Identify affected individuals.
Look for shaded symbols first.
What is a pedigree in AP Biology?
A pedigree is a family tree used to track how a trait is inherited across generations. In AP Biology, pedigrees help identify inheritance patterns such as autosomal dominant, autosomal recessive, and X-linked recessive by showing which family members are affected, unaffected, or carriers.
Say it fast
Pedigrees use family patterns to infer inheritance.
AP exam tip: Do not guess the pattern from one person. Use multiple generations, affected parents, affected offspring, sex bias, and carrier clues.
Pedigree Symbols You Need to Know
Pedigrees use simple symbols to show sex, trait status, and relationships. AP Biology questions often expect you to read symbols quickly before analyzing inheritance.
| Symbol | Meaning |
|---|---|
| Square | Male |
| Circle | Female |
| Shaded symbol | Affected individual |
| Unshaded symbol | Unaffected individual |
| Half-shaded symbol | Carrier, if shown |
| Horizontal line | Mating / parents |
| Vertical line | Offspring |
| Roman numerals | Generation |
| Arabic numerals | Individual within a generation |
How to Read a Pedigree Step by Step
Check whether the trait skips generations.
Skipping generations often suggests recessive inheritance.
Compare males and females.
A strong male bias may suggest X-linked recessive inheritance.
Look for affected children with unaffected parents.
This strongly suggests recessive inheritance.
Check whether every affected child has an affected parent.
This can suggest dominant inheritance.
Use genotype logic.
Assign possible genotypes and test whether the pattern works.
AP exam warning: A pedigree pattern is evidence, not a guess. Always justify your answer with specific family relationships.
Before analyzing patterns, review Mendelian genetics for dominant and recessive allele logic.
Autosomal Dominant vs Autosomal Recessive Pedigrees
| Feature | Autosomal Dominant | Autosomal Recessive |
|---|---|---|
| Generation pattern | Often appears every generation | Can skip generations |
| Affected parents | Often have affected children | Unaffected carriers can have affected children |
| Carrier idea | Usually not hidden in the same way | Carriers can be unaffected |
| Sex pattern | Usually similar in males and females | Usually similar in males and females |
| AP clue | Affected child often has affected parent | Affected child can have unaffected parents |
Autosomal Dominant Pedigree Clues
An autosomal dominant trait is controlled by a dominant allele on a non-sex chromosome. Affected individuals usually have at least one affected parent. The trait often appears in every generation, although small family size can make patterns less obvious.
- Trait appears in every generation.
- Affected child usually has an affected parent.
- Males and females are affected at similar rates.
- Two unaffected parents usually do not have affected children.
- Affected heterozygous parent can pass the trait to about half of offspring.
Notation:
- A = dominant trait allele
- a = unaffected allele
- Affected: AA or Aa
- Unaffected: aa
Autosomal Recessive Pedigree Clues
An autosomal recessive trait appears only when an individual inherits two recessive alleles. Unaffected parents can have an affected child if both parents are carriers. This is why autosomal recessive traits can skip generations.
- Trait can skip generations.
- Unaffected parents can have affected offspring.
- Males and females are affected at similar rates.
- Carriers are heterozygous and usually unaffected.
- Affected individuals are homozygous recessive.
Notation:
- A = normal dominant allele
- a = recessive trait allele
- Carrier: Aa
- Affected: aa
- Unaffected non-carrier: AA
Some recessive patterns overlap with topics in non-Mendelian genetics, but classic autosomal recessive pedigrees follow simple Mendelian ratios.
X-Linked Recessive Pedigree Clues
An X-linked recessive trait is caused by a recessive allele on the X chromosome. These traits often appear more often in males because males usually have only one X chromosome. Carrier females can pass the recessive allele to sons.
- More affected males than females.
- Carrier mother can have affected sons.
- Affected father does not pass his X-linked allele to sons.
- Daughters of affected males receive his X chromosome.
- Trait may appear to pass through unaffected carrier females.
Review Sex-Linked Traits for full X-linked Punnett square method and carrier female reasoning.
