Pedigree analysis worksheet for studying genetic traits and inheritance.

Let's solve the pedigree problem step by step based on the image you provided.

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Pedigree Analysis

The pedigree shows inheritance of a genetic trait across four generations (I to IV). We are given symbols:

- Square = Male
- Circle = Female
- Shaded = Affected individual
- Unshaded = Unaffected individual
- Lines between squares and circles = Marriage lines
- Vertical lines from parents to children = Offspring

We’ll analyze the pattern of inheritance and answer each question.

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Step 1: Identify the Inheritance Pattern

Looking at the pedigree:

- Individual I-1 is affected (male), I-2 is unaffected (female).
- Their offspring:
- II-1: Affected female
- II-2: Affected male
- II-3: Unaffected male
- II-4: Unaffected female

So, an affected father (I-1) has both affected and unaffected children — this suggests not Y-linked, since only males would be affected.

Also, II-1 (affected female) has an unaffected mother (I-2), so if it were X-linked recessive, she would need two copies of the allele. But her father is affected (has one copy), and mother is unaffected — so she could have inherited the mutant allele from her father and possibly a normal allele from mother. That’s possible.

But let’s look further.

Now consider III-1 (affected male) — his mother (II-1) is affected, father (II-2) is unaffected.

Then IV-1 is unaffected, but born to III-1 (affected male) and III-2 (unaffected female).

Wait — IV-1 is not shaded → unaffected.

Now check II-5 and II-6: both affected. Their children:
- III-3: affected
- III-4: unaffected
- III-5: affected

So both parents affected → some children affected, some not → suggests autosomal dominant or recessive?

But in autosomal recessive, two affected parents should have all affected children unless they're heterozygous — but that doesn’t make sense if they’re affected.

Wait — if the trait is autosomal recessive, then affected individuals must be homozygous recessive. So two affected parents should have all affected children (since both contribute recessive alleles).

But here, II-5 and II-6 are both affected, yet their daughter III-4 is unaffected → this contradicts autosomal recessive.

So it cannot be autosomal recessive.

Now consider autosomal dominant:

- Affected individuals have at least one copy of the mutant allele.
- If both parents are affected (heterozygous), then 75% chance of affected offspring — which matches: III-3 (affected), III-4 (unaffected), III-5 (affected).

Also, I-1 is affected, I-2 is unaffected → children: some affected, some not → consistent with dominant (I-1 heterozygous, I-2 homozygous normal).

And II-1 is affected (female), her mother (I-2) is unaffected → so she must have inherited the allele from her father (I-1), which is fine.

Now, III-1 is affected male, married to III-2 (unaffected female). Their child IV-1 is unaffected → consistent with dominant inheritance (III-1 is heterozygous, III-2 is homozygous normal → 50% chance of affected offspring; IV-1 is unaffected).

So far, autosomal dominant fits best.

Check for X-linked? Let's test:

If X-linked dominant, then:
- Affected males pass the trait to all daughters, but no sons.
- But here, I-1 is affected male → his son II-2 is affected → that would mean he passed the X chromosome to son — impossible. Males pass Y to sons, X to daughters.

So I-1 (affected male) → II-2 (son) is affected → this cannot happen in X-linked inheritance.

Therefore, not X-linked.

So we conclude: Autosomal Dominant inheritance.

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Answering Questions

#### 1. How many generations are in this pedigree?
- Generation I: I-1, I-2
- Generation II: II-1 to II-6
- Generation III: III-1 to III-5
- Generation IV: IV-1

✔ Answer: 4 generations

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#### 2. How many marriages are in this pedigree?
Count the marriage lines:

- I-1 & I-2 → 1
- II-1 & II-2 → 2
- II-5 & II-6 → 3
- III-1 & III-2 → 4

Note: II-3 and II-4 are not shown marrying anyone.

✔ Answer: 4 marriages

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#### 3. How many individuals are affected?
Count shaded individuals:

- I-1 → yes
- II-1 → yes
- II-2 → yes
- II-5 → yes
- II-6 → yes
- III-1 → yes
- III-3 → yes
- III-5 → yes

That’s 8.

