Mendelian inheritance Practice Questions
20 free Mendelian inheritance 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.
A healthy couple has a child with an autosomal recessive metabolic disorder. Neither parent is affected. What is the probability that their next child will also be affected?
- A 0%
- B 25%
- C 50%
- D 75%
Correct answer: 25%
Autosomal recessive disorders require inheritance of two mutant alleles. Two carriers have a 25% chance of producing an affected child. The remaining offspring are either carriers or unaffected.
If a father has an X-linked recessive disorder, which of his children are guaranteed to inherit the mutant allele?
- A All sons
- B All daughters
- C Half of sons
- D Half of daughters
Correct answer: All daughters
Fathers pass their X chromosome exclusively to all daughters. Sons inherit the Y chromosome instead. Thus, all daughters are obligate carriers unless the disorder is dominant.
A woman who is a carrier for an X-linked recessive disease has a child with a healthy male partner. What is the chance that she will have an affected son?
- A 25%
- B 50%
- C 75%
- D 100%
Correct answer: 50%
A carrier mother has a 50% chance of passing the mutant X allele. Sons who inherit the allele will be affected because they have only one X chromosome.
A child expresses a phenotype that neither parent shows, but both parents are carriers. Which inheritance pattern best explains this?
- A Autosomal dominant inheritance
- B Autosomal recessive inheritance
- C X-linked dominant inheritance
- D Mitochondrial inheritance
Correct answer: Autosomal recessive inheritance
Autosomal recessive traits require two copies of the mutant allele, so unaffected carriers may pass the disorder to offspring. This results in a classic 'skipping generations' pattern. Dominant traits would appear in at least one parent.
A mitochondrial disorder is suspected in a family. Which of the following inheritance patterns is expected?
- A Affected fathers transmit disease to daughters only
- B Affected mothers transmit disease to all children
- C Affected mothers transmit disease to sons only
- D Affected fathers transmit disease to all children
Correct answer: Affected mothers transmit disease to all children
Mitochondrial DNA is inherited exclusively from the mother. Therefore, all children of an affected mother can inherit the condition. Fathers cannot transmit mitochondrial disorders.
A trait shows vertical transmission across generations, and every affected individual has at least one affected parent. Which inheritance pattern does this describe?
- A Autosomal recessive
- B Autosomal dominant
- C X-linked recessive
- D Mitochondrial
Correct answer: Autosomal dominant
Autosomal dominant inheritance displays vertical transmission, meaning the trait appears in multiple consecutive generations. Affected individuals must have one affected parent. Recessive traits typically skip generations.
If a homozygous recessive individual mates with a heterozygous carrier, what percentage of their offspring are expected to be affected?
- A 0%
- B 25%
- C 50%
- D 100%
Correct answer: 50%
The cross rr × Rr results in 50% rr offspring. These individuals express the recessive phenotype. The remaining 50% are carriers.
A father is affected with an autosomal dominant disorder. The mother is unaffected and does not carry the mutation. What is the chance their child will inherit the disorder?
- A 0%
- B 25%
- C 50%
- D 100%
Correct answer: 50%
Autosomal dominant disorders require only one affected allele. A heterozygous affected parent passes the allele to 50% of children. The remaining children inherit the normal allele.
A disorder is passed from affected mothers to all children, but affected fathers do not transmit it to any offspring. This pattern most strongly suggests:
- A Autosomal dominant inheritance
- B Autosomal recessive inheritance
- C Mitochondrial inheritance
- D X-linked dominant inheritance
Correct answer: Mitochondrial inheritance
Mitochondrial disorders follow strict maternal inheritance. Fathers do not transmit mitochondrial DNA. All children of an affected mother may inherit the mutation.
A patient with an autosomal dominant condition displays variable expressivity. Which best describes this phenomenon?
- A The same mutation produces differing severity among individuals
- B The trait skips generations before reappearing in descendants
- C Multiple different genes independently produce the same phenotype
- D The trait is expressed only in homozygous affected individuals
Correct answer: The same mutation produces differing severity among individuals
Variable expressivity means the same genotype produces a range of phenotypic severities. Penetrance concerns whether the trait is expressed at all, not its degree. Skipping generations is a hallmark of recessive inheritance.
