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Here is the complete solution and explanation for Pedigree Worksheet 3: X-Linked Dominant Traits.

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## 1. Hypophosphatemic rickets (R) is an X-linked dominant condition...

a. Number each individual within each generation.

We number individuals left to right, top to bottom, within each generation.

- Generation I (Top):
- I-1: Affected male (green square)
- I-2: Unaffected female (white circle)

- Generation II:
- II-1: Affected female (green circle)
- II-2: Unaffected male (white square)
- II-3: Unaffected female (white circle)
- II-4: Affected female (green circle)
- II-5: Affected male (green square)

*(Note: The pedigree has two branches from I-1 and I-2 — one leading to II-1, II-2, II-3; the other to II-4, II-5)*

- Generation III:
- Left branch (from II-1 & II-2):
- III-1: Unaffected female
- III-2: Affected male
- III-3: Affected male
- III-4: Affected female
- Right branch (from II-4 & II-5):
- III-5: Affected female
- III-6: Affected female
- III-7: Affected female
- III-8: Unaffected male
- III-9: Unaffected female

*(Note: III-7 is the 7th individual of Generation III — as referenced in part d)*

- Generation IV:
- From III-1 & III-2:
- IV-1: Unaffected male
- IV-2: Affected female
- IV-3: Affected female
- From III-7 & III-8:
- IV-4: Unaffected female
- IV-5: Unaffected female
- IV-6: Unaffected male
- IV-7: Unaffected female

✔ Numbering completed.

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b. How many generations are represented within this pedigree?

There are 4 generations shown: I, II, III, and IV.

> Answer: 4

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c. Why are there no carriers represented in this pedigree?

In X-linked dominant inheritance:

- A single copy of the mutant allele on the X chromosome is sufficient to cause the disease.
- Therefore, there are no “carriers” — individuals either have the trait (if they have the mutant allele) or they don’t.
- In contrast, X-linked *recessive* disorders can have female carriers (heterozygous females who do not show symptoms).

> Answer: Because it’s X-linked dominant — only one mutant allele is needed to express the trait, so heterozygous individuals are affected, not carriers.

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d. The 7th individual of the 3rd generation has children with an unaffected male individual. Why are no male children of the next generation impacted?

- Individual III-7 is an affected female → genotype: XᴿXʳ (assuming R = mutant dominant allele, r = normal recessive allele).
- She marries an unaffected male → genotype: XʳY.

Their sons inherit:
- X chromosome from mother → 50% chance of Xᴿ (affected), 50% chance of Xʳ (unaffected)
- Y chromosome from father

But in the pedigree, all sons (IV-6) are unaffected → meaning they inherited the normal Xʳ from their mother.

> Answer: The affected female (III-7) is heterozygous (XᴿXʳ). Her sons inherit either her Xᴿ or Xʳ. Since all sons are unaffected, they must have inherited the normal Xʳ allele from her. It’s possible by chance that none inherited the mutant Xᴿ.

*(Note: The question says “why are no male children impacted?” — the answer is because they inherited the normal X from mom. Statistically, 50% should be affected, but in this family, by chance, all sons got the normal allele.)*

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e. Individuals 1 and 2 of the 2nd generation have children, 25% of which are unaffected. How is this possible?

- II-1: Affected female → genotype: XᴿXʳ (must be heterozygous, since she has unaffected children)
- II-2: Unaffected male → genotype: XʳY

Cross: XᴿXʳ × XʳY

Offspring possibilities:

| Mother’s Egg | Father’s Sperm | Child Genotype | Phenotype |
|--------------|----------------|----------------|-----------------|
| Xᴿ | Xʳ | XᴿXʳ | Affected female |
| Xᴿ | Y | XᴿY | Affected male |
| Xʳ | Xʳ | XʳXʳ | Unaffected female |
| Xʳ | Y | XʳY | Unaffected male |

→ 50% affected, 50% unaffected.

But the question says 25% are unaffected — wait, that contradicts the cross above.

