Pedigree analysis Practice Questions
19 free Pedigree analysis practice questions for the USMLE Step 1. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
In a pedigree chart, a filled square usually represents what?
- A An unaffected male
- B An affected female
- C An affected male
- D An unaffected female
Correct answer: An affected male
In standard pedigree symbolism, squares represent males and filled (shaded) shapes denote affected individuals — hence a filled square indicates an affected male.
Which mode of inheritance is most consistent with a trait that appears in every generation and affects both sexes equally?
- A Autosomal recessive
- B Autosomal dominant
- C X-linked recessive
- D Mitochondrial
Correct answer: Autosomal dominant
Autosomal dominant traits typically appear in every generation and affect males and females equally, because a single mutant allele on an autosome is enough for expression.
If two unaffected parents have an affected child, which mode of inheritance is most likely?
- A Autosomal dominant
- B Autosomal recessive
- C X-linked dominant
- D Y-linked
Correct answer: Autosomal recessive
When two phenotypically unaffected parents produce an affected child, autosomal recessive inheritance is most likely, since both parents can be asymptomatic carriers.
In a pedigree for an X-linked recessive disease, which of the following patterns is most suggestive?
- A Both sexes affected about equally in every generation
- B Only females affected, with all males remaining unaffected
- C Mostly males affected, with no father-to-son transmission
- D All daughters of an affected father are themselves affected
Correct answer: Mostly males affected, with no father-to-son transmission
X-linked recessive disorders affect males more often, because males are hemizygous for the X chromosome. A key clue is the absence of male-to-male transmission, since fathers pass a Y (not their X) to sons.
Which clue argues against X-linked dominant inheritance in a pedigree?
- A An affected male has all affected daughters and no affected sons
- B The trait appears in every generation without ever skipping
- C Generations are skipped: unaffected parents have an affected child
- D Affected females clearly outnumber affected males overall
Correct answer: Generations are skipped: unaffected parents have an affected child
X-linked dominant traits rarely skip generations. Two unaffected parents producing an affected child indicates a recessive-type pattern, arguing against X-linked dominant inheritance. The other options are all consistent with X-linked dominant.
Which assumption is typically made when performing classic pedigree analysis under Mendelian inheritance models?
- A The disease allele is at high frequency in the population
- B Incomplete penetrance is assumed to be present in every case
- C The trait is rare, so those marrying in seldom carry the allele
- D Many different genes each contribute equally to the phenotype
Correct answer: The trait is rare, so those marrying in seldom carry the allele
Classic Mendelian pedigree analysis assumes the trait is rare, so unrelated individuals who marry into the family are unlikely to carry the mutant allele. This simplifies inferring the mode of inheritance.
In autosomal dominant inheritance with full penetrance, what is the probability a child will be affected if one parent is heterozygous (Aa) and the other is homozygous normal (aa)?
- A 0%
- B 25%
- C 50%
- D 100%
Correct answer: 50%
In autosomal dominant inheritance, a heterozygous parent (Aa) crossed with a homozygous normal parent (aa) gives each child a 50% chance of inheriting the mutant allele and being affected.
In a pedigree, if an affected father transmits the trait to all his daughters but none of his sons, which inheritance pattern is most likely?
- A Autosomal dominant
- B Autosomal recessive
- C X-linked dominant
- D X-linked recessive
Correct answer: X-linked dominant
In X-linked dominant inheritance, an affected father passes his X chromosome to all daughters (who are affected) and his Y to sons (who are unaffected), giving exactly this pattern.
Which inheritance pattern would you suspect if multiple siblings are affected but neither parent shows the trait and there is consanguinity?
- A Autosomal dominant with reduced penetrance
- B Autosomal recessive
- C X-linked dominant
- D Y-linked
Correct answer: Autosomal recessive
Consanguinity raises the chance that both parents carry the same recessive allele, so affected children can appear from unaffected parents — a hallmark of autosomal recessive inheritance.
Which mode of inheritance is nearly ruled out if affected males are found in consecutive generations and pass the trait to their sons?
- A Autosomal dominant
- B Autosomal recessive
- C X-linked dominant
- D Y-linked
Correct answer: X-linked dominant
Male-to-male (father-to-son) transmission argues against X-linked inheritance, because a father passes his Y — not his X — to sons. Autosomal and Y-linked patterns all permit father-to-son transmission, so X-linked dominant is the one nearly ruled out.
