Pedigree Decoder Worksheet
9th Grade Biology • Genetics Lab
Cracking the Pedigree Code
How geneticists trace inherited traits across family generations
Name:
Period:
Date:
Partner:
Prior Knowledge Reminder:
Dominant (A): Capital letter (shows if AA or Aa) Recessive (a): Lowercase letter (only shows if aa) Heterozygous: Aa (Carrier)
Part 1: The Visual Symbol Key (four essential pedigree symbols and lines)
Square Male
Circle Female
Shaded Has the trait
Unshaded Normal / unaffected
Lines Across = parents • Down = children
Part 2: Reading Pedigree "Addresses"
Every person in a pedigree has an address. Generations are labeled with Roman numerals (I, II, III) on the left. Individuals within each generation are numbered from left to right with numbers (1, 2, 3...) written below their symbol.
II-3 means Generation II, the 3rd person from the left.
Try it: What is the address of the 1st person in Generation I?
Part 3: Guided Case Study • Recessive Hitchhiker's Thumb Trait allele: h (recessive) | Straight thumb: H (dominant)
Family Pedigree Diagram I II 1 2 1 2 3
Detective Clue: "The Hidden Recessive Trait"
Notice that neither parent in Generation I has hitchhiker's thumb, but their son II-2 does have it!
The Golden Rule of Recessive Traits: If two unaffected parents have a child who expresses the trait, the trait must be recessive and both parents must be heterozygous carriers (Hh).
1
Which individual is affected with hitchhiker's thumb? Write their official pedigree address:
Address: Gender of this person:
2
Since hitchhiker's thumb is recessive (h), what is individual II-2's genotype? (Circle one • remember recessive traits use lowercase letters)
HH Hh hh
3
Individual II-2 received one allele from Mom (I-2) and one from Dad (I-1). What are the genotypes of the parents?
Dad (I-1): Mom (I-2): (Hint: They look normal, but each carries a hidden lowercase allele)
Turn over for Part 4: 3-Generation Family Practice • Page 1 of 2 Continue to Back
Genetics Mystery Lab
Part 4: Independent Pedigree Practice
Target trait: Albinism Normal pigment: A (dominant) • Albinism: a (recessive)
Family Case: 3-Generation Albinism Pedigree
Shaded = has albinism (aa) • Unshaded = normal pigmentation (A_)
I II III 1 2 1 2 3 4 5 1 2 3
1. Pedigree Anatomy • Count and Record:
a) Total generations:
b) Total males:
c) Total females:
d) Total affected (shaded):
2. Addresses & Family Relationships:
a) What is the address of the affected female in Generation III?
b) What is the relationship of II-1 to II-2?
c) How is III-1 related to individual II-2?
d) Did II-3 inherit her alleles from I-1 and I-2? (Yes/No):
3. Genotype Deduction • Use dominant A and recessive a:
a) Individual II-2 has albinism. What is his exact genotype? Genotype:
b) Parents I-1 and I-2 have normal pigment, but had an affected son (II-2). What must their genotypes be?
I-1: I-2:
c) Daughter III-2 has albinism (aa). She got one a from dad (II-2). What must mom (II-3) be? Mom II-3:
d) Individual II-1 has normal pigmentation. Can you be 100% sure if she is AA or Aa? Explain why:
4. Genetics Probability Connection:
If parents II-2 (genotype: aa) and II-3 (genotype: Aa) have another child:
Fill out the Punnett square to the right to find the possible genotypes for their offspring.
What is the probability this child will have albinism (aa)? %
Punnett Square
A
a
a
a
Pedigree Decoder Worksheet • Biology Inclusion Support Page 2 of 2