Let’s reexamine:

Actually, looking at the pedigree:

- II-1 and II-2 have 4 children: III-1 (unaffected female), III-2 (affected male), III-3 (affected male), III-4 (affected female)

→ That’s 1 out of 4 = 25% unaffected.

This matches the cross: each child has a 50% chance of being unaffected, but in this specific family, only 1 out of 4 is unaffected — which is statistically possible (though unlikely, it can happen by chance).

> Answer: Although each child has a 50% chance of being unaffected, in this particular family, only 1 out of 4 children (25%) is unaffected — this is due to random segregation of alleles during meiosis.

*(Note: The 25% refers to the actual observed proportion in this family, not the expected probability.)*

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## 2. Using your monohybrid crossing analytical skills, determine and list the genotypes of each individual in the pedigree above.

Let’s define:
- Xᴿ = mutant dominant allele (causes disease)
- Xʳ = normal recessive allele
- Males: XᴿY (affected), XʳY (unaffected)
- Females: XᴿXᴿ, XᴿXʳ (both affected), XʳXʳ (unaffected)

Now assign genotypes:

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Generation I:

- I-1: Affected male → XᴿY
- I-2: Unaffected female → XʳXʳ

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Generation II:

- II-1: Affected female → must be XᴿXʳ (since her mother I-2 is XʳXʳ, she must have gotten Xʳ from mom and Xᴿ from dad I-1)
- II-2: Unaffected male → XʳY
- II-3: Unaffected female → XʳXʳ (got Xʳ from both parents)
- II-4: Affected female → XᴿXʳ (same logic as II-1)
- II-5: Affected male → XᴿY (inherited Xᴿ from mother I-2? Wait — contradiction!)

⚠️ Problem here!

I-2 is unaffected female → XʳXʳ. She can only give Xʳ to offspring.

So how can II-5 be an affected male (XᴿY)? He must have gotten Xᴿ from his mother — but his mother (I-2) is XʳXʳ → impossible!

This suggests an error in the pedigree or our assumption.

Wait — let’s double-check the pedigree structure.

Looking again: I-1 (affected male, XᴿY) and I-2 (unaffected female, XʳXʳ) have children:

- II-1: affected female → XᴿXʳ (got Xᴿ from dad, Xʳ from mom) ✔
- II-2: unaffected male → XʳY (got Xʳ from mom, Y from dad) ✔
- II-3: unaffected female → XʳXʳ ✔
- II-4: affected female → XᴿXʳ ✔
- II-5: affected male → XᴿY — but he must have gotten Xᴿ from mom — but mom is XʳXʳ → impossible!

👉 Conclusion: There is likely a mistake in the pedigree drawing or labeling.

However, since this is a worksheet, we assume the pedigree is correct and perhaps II-5 is mislabeled or there’s a new mutation.

But biologically, II-5 cannot be affected if his mother is unaffected and the trait is X-linked dominant — unless there’s a de novo mutation.

For the sake of completing the worksheet, we’ll assume:

- II-5: XᴿY (with a de novo mutation, or perhaps the pedigree has an error)

Alternatively, maybe II-5 is not the son of I-1 and I-2? But the lines suggest he is.

Given standard pedagogy, we proceed assuming the pedigree is drawn correctly and II-5 is affected — so we must conclude:

> II-5: XᴿY — implying a new mutation occurred in him, or the pedigree has an inconsistency.

But since the worksheet asks us to solve based on the diagram, we’ll assign:

- II-5: XᴿY

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Generation III:

Left branch (II-1 XᴿXʳ × II-2 XʳY):

- III-1: Unaffected female → XʳXʳ
- III-2: Affected male → XᴿY
- III-3: Affected male → XᴿY
- III-4: Affected female → XᴿXʳ

Right branch (II-4 XᴿXʳ × II-5 XᴿY):

- III-5: Affected female → could be XᴿXᴿ or XᴿXʳ
- Since father is XᴿY → gives Xᴿ to daughters
- Mother is XᴿXʳ → gives Xᴿ or Xʳ
- So daughter could be XᴿXᴿ or XᴿXʳ — both affected → we can write Xᴿ_ (unknown second allele)
- But to be precise, since mother can give Xʳ, and we don’t know, we’ll say XᴿXʳ (most likely, unless specified otherwise)

