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Abnormal Foundations of Genetics & Molecular Biology

CPR APPLICATIONS FOUNDATION · CHAPTER 1 · ABNORMAL

Abnormal Foundations of Genetics & Molecular Biology

Use a chapter-level CPR map to organize the recurring ways normal function breaks and how those deviations present.

Supplemental foundation map: this page orients the detailed CPR Applications for Chapter 1. It is not part of the canonical CPR object inventory.

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1.12 · SOURCE CPR FOUNDATION

Concept

Genetic disease on Step 1 asks you to identify what level of genetic information is disrupted (chromosome number/structure, single-gene behavior, repair, repeats, imprinting, metabolism) and then predict the inheritance, severity, variability, and downstream consequences. Inheritance patterns describe how traits transmit across generations.

Pattern

Organize abnormalities by “where the error lives”: Chromosome number (dosage) vs chromosome structure (rearrangement) vs single-gene behavior (penetrance/expressivity, LOF/GOF) vs maintenance errors (repair, repeats, imprinting) vs biochemical pathway errors (inborn metabolism). Pedigrees repeatedly fall into recognizable shapes: vertical transmission, sibling clustering, sex bias, and maternal-only inheritance.

Rule

  1. Locate where the genetic error lives: decide whether the problem is chromosome dosage, structural rearrangement, mutation effect, repair failure, imprinting, repeat expansion, or metabolic pathway block.
  2. Classify the inheritance pattern before naming the syndrome: vertical transmission, sibling clustering, sex bias, and maternal-only transmission each narrow the mechanism.
  3. Use the mechanism to predict severity and timing: enzyme loss often presents early and severe; dominant structural or regulatory defects may present later or with variable severity.

Applications

  • Marfan syndrome → single-gene dominant mutation → abnormal fibrillin → tall habitus, aortic root dilation
  • Cystic fibrosis → single-gene loss-of-function → defective CFTR → thick secretions, recurrent infections
  • Lynch syndrome → DNA mismatch repair defect → microsatellite instability → early colon and endometrial cancers
  • Prader-Willi vs Angelman → imprinting defect on chromosome 15 → different phenotypes depending on parental origin
  • Phenylketonuria → metabolic pathway defect → phenylalanine buildup → intellectual disability if untreated

CPR Applications · Active reasoning

Locate the genetic defect before naming the disease

Distinguish inheritance, defective molecular machinery, and parent-of-origin effects. Similar phenotypes can arise from very different defects. Work through each decision before revealing the reasoning. Your responses stay in this page; there is no score or submission.

Decision 1 of 3

Two unaffected parents have two children of different sexes with early pancreatic insufficiency and recurrent pulmonary infections. A CFTR-associated disorder is suspected. Which model most directly explains the sibling pattern?
Reveal the worked reasoning (also works without scripts)

Best answer: Biallelic loss-of-function with autosomal-recessive inheritance. Two carrier parents can transmit pathogenic CFTR alleles to children of either sex. The phenotype reflects loss of CFTR-mediated chloride/bicarbonate transport.

Transfer: Would an unaffected sibling necessarily lack both family variants? No; an unaffected sibling may be a carrier.

Decision 2 of 3

A colorectal tumor exhibits instability at multiple short tandem repeat loci. Which molecular failure most directly explains the accumulated replication errors?
Reveal the worked reasoning (also works without scripts)

Best answer: Mismatch-repair deficiency. Mismatch repair normally resolves base-pair mismatches and short insertion/deletion loops during replication; its failure produces microsatellite instability.

Transfer: Does microsatellite instability alone prove inherited Lynch syndrome? No; sporadic mismatch-repair-deficient tumors also occur.

Decision 3 of 3

A child has severe neonatal hypotonia followed later by hyperphagia. Testing shows loss of the normally expressed paternal gene contribution at 15q11-q13. Which syndrome best fits?
Reveal the worked reasoning (also works without scripts)

Best answer: Prader–Willi syndrome. Loss of paternal expression in this imprinted region is characteristic of Prader–Willi syndrome; multiple molecular mechanisms can cause that loss.

Transfer: Could uniparental disomy also cause a parent-of-origin expression defect? Yes; deletion is not the only mechanism.

Explain it back

Describe how the same region on chromosome 15 can produce different clinical syndromes depending on which parent’s functional contribution is lost.

Compare your explanation with the reasoning checklist
  • Imprinted genes depend on parental origin rather than simple diploid dosage alone.
  • Loss of paternal 15q11-q13 expression is linked to Prader–Willi syndrome.
  • Loss of maternal neuronal UBE3A expression is linked to Angelman syndrome.

Educational mechanism exercises only. These do not establish individual clinical recommendations or psychometric performance.

How to use this foundation map

  1. Read the Concept and identify the organizing mechanism.
  2. Use the Pattern to recognize how the mechanism tends to appear in Step 1-style reasoning.
  3. Restate the Rule without looking, then test whether each application follows from it.
  4. Move to the adjacent foundation layer or use question practice to test transfer.

When applying the organized knowledge to a case or question, pair CPR with the 5P Approach™.

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Back to Chapter 1 in the Foundations Library · USMLE Step 1 resources · Interactive CPR demonstration