Two unaffected parents have a child with an autosomal recessive disease. What is the probability that their next child will be a carrier?
- A 25%
- B 33%
- C 50%
- D 75%
Correct answer: 50%
Two carriers have offspring with genotypes: 25% affected, 50% carriers, 25% unaffected non-carriers. Thus, there is a 50% chance the next child is a carrier.
A male has an X-linked dominant disorder. Which of his children are expected to inherit the condition?
- A Only sons
- B Only daughters
- C Half of all children
- D All children
Correct answer: Only daughters
X-linked dominant disorders are transmitted from affected fathers to all daughters. Sons inherit the Y chromosome and therefore are not affected.
A lethal autosomal dominant mutation reduces survival before reproduction. What is expected in the population?
- A Allele frequency increases
- B Allele frequency remains constant
- C Allele frequency decreases
- D No effect on allele frequency
Correct answer: Allele frequency decreases
A lethal dominant mutation prevents affected individuals from reproducing, reducing allele frequency. Such mutations must arise de novo to persist. This is an example of strong negative selection.
A pedigree shows affected individuals in every generation, but some individuals who carry the mutant allele do not express the phenotype. This is an example of:
- A Variable expressivity
- B Incomplete penetrance
- C Genetic anticipation
- D Codominance
Correct answer: Incomplete penetrance
Incomplete penetrance means individuals with the mutation may or may not express the phenotype. Variable expressivity affects severity, not presence. Anticipation involves earlier onset over generations.
In codominance, which statement is true?
- A Only the dominant allele is expressed in the phenotype
- B Both alleles are fully expressed in the phenotype
- C Neither allele is expressed in the resulting phenotype
- D The phenotype skips generations in the pedigree
Correct answer: Both alleles are fully expressed in the phenotype
Codominance occurs when both alleles are fully and simultaneously expressed, as in ABO blood types. This differs from incomplete dominance, where the two phenotypes blend. Codominance does not skip generations.
A child inherits two different mutant alleles of the same gene, one from each parent, resulting in disease expression. What is this called?
- A Codominant allele expression
- B Genetic anticipation over generations
- C Compound heterozygosity
- D Incomplete penetrance of the allele
Correct answer: Compound heterozygosity
Compound heterozygosity occurs when two different pathogenic alleles of the same gene cause disease. This is common in recessive disorders. It is distinct from homozygosity for a single mutation.
A woman affected by an autosomal dominant condition has a child with an unaffected man. What is the probability that their child will be unaffected and also a non-carrier?
- A 25%
- B 50%
- C 75%
- D 100%
Correct answer: 50%
A heterozygous affected mother passes the mutant allele to 50% of children. The other 50% inherit the normal allele and are fully unaffected. There are no carriers in dominant conditions.
A pedigree shows only males affected, with no male-to-male transmission. This most likely represents:
- A Autosomal dominant
- B Autosomal recessive
- C X-linked recessive
- D Mitochondrial
Correct answer: X-linked recessive
X-linked recessive traits affect males disproportionately and lack male-to-male transmission. Females are usually carriers. Autosomal traits do not show this pattern.
A mother with an X-linked dominant disorder has a child with an unaffected partner. What proportion of daughters will be affected?
- A 0%
- B 25%
- C 50%
- D 100%
Correct answer: 50%
A heterozygous affected mother passes the dominant allele to 50% of daughters. Both sons and daughters have a 50% chance of inheriting the mutation. The unaffected parent contributes a normal allele.
A disease shows increasing severity and earlier onset in successive generations. Which genetic concept explains this pattern?
- A Genetic anticipation
- B Variable expressivity
- C Incomplete penetrance
- D X-linked dominance
Correct answer: Genetic anticipation
Genetic anticipation refers to worsening disease severity or earlier onset across generations, often related to trinucleotide repeat expansion. Expressivity deals with severity variability without generational change.