- III-6: Affected female → same → XᴿXʳ
- III-7: Affected female → XᴿXʳ
- III-8: Unaffected male → XʳY
- III-9: Unaffected female → XʳXʳ

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Generation IV:

From III-1 (XʳXʳ) × III-2 (XᴿY):

- IV-1: Unaffected male → XʳY
- IV-2: Affected female → XᴿXʳ
- IV-3: Affected female → XᴿXʳ

From III-7 (XᴿXʳ) × III-8 (XʳY):

- IV-4: Unaffected female → XʳXʳ
- IV-5: Unaffected female → XʳXʳ
- IV-6: Unaffected male → XʳY
- IV-7: Unaffected female → XʳXʳ

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✔ Genotype List Summary:

| Individual | Genotype |
|------------|----------|
| I-1 | XᴿY |
| I-2 | XʳXʳ |
| II-1 | XᴿXʳ |
| II-2 | XʳY |
| II-3 | XʳXʳ |
| II-4 | XᴿXʳ |
| II-5 | XᴿY *(assumed, though biologically problematic)* |
| III-1 | XʳXʳ |
| III-2 | XᴿY |
| III-3 | XᴿY |
| III-4 | XᴿXʳ |
| III-5 | XᴿXʳ |
| III-6 | XᴿXʳ |
| III-7 | XᴿXʳ |
| III-8 | XʳY |
| III-9 | XʳXʳ |
| IV-1 | XʳY |
| IV-2 | XᴿXʳ |
| IV-3 | XᴿXʳ |
| IV-4 | XʳXʳ |
| IV-5 | XʳXʳ |
| IV-6 | XʳY |
| IV-7 | XʳXʳ |

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## 3. If this diagram were representative of an autosomal dominant disorder:

a. What change would you expect to observe?

In autosomal dominant inheritance:

- Trait appears in every generation (like X-linked dominant)
- Males and females are equally likely to be affected
- Affected individuals have at least one affected parent (unless new mutation)
- Unaffected individuals do not transmit the trait
- No sex bias — so we’d expect roughly equal numbers of affected males and females

In this pedigree, we see:
- More affected females than males (common in X-linked dominant because females have two X chromosomes)
- Affected males pass trait to ALL daughters (which is true for X-linked dominant, but NOT for autosomal dominant)

> Answer: We would expect to see affected males and females in approximately equal proportions, and affected males would not necessarily pass the trait to all their daughters (since autosomal traits are not sex-linked). Also, an affected male could have unaffected daughters, which is impossible in X-linked dominant.

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b. What change would you not expect to observe?

We would NOT expect:

- Skipping generations — autosomal dominant traits typically appear in every generation.
- No male-to-male transmission — in autosomal dominant, affected fathers can pass the trait to sons (unlike X-linked, where males pass X to daughters only).
- All daughters of an affected male being affected — this is characteristic of X-linked dominant, not autosomal.

> Answer: You would not expect to see the pattern where all daughters of an affected male are affected — that is specific to X-linked dominant inheritance. Also, you would not expect to see the trait skipping generations.

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## ✔ Final Answers Summary:

1.

a. Numbered as above (I-1 to IV-7)
b. 4 generations
c. No carriers because X-linked dominant traits require only one mutant allele to express — heterozygotes are affected, not carriers.
d. The affected female (III-7) is heterozygous; her sons inherited her normal Xʳ allele by chance.
e. Each child has 50% chance of being unaffected; 25% unaffected in this family is due to random chance in small sample size.

2. Genotypes listed above.

3.

a. Equal male/female affected ratios; affected males can have unaffected daughters; no mandatory father-to-daughter transmission.
b. You would NOT expect to see all daughters of an affected male being affected — that’s unique to X-linked dominant.

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Let me know if you’d like a visual chart or Punnett squares for any part!