Pedigree Analysis- Activity Answer Key

What Is Pedigree Analysis?

Pedigree analysis is a way to trace how a genetic trait or disease passes through generations. You look at a family tree diagram filled with standard symbols and figure out the genotype of each person.

That's it. No magic. Just logic and basic Mendelian rules.

If you're stuck on a pedigree activity or need to check your answers, this guide walks through the most common problems students face with clear explanations and answer keys.

Standard Pedigree Symbols You Must Know

Before you can solve anything, you need to recognize what you're looking at. These symbols are universal in genetics textbooks and exams.

Square (■) = Male
Circle (●) = Female
Shaded/Filled = Affected by the trait
Half-shaded = Carrier (heterozygous)
Unshaded = Unaffected
Horizontal line = Parents (connected by marriage line)
Vertical line + horizontal bar = Children (offspring)

If you mix these up, every answer you write will be wrong. Memorize them now.

Three Types of Inheritance You'll Encounter

Autosomal Recessive

The disease or trait only shows up when someone has two copies of the recessive allele (aa).

Key patterns to look for:

Affected individuals often have unaffected parents
Parents of affected children are carriers (Aa × Aa)
Trait can skip generations
Males and females affected equally

Autosomal Dominant

The disease shows up if someone has even one copy of the dominant allele (AA or Aa).

Key patterns:

Every affected person has at least one affected parent
Unaffected parents produce only unaffected children
Both sexes affected equally
Vertical transmission through generations

X-Linked Recessive

The gene is on the X chromosome. Males are more frequently affected because they only have one X chromosome.

Key patterns:

Affected males inherit the allele from their carrier mothers
Affected females must have an affected father + carrier/expressed mother
No male-to-male transmission
Females are usually carriers, not affected

Pedigree Analysis Activity: Practice Problems

Problem 1: Is This Autosomal Recessive or Dominant?

Look at this pedigree scenario:

Generation I: Both parents unaffected (I-1 and I-2).
Generation II: Three children — two unaffected, one affected female (II-3).
Generation III: The affected female (II-3) has two children — one affected son, one unaffected daughter.

Analysis:

Affected child born to unaffected parents = recessive inheritance. If it were dominant, at least one parent would show the trait.

Since both parents are unaffected but produced an affected child, they must both be carriers (Aa × Aa).

Answer Key:

Inheritance pattern: Autosomal recessive
Generation I parents: Both carriers (Aa)
Affected female (II-3): aa
Her affected son: aa
Her unaffected daughter: AA or Aa

Problem 2: X-Linked or Autosomal?

Scenario:

An affected father (I-1) has four children with an unaffected mother (I-2). Two sons are affected, two daughters are unaffected. One affected daughter (II-4) marries an unaffected man and has two sons — both affected.

Analysis:

The father passes his X chromosome to all daughters, so if this were X-linked recessive, all daughters would be carriers. The affected daughter (II-4) must have gotten her father's X with the recessive allele.

Her sons inherited that X chromosome, which is why both are affected.

Answer Key:

Inheritance pattern: X-linked recessive
Father (I-1): X^aY (affected)
Mother (I-2): XX (unaffected, likely carrier)
Affected daughter (II-4): X^aX (carrier)
Her sons: X^aY (affected)

Quick Reference Table: Identifying Inheritance Patterns

Pattern	Affected Both Sexes?	Skips Generations?	Male-to-Male Transmission?	Key Clue
Autosomal Recessive	Yes	Yes	Yes	Unaffected parents, affected child
Autosomal Dominant	Yes	Rarely	Yes	Affected parent always passes to child
X-Linked Recessive	Mostly males	Yes	No	Fathers can't pass to sons
X-Linked Dominant	Mostly females	No	No	Affected fathers → all daughters affected

How to Solve Any Pedigree Problem in 5 Steps

Step 1: Identify the Symbols

Label every individual as affected or unaffected, male or female. If you misread a symbol, you'll chase the wrong answer through the entire problem.

Step 2: Look for the Inheritance Pattern

Ask yourself:

Are both sexes affected equally? → Points to autosomal
Are males affected more than females? → Points to X-linked
Do affected parents always have affected children? → Points to dominant
Can unaffected parents have affected children? → Points to recessive

Step 3: Assign Genotypes to Clear Cases

Start with individuals you can definitively genotype. An unaffected person in a recessive disease must be homozygous dominant (AA). An affected person in a dominant disease must have at least one dominant allele.

Step 4: Work Through Uncertain Individuals

Use the information from known genotypes to deduce unknowns. If both parents are carriers (Aa × Aa), their offspring have a 25% chance of being aa.

Step 5: Check Your Work

Trace each genotype back through the pedigree. If any genotype creates a contradiction, you made an error somewhere. Every individual must fit the pattern.

Common Mistakes Students Make

Assuming dominant when it's recessive — Just because someone shows a trait doesn't mean the allele is dominant. Recessive traits persist in populations because carriers show no symptoms.
Ignoring the possibility of new mutations — In real genetics, sometimes a trait appears spontaneously. In textbook problems, assume the given information is accurate unless stated otherwise.
Miscrossing horizontal and vertical lines — Horizontal lines connect parents. Vertical lines connect parents to children. Mixing these up will destroy your analysis.
Forgetting that males are XY — In X-linked problems, males only have one allele to consider. If they're affected, their genotype is simple: X^aY.

Sample Pedigree Problem with Complete Answer Key

Given: A family tree shows a rare trait. Generation I: Father affected, mother unaffected. Generation II: All three daughters are affected. Generation III: Two of the daughters have children with unaffected men. One daughter (II-1) has two affected sons and one unaffected daughter. Another daughter (II-2) has three unaffected children.

Question 1: What is the inheritance pattern?

Answer: X-linked dominant. The father passes his X chromosome to all daughters, so if the allele is on the X, every daughter inherits it and is affected.

Question 2: What are the genotypes?

I-1 (affected father): X^AY
I-2 (unaffected mother): XX
II-1 (affected daughter): X^AX (got A from father)
II-1's sons: X^AY (inherited mother's X)
II-2 (affected daughter): X^AX — but wait, if she has unaffected children, she must be heterozygous and her sons got her X without the allele, meaning she passed X without A. This is impossible under X-linked dominant. Re-evaluate.

Revised Answer: This cannot be X-linked dominant if II-2 has unaffected sons. The trait must be autosomal dominant with incomplete penetrance, or the problem contains an error. In most standard problems, autosomal dominant traits show up in every generation with vertical transmission.

The point: always test your genotypes against the phenotype data. If it doesn't fit, the pattern assumption was wrong.

Final Answer Key Summary

Autosomal recessive: Unaffected parents can have affected children. Those children are aa. Parents are carriers (Aa).
Autosomal dominant: Affected parents usually have affected children. At least one parent is Aa or AA.
X-linked recessive: Males are affected more often. An affected male's daughters are always carriers.
Always assign obvious genotypes first, then work outward.
When in doubt, test your answer by tracing genotypes through the entire pedigree.

Pedigree analysis is pattern recognition. Once you know what each inheritance pattern looks like on paper, you can solve any problem they throw at you.