List:
- I-1
- II-1, II-2, II-5, II-6
- III-1, III-3, III-5

✔ Answer: 8 individuals

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#### 4. What is the sex of II-4?
II-4 is a circle → female

✔ Answer: Female

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#### 5. How many children did couple I-1 and I-2 have?
They are parents of II-1, II-2, II-3, II-4 → 4 children

✔ Answer: 4 children

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#### 6. Why did II-2 marry?
This is a bit ambiguous, but likely asking why II-2 married II-1 (since they are connected).

But more likely, the question means: "Why did II-2 marry II-1?" — but there’s no indication of reason.

Alternatively, perhaps it's a typo or misphrasing.

But looking again: “Why did II-2 marry?” — seems incomplete.

Possibly it’s asking: “Why did II-2 marry II-1?” — but no information about motivation.

But maybe it’s asking: What is the reason for the marriage in terms of genetics? — unlikely.

Alternatively, it might be a trick: perhaps the question is “Why did II-2 marry?” — but he didn't necessarily marry for any specific reason.

Wait — actually, II-2 is affected, and II-1 is also affected — both have the disorder.

But II-3 and II-4 are unaffected.

So perhaps the question is meant to be: “Why did II-2 marry II-1?” — but again, no real explanation.

Alternatively, maybe the question is asking: "Why did II-2 marry?" as in, what was the consequence?

But that doesn’t make sense.

Wait — perhaps it's a typo, and it's supposed to be: "Why did II-2 and II-1 marry?" — but still, no data.

Alternatively, maybe the question is asking: "Why did II-2 marry someone who is also affected?" — but again, no info.

Perhaps it's just poorly worded.

But let's look at the context: the question says “Why did II-2 marry?” — but II-2 is married to II-1, and they have children (III-1, III-2, III-3, III-4, III-5).

So perhaps the intended meaning is: "Why did II-2 marry II-1?" — but we can’t say.

Alternatively, maybe the question is: "Why did II-2 marry?" — implying he did marry — and the answer is simply because he had children.

But that’s trivial.

Alternatively, maybe it's a joke or misprint.

But let’s assume it's asking: "Why did II-2 marry?" — and the answer is: He married to have children — but that’s speculative.

Wait — perhaps the question is: "Why did II-2 marry?" — and the answer is because he is affected, but that doesn't explain.

Alternatively, maybe the question is: "Why did II-2 marry II-1?" — and the answer is they are both affected, so perhaps for companionship.

But none of this is supported.

Wait — perhaps it's a typo, and it's supposed to be: "Why did II-2 and II-1 marry?" — but still.

Alternatively, maybe the question is: "Why did II-2 marry?" — and the answer is to produce offspring, but that's not specific.

Given the context, perhaps it's better to interpret as: "Why did II-2 marry II-1?" — and the answer is because they are both affected, but that's not a reason.

Alternatively, maybe the question is: "Why did II-2 marry?" — and the answer is he did marry, and that's it.

But that doesn't help.

Wait — perhaps it's asking: "Why did II-2 marry?" — and the answer is to have children, and the fact that he is affected may influence reproductive decisions.

But without more context, it's unclear.

Alternatively, maybe it's a mistake, and the intended question is: "Why is II-2 affected?" — but that’s not what it says.

Given the ambiguity, and since this is likely a student worksheet, perhaps the intended answer is:

> Because he is affected, and wants to have children — but this is speculative.

But let's skip and go to the next one.

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#### 7. What is the genotype of individual I-2?

We determined the trait is autosomal dominant.

- I-1 is affected → genotype: Aa (assuming A = dominant allele, a = normal)
- I-2 is unaffected → so genotype must be aa (homozygous recessive)

Because unaffected individuals do not carry the dominant allele.

✔ Genotype of I-2: aa

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Now, II-31 — wait, that’s not in the diagram.

Wait: the question says:

> 7. What is the genotype of individual I-2? II-31? III-1?

But II-31 is not a valid label.

Look at the pedigree:

- Generation II: II-1, II-2, II-3, II-4, II-5, II-6
- No II-31

Likely a typo.

