Solving Pedigree Analysis in 3 steps | Epomedicine - Free Printable

The image you've uploaded presents four different pedigree patterns for genetic inheritance, each illustrating a possible mode of inheritance: Autosomal Dominant, Autosomal Recessive, X-Linked Dominant, and X-Linked Recessive. Below is a detailed explanation of each pedigree and the reasoning behind the conclusions drawn.

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🔹 1. Autosomal Dominant Inheritance

Pedigree Pattern:
- Both parents are affected (filled symbols).
- One child is unaffected (empty symbol), one is affected.
- The trait appears in every generation.

Explanation:
- In autosomal dominant inheritance, only one copy of the mutated gene (from either parent) is sufficient to cause the disorder.
- If both parents are affected, they must be heterozygous (Aa) — meaning they carry one mutant allele and one normal allele.
- An unaffected child (aa) can result from a cross between two heterozygotes (Aa × Aa), which has a 25% chance of producing an unaffected individual.
- This pattern cannot be recessive, because if it were recessive, two affected parents (aa) would have all affected offspring (since both parents would pass on the recessive allele). But here, an unaffected child exists — so it cannot be recessive.

✔ Conclusion: This supports autosomal dominant inheritance.

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🔹 2. Autosomal Recessive Inheritance

Pedigree Pattern:
- Both parents are unaffected (empty symbols).
- One child is affected (filled symbol), others are unaffected.
- The trait skips generations.

Explanation:
- In autosomal recessive inheritance, an individual must inherit two copies of the mutant allele (one from each parent) to be affected.
- Unaffected parents who have an affected child must be carriers (heterozygous, Aa).
- The affected child is homozygous recessive (aa).
- This pattern cannot be dominant, because if the trait were dominant, at least one parent would need to be affected (since only one copy is needed). But here, both parents are unaffected — so the trait cannot be dominant.

✔ Conclusion: This supports autosomal recessive inheritance.

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🔹 3. X-Linked Dominant Inheritance

Pedigree Pattern:
- Affected father (filled square) mates with unaffected mother (empty circle).
- All daughters are affected (filled circles), all sons are unaffected (empty squares).

Explanation:
- In X-linked dominant inheritance:
- Males (XY) have one X chromosome; if it carries the mutant allele, they are affected.
- Females (XX) need only one mutant allele to be affected.
- An affected father passes his X chromosome to all daughters (and Y to sons).
- So, all daughters will inherit the mutant X and be affected.
- Sons receive the Y chromosome from the father and the X from the mother (who is unaffected), so they are not affected.
- This pattern shows 100% of daughters affected when the father is affected — a hallmark of X-linked dominant inheritance.
- However, this alone does not confirm X-linkage definitively (other factors could mimic this), but it is strongly suggestive.

⚠️ Note: The caption says "Mode of inheritance cannot be confirmed" — this is cautious, as other possibilities (like mitochondrial or de novo mutation) could also explain such a pattern, but X-linked dominant is highly likely.

✔ Conclusion: Strongly suggests X-linked dominant inheritance.

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🔹 4. X-Linked Recessive Inheritance

Pedigree Pattern:
- Unaffected parents (both empty symbols).
- Multiple sons are affected (filled squares), daughters are unaffected.
- No male-to-male transmission.

Explanation:
- In X-linked recessive inheritance:
- Males are more frequently affected because they have only one X chromosome.
- An affected son inherits the mutant allele from his mother, who is typically a carrier (heterozygous, X^A X^a).
- The father contributes the Y chromosome and does not pass X to sons.
- The high incidence of affected sons with unaffected parents suggests the mother is a carrier.
- Daughters are usually unaffected unless the father is affected (which he isn't here), so they get one normal X from father and one possibly mutant X from mother — making them carriers or unaffected.
- The absence of male-to-male transmission is consistent with X-linkage (males pass Y to sons, not X).

⚠️ Note: The caption says "Sex linkage cannot be confirmed" — again, this is conservative. While other mechanisms might explain some cases, the high incidence of affected sons with unaffected parents strongly points to X-linked recessive inheritance.

✔ Conclusion: Suggests X-linked recessive inheritance.

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✔ Summary Table:

| Inheritance Type | Key Features | Reasoning |
|-------------------------|------------------------------------------------------------------------------|----------|
| Autosomal Dominant | Affected parents → unaffected child; trait in every generation | Cannot be recessive (affected parents would not produce unaffected children) |
| Autosomal Recessive | Unaffected parents → affected child | Cannot be dominant (unaffected parents wouldn’t have affected kids) |
| X-Linked Dominant | Affected father → all daughters affected, no sons affected | Father passes X to daughters; strong evidence for X-linkage |
| X-Linked Recessive | Unaffected parents → multiple affected sons | High male incidence; mother likely carrier |

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🧠 Final Takeaway:

These pedigrees illustrate how family history helps determine the mode of inheritance of a genetic trait. By analyzing who is affected, gender distribution, and transmission across generations, we can infer whether a condition is autosomal or sex-linked, and dominant or recessive.

Let me know if you'd like help interpreting a real pedigree or applying these principles!