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Unique Rare Genetic Conditions Explained

May 21, 2025

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Lecture Notes

Topic: Rare Genetic Conditions

1. Chimerism

  • Definition: Condition where an individual has two different sets of DNA.
  • Cause: Occurs when one absorbs their twin in the womb.
  • Characteristics:
    • Different parts of the body can have different DNA.
    • Visible signs include different colored eyes or skin patches.
    • Some may have two different blood types.
  • Case Studies:
    • Woman almost lost custody of her kids due to mismatched DNA.
    • Model Taylor Mull has a visible line dividing different skin colors on her torso.
  • Prevalence: Likely more common than realized; often undiagnosed without genetic tests.

2. Uncombable Hair Syndrome

  • Definition: Genetic condition where hair cannot be combed flat.
  • Characteristics:
    • Hair grows in all directions and refuses to lie flat.
    • Usually silvery blonde or straw-colored.
  • Genetic Cause: Mutations in TCHH, PAI3, TGM3 genes.
  • Prevalence: Extremely rare, about 100 cases documented.
  • Prognosis:
    • Often improves during puberty.
    • Harmless aside from unique appearance.

3. RH Null Blood (Golden Blood)

  • Definition: Blood type with no Rh proteins, extremely rare.
  • Characteristics:
    • Only about 50 people in the world have this type.
    • Can donate to anyone with rare Rh blood types, but can only receive from another RH null.
  • Discovery: First identified in 1961.
  • Complications: Risky due to difficulty finding matching donors.

4. Heterochromia

  • Definition: Having two different colored eyes.
  • Types:
    • Complete: Each eye a different color.
    • Segmental: Parts of the same eye different colors.
    • Central: Ring of different color around the pupil.
  • Causes:
    • Genetic mutations or injuries/illnesses.
  • Cultural Beliefs: Historically associated with magical powers or mystical abilities.

5. Anonychia Congenita

  • Definition: Condition of being born without fingernails or toenails.
  • Genetic Cause: Mutations in RSPO4 gene.
  • Characteristics: Totally normal fingers and toes aside from lack of nails.
  • Prevalence: Very rare, not considered a disability.

6. Distichiasis

  • Definition: Having two rows of eyelashes.
  • Characteristics:
    • Second row often grows inward, causing irritation.
  • Causes: Genetic mutation or acquired after eye injuries/infections.
  • Treatment: Laser treatments or manual removal to alleviate symptoms.

7. Cervical Rib

  • Definition: An extra rib growing from neck vertebrae.
  • Prevalence: About 1 in 100 people have it.
  • Complications: Can compress nerves or vessels, causing pain and numbness.
  • Historical Note: Also found in woolly mammoths.

8. Single Palmar Crease

  • Definition: Having one line across the palm instead of three.
  • Prevalence: 1 in 30 people; more common in men and certain ethnic groups.
  • Associations: Sometimes linked to genetic conditions like Down syndrome, but often benign.

9. Pre-auricular Pit

  • Definition: Small pit near the ear.
  • Prevalence: Found in about 1% of the US population but can be as high as 10% in Asia/Africa.
  • Origin: Developmental variation during the formation of the outer ear.
  • Complications: Rarely, pits can become infected.

Note: The lecture emphasized the rarity and unique nature of these genetic conditions, often highlighting that they are not typically harmful and that many individuals live normal lives despite them.

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