Possibly meant to be II-3?

Similarly, III-1 is present.

So probably: I-2, II-3, III-1

Let’s assume:

> What is the genotype of individual I-2? II-3? III-1?

Let’s solve those.

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I-2: Unaffected → aa

II-3: Unaffected male → aa

III-1: Affected male → must have at least one A allele.

His parents: II-1 (affected) and II-2 (affected)

Both are affected → so both could be Aa or AA

But since they have unaffected children (e.g., III-4), that means both must be Aa (heterozygous), because if either were AA, all children would be affected.

So:
- II-1: Aa
- II-2: Aa
- III-1: affected → genotype: Aa or AA

But since III-4 is unaffected (aa), then both parents must have contributed a allele.

So III-1 could be Aa or AA, but since he has an unaffected sibling, and he is affected, most likely Aa.

But we can’t be 100% sure — he could be AA, but that’s less likely.

But in standard pedigree analysis, when two heterozygous parents have an unaffected child, the affected children are assumed to be heterozygous unless otherwise stated.

So III-1: Aa

But let’s confirm:

- II-1: Aa
- II-2: Aa
- Child III-4: aa → unaffected
- So III-1: affected → could be AA or Aa

But since both parents are Aa, probability of AA is 1/4, Aa is 1/2, aa is 1/4.

But III-1 is affected, so he is AA or Aa.

But we don’t know which.

However, in most cases, unless specified, we assume heterozygous.

But the question asks for genotype — we can write possibilities.

But since III-1 has an unaffected child (IV-1), we can deduce more.

III-1 is affected, III-2 is unaffected → aa

Their child IV-1 is unaffected → aa

So IV-1 must have received a from both parents.

So III-1 must have contributed a → so III-1 must be Aa, not AA.

✔ Therefore, III-1 is Aa

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So final genotypes:

- I-2: aa
- II-3: aa
- III-1: Aa

(We assume "II-31" was a typo for II-3)

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Now, back to Question 6: “Why did II-2 marry?”

Given the context, perhaps it’s a typo or misphrasing.

But maybe the question is: "Why did II-2 marry II-1?" — and the answer is because they are both affected, but that’s not a reason.

Alternatively, perhaps the question is: "Why did II-2 marry?" — and the answer is to have children, but that’s vague.

But in the context of genetics, perhaps the intended answer is:

> Because he is affected and wants to pass on the trait — but that’s not ethical.

Alternatively, maybe it's a trick: "Why did II-2 marry?" — because he is alive and able to marry.

But that’s silly.

Alternatively, perhaps it's asking: "Why did II-2 marry?" — and the answer is he didn’t choose not to, but that’s not helpful.

Given the ambiguity, and since it’s likely a typo, perhaps the intended question is: "Why is II-2 affected?" — but that’s not what it says.

Alternatively, maybe it's: "Why did II-2 and II-1 marry?" — and the answer is they are both affected, so they might be in a similar situation.

But without more context, we can’t say.

Perhaps the best answer is:

> II-2 married because he is an adult and capable of marriage, and he chose to have a family.

But that’s not scientific.

Alternatively, perhaps the question is: "Why did II-2 marry II-1?" — and the answer is because they are both affected, so they may understand each other’s condition.

But again, speculative.

Given that, and since this is likely a student worksheet, perhaps the intended answer is:

> Because they are both affected, so they may have chosen to marry due to shared experience.

But that’s not evidence-based.

Alternatively, maybe the question is misprinted, and it’s supposed to be: "Why is II-2 affected?" — then answer: because he inherited the dominant allele from his father, I-1.

That makes sense.

But as written: "Why did II-2 marry?" — it’s odd.

Perhaps it’s a typo for: "Why did II-2 and II-1 marry?" — but still.

Another possibility: "Why did II-2 marry?" — and the answer is to have children, and the fact that he is affected may influence the decision.

But again, not clear.

Given the confusion, and since the rest of the questions are clear, perhaps we can assume it’s a typo and move on.

But let’s try to answer as best as possible.

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Final Answers:

1. How many generations are in this pedigree?
✔ 4

2. How many marriages are in this pedