
who have more than four sons, with none exhibiting hemophilia, are likely to be the genotype X HXH. If she
has had four or fewer sons her genotype is less certain. In such cases her genotype is labeled XHX?.
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1
3
a.
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21
6
14
13
12
11
19
5
4
22
7
15
23
A
8
16
9
17
10
18
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B
C
D
How many males represented by this pedigree have hemophilia? __________
b. How many females in this pedigree have hemophilia? _________
c. How many females in this pedigree are normal? ________
d. How many children did the first couple have? _________
e. How many generations are represented in this pedigree? __________
f.
How many males have the genotype XHY? ________
g. How many women have the genotype XhXh in this pedigree? ___________
h. How many females have the genotype XHXH? _________
i.
What do the letters A. B, C, and D represent in the pedigree above? __________________________________
3.
The next pedigree depicts a family where a number of individuals have red-green colorblindness.
Fill in the genotypes of each individual in the pedigree and then fill in the data table below. Females
who have more than four sons, with none who are colorblind, are likely to be the genotype XRXR. If
she has had four or fewer sons her genotype is less certain. In such cases her genotype is labeled
XRX?.
1
2
8
3
4
5
6
9
7
16
11
12
13
14
15
17
Questions about Red-Green Colorblindness Pedigree
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18
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Answer
number of generations
number of men that are colorblind
number of women not colorblind
number of marriages
number of men with genotype XRY
number of women with genotype XrXr
number of single women
number of people that never married
number of women with genotype XRX?
number of couples with only one child
Analysis and Conclusions
1. What is meant by a “sex-linked” trait?
2. Which sex usually inherits a sex-linked condition? Explain why.
3. How can you tell whether a female has a genotype XHXH or XHXh?
4. How could you tell by looking at a pedigree is a trait is sex-linked?
5. Create your own pedigrees of your own family using the following traits: Middle digit hair
(Dominant); Hitchhikers thumb (Dominant); Tongue rolling (Dominant); Ear lobe free
(Dominant).
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