Which scenario describes pseudodominance that might mislead a pedigree analyst?
- A An autosomal dominant trait that skips an entire generation
- B A recessive trait recurring across generations via inbreeding
- C An X-linked dominant trait expressed only in affected females
- D A mitochondrial trait transmitted only through affected males
Correct answer: A recessive trait recurring across generations via inbreeding
Pseudodominance is when a recessive trait mimics dominant inheritance — for example, appearing in successive generations in consanguineous families or where the allele is common — which can mislead a pedigree analyst.
In pedigree analysis, which of the following features strongly suggests an autosomal recessive rather than autosomal dominant inheritance for a rare disorder?
- A Trait appears in every generation
- B Affected individuals always have an affected parent
- C Unaffected parents produce rare affected children
- D Equal number of affected males and females in every generation
Correct answer: Unaffected parents produce rare affected children
A hallmark of autosomal recessive inheritance is that unaffected carrier parents can have affected children, even when earlier generations show no disease.
Which pattern on a pedigree might suggest mitochondrial inheritance?
- A Affected mothers pass it to all offspring; fathers pass to none
- B Male-to-male transmission is seen in every generation
- C The trait is expressed only among the male relatives
- D Both sexes transmit it equally, regardless of parent sex
Correct answer: Affected mothers pass it to all offspring; fathers pass to none
Mitochondrial DNA is maternally inherited: an affected mother transmits the trait to all her children, while affected fathers transmit it to none. This maternal pattern is distinctive on a pedigree.
If a pedigree shows an affected male whose mother is unaffected but paternal grandfather is affected, which inheritance pattern is least likely?
- A Autosomal dominant
- B Autosomal recessive
- C X-linked recessive
- D X-linked dominant
Correct answer: X-linked dominant
X-linked dominant inheritance would require the mother to carry a mutant X. An affected male with an unaffected mother argues against X-linked dominant, making it the least likely pattern here.
When analyzing a pedigree, why do geneticists often assume “complete penetrance” and “rare allele frequency” for a Mendelian trait?
- A To simplify inference of the mode of inheritance
- B Because all real traits are always fully penetrant and rare
- C To allow analysis without knowing any actual genotypes
- D Because environmental factors never influence gene expression
Correct answer: To simplify inference of the mode of inheritance
Assuming complete penetrance and a rare allele simplifies pedigree analysis by removing confounders such as variable expression, incomplete penetrance, and common carriers. It is a simplifying assumption, not a claim about all real traits.
A pedigree shows only males affected in multiple generations, and no daughters are affected even when fathers are affected. Which pattern is the best fit?
- A Autosomal dominant
- B X-linked recessive
- C X-linked dominant
- D Autosomal recessive
Correct answer: X-linked recessive
When only males are affected across generations and no daughters are affected even from affected fathers, X-linked recessive inheritance is the best fit.
Which inheritance pattern would you suspect if all children — male and female — of an affected male are affected?
- A Autosomal dominant
- B Autosomal recessive
- C X-linked dominant
- D X-linked recessive
Correct answer: X-linked dominant
If an affected male transmits the trait to all his children (both sexes), this suggests X-linked dominant inheritance, since he passes his mutant X chromosome to all daughters and mutant Y (non-trait) to sons only, but daughters receive the trait, and if trait is fully penetrant, all children may be affected depending on pattern.
Which feature on a pedigree would argue for autosomal rather than X-linked inheritance when both sexes show roughly equal affected frequency?
- A Only male family members are ever affected
- B Affected fathers produce only affected daughters
- C Both sexes affected in similar proportions
- D The trait appears only among siblings of one sex
Correct answer: Both sexes affected in similar proportions
Roughly equal numbers of affected males and females across generations point to autosomal inheritance, whereas X-linked patterns usually show a sex bias.
In pedigree analysis with a rare autosomal recessive trait, what is the chance that two carrier parents will have an unaffected child who is also a carrier?
- A 25%
- B 50%
- C 75%
- D 100%
Correct answer: 50%
For a cross of two carriers (Aa × Aa), offspring are 1 AA : 2 Aa : 1 aa, so any given child has a 50% chance of being an unaffected carrier (Aa).