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PEDIGREE ANALYSIS
INTRODUCTION
Traits in humans are often difficult to study for several reasons. Unlike Drosophila, which
produce large numbers of offspring very quickly, humans reproduce slowly and produce few
offspring at one time. So traits in humans must be studied through pedigree analysis.
PROCEDURE
Part A The Basics
1. The diagram below shows a pedigree. A pedigree shows a family’s pattern of
inheritance for a specific trait.
2. In a pedigree, each person has a number and each generation is represented by a
Roman numeral. In this way, each individual is identified by a Roman and an Arabic
numeral. Females are represented by circles and males by squares. Shaded symbols
indicate individuals homozygous recessive for the trait.
3. The trait shown in the pedigree is the ability to taste PTC paper.
The dominant taster trait is represented by “T.”
4. What is the genotype of person I-2? _____________
Explain. ________________
_____________________________________________________________________
5. What is the genotype of persons II-1, II-2, II-3, and II-4? ____________ Explain.
_____________________________________________________________________
6. What are the two possible genotypes for person II-6? _________________________
Explain. _____________________________________________________________
1
1
2
2
3
I
4
5
6
1
II
III
Part B Numbering and Labeling
1. Write the Roman numeral for each generation.
2. Write the Arabic number for each individual.
3. Assume the shaded symbols represent the genotype “rr.”
Write the genotype below each arabic number.
Roman numeral
Arabic number
_____
_____
genotype
_____
_____
_____
Arabic number
_____ _____
_____
_____
_____
_____
genotype
_____ _____
_____
_____
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_____
Arabic number
_____ _____
_____
genotype
_____ _____
_____
_____
_____
Part C The Trait of Albinism
1. Write the Roman numeral for each generation.
2. Write the Arabic number for each individual.
3. Assume individuals I-1 and I-2 are homozygous for normal pigmentation.
Let “A” represent the dominant gene.
4. Write the genotype below each arabic number.
Roman
numeral
Arabic
number
____
____
____
____
____
____
____
____
genotype
____
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Arabic
number
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genotype
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Arabic
number
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genotype
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Part D The Trait of Night Blindness
1. Fill in the Roman numerals and Arabic numbers for the pedigree.
2. Let “N” represent the dominant gene for normal vision.
Let “n” represent the recessive gene for night blindness.
3. Assume that night blindness is a sex-linked trait. Therefore you will need to write
the X and Y chromosomes and use superscripts for the genes. (i.e. XnY)
4. Note that individuals I-8, II-5 and III-3 are homozygous.
Individuals I-4 and III-2 are heterozygous.
Roman
numeral
Arabic
number
____
____
____
____
____
____
____
____
genotype
____
____
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____
____
____
____
____
Arabic
number
___
___ ___ ___
___
___
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___
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___
___
genotype
___
___ ___ ___
___
___
___
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Arabic
number
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genotype
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Arabic
number
genotype
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