How to Find Carriers in a Pedigree
A carrier has one recessive allele but does not show the recessive phenotype. In autosomal recessive pedigrees, unaffected parents of an affected child must usually be carriers. In X-linked recessive pedigrees, unaffected mothers of affected sons may be carriers.
- If two unaffected parents have an affected child, both parents are carriers for an autosomal recessive trait.
- If a male has an X-linked recessive trait, his recessive X chromosome came from his mother.
- If a female is affected by an X-linked recessive trait, she must have inherited recessive X alleles from both parents.
- Do not label someone a carrier unless the pedigree evidence supports it.
Pedigree Probability and Genotype Reasoning
Some AP Biology pedigree questions ask for the probability that a child will inherit a trait. First infer the likely genotypes from the pedigree, then use Punnett square logic.
Example: If two carrier parents for an autosomal recessive trait are crossed (Aa × Aa monohybrid cross):
Expected offspring:
- 25% AA
- 50% Aa
- 25% aa
Conclusion: There is a 25% chance of an affected child.
Review Punnett Squares for step-by-step probability calculations.
Pedigree vs Punnett Square
| Tool | What It Shows | Best Use |
|---|---|---|
| Pedigree | Family inheritance across generations | Identify inheritance pattern |
| Punnett square | Possible offspring genotypes from known parents | Calculate probabilities |
| Chi-square | Observed vs expected counts | Test whether data fit expected ratios |
Punnett Squares · Chi-Square Test Genetics
During meiosis, gametes receive one allele per gene, which is why carrier parents can produce affected offspring when both pass recessive alleles.
AP Bio Exam Clues for Pedigrees
“Trait skips generations”
→ recessive clue
“Affected child has unaffected parents”
→ autosomal recessive clue
“Every affected person has affected parent”
→ dominant clue
“More affected males”
→ X-linked recessive clue
“Father does not pass trait to sons”
→ X-linked clue
“Carrier female”
→ X-linked or autosomal recessive context
“Males and females equally affected”
→ likely autosomal
“Need probability”
→ infer genotypes, then use Punnett logic
Common Pedigree Analysis Mistakes
Guessing from one affected person
Fix: Use multiple generations and family relationships.
Assuming all skipped traits are X-linked
Fix: Autosomal recessive traits can also skip generations.
Forgetting carriers
Fix: Unaffected people can carry recessive alleles.
Confusing autosomal and sex-linked
Fix: Check sex bias and father-to-son inheritance.
Ignoring the denominator
Fix: A probability among sons is different from probability among all children.
Treating pedigree clues as absolute rules
Fix: Small families can make patterns incomplete. Use the best-supported pattern.
Identify the Pedigree Pattern
Revealed: 0 of 5 scenarios
Two unaffected parents have an affected child.
Reveal: This strongly suggests recessive inheritance. For autosomal recessive, both parents are likely carriers.
A trait appears in every generation and affected children usually have affected parents.
Reveal: This suggests autosomal dominant inheritance.
A recessive trait appears more often in males.
Reveal: This suggests X-linked recessive inheritance.
An affected father has unaffected sons but all daughters receive his X chromosome.
Reveal: This supports X-linked reasoning.
Males and females are affected at similar rates, and the trait skips generations.
Reveal: This suggests autosomal recessive inheritance.
Pedigree Practice Questions
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Pedigree FRQ Practice
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Prompt
A pedigree shows two unaffected parents with an affected child. The trait appears in both males and females. Explain which inheritance pattern is most likely and justify your answer.
Scoring rubric
- Unaffected parents with affected child suggest recessive inheritance.
- Both parents are likely carriers.
- Trait appears in both males and females, supporting autosomal rather than sex-linked.
- Most likely pattern is autosomal recessive.
- Use genotype reasoning such as Aa × Aa producing aa offspring.
Sample response
The most likely pattern is autosomal recessive inheritance. Two unaffected parents having an affected child suggests that both parents are carriers of a recessive allele. Because the trait appears in both males and females, it is more likely autosomal than sex-linked. If both parents are Aa, they can produce an aa child who shows the recessive phenotype.
Status: Draft your answer first—then open the rubric or sample.
Prompt
A pedigree shows a recessive trait appearing more often in males than females. An affected son has an unaffected mother. Explain why this pattern supports X-linked recessive inheritance.
Scoring rubric
- X-linked recessive traits often appear more often in males.
- Males usually have one X chromosome.
- A recessive allele on the X chromosome is expressed in males.
- The unaffected mother may be a carrier.
- The affected son inherited the recessive X chromosome from his mother.
Sample response
The pattern supports X-linked recessive inheritance because recessive X-linked traits often appear more often in males. A male usually has only one X chromosome, so if that X chromosome carries the recessive allele, he shows the trait. The affected son's mother can be unaffected if she is a carrier, because her other X chromosome has the dominant allele. The son likely inherited the recessive X chromosome from his mother.
Status: Draft your answer first—then open the rubric or sample.
Continue the Unit 5 Heredity Path
Pedigree FAQs
What is a pedigree in AP Biology?
A pedigree is a family tree used to track how a trait is inherited across generations. In AP Biology, pedigrees help identify inheritance patterns such as autosomal dominant, autosomal recessive, and X-linked recessive by showing which family members are affected, unaffected, or carriers.
How do you read a pedigree?
Start by identifying affected individuals, then check whether the trait skips generations, compare males and females, look for affected children with unaffected parents, and test whether every affected child has an affected parent. Use genotype logic to see which inheritance pattern best fits the family evidence.
What do pedigree symbols mean?
Squares usually represent males, circles represent females, shaded symbols show affected individuals, unshaded symbols show unaffected individuals, and half-shaded symbols may show carriers. Horizontal lines connect mates and vertical lines connect parents to offspring.
How can you tell if a pedigree is autosomal dominant?
Autosomal dominant pedigrees often show the trait in every generation, affected children usually have at least one affected parent, and males and females are affected at similar rates. Two unaffected parents usually do not have affected children.
How can you tell if a pedigree is autosomal recessive?
Autosomal recessive pedigrees often show skipped generations, unaffected parents can have affected children, and males and females are affected at similar rates. Carriers are usually unaffected heterozygotes.
How can you tell if a pedigree is X-linked recessive?
X-linked recessive pedigrees often show more affected males than females, carrier mothers can have affected sons, and affected fathers do not pass the trait to sons. The trait may appear to pass through unaffected carrier females.
How can you tell if a pedigree is autosomal or X-linked?
Autosomal traits are on non-sex chromosomes and usually affect males and females at similar rates. X-linked recessive traits often affect more males, can pass through carrier females, and usually do not show father-to-son transmission because fathers pass Y chromosomes to sons.
How do you find carriers in a pedigree?
For autosomal recessive traits, unaffected parents of an affected child are usually carriers. For X-linked recessive traits, unaffected mothers of affected sons may be carriers. Only label someone a carrier when the pedigree evidence supports it.
What does it mean if a trait skips generations?
A skipped generation often suggests recessive inheritance, because unaffected carriers can pass a recessive allele without showing the trait themselves. Both autosomal recessive and X-linked recessive patterns can skip generations.
What is the difference between a pedigree and a Punnett square?
A pedigree shows family inheritance across generations and helps identify the inheritance pattern. A Punnett square predicts offspring genotypes and probabilities from known parent genotypes. Pedigrees show evidence; Punnett squares calculate expected ratios.
Why do pedigree questions sometimes seem ambiguous?
Small family trees may not show every clue clearly. Different inheritance patterns can look similar when only a few individuals are shown. AP Biology answers should choose the best-supported pattern and cite specific pedigree evidence.
What are the most common AP Biology pedigree mistakes?
Common mistakes include guessing from one affected person, assuming all skipped traits are X-linked, forgetting carriers, confusing autosomal and sex-linked patterns, ignoring the correct probability denominator, and treating pedigree clues as absolute rules.
How should I answer pedigree FRQs?
State the most likely inheritance pattern, cite specific pedigree evidence such as affected parents or sex bias, explain genotypes when helpful, and connect the pattern to Mendelian or sex-linked reasoning. Use evidence from the family tree before giving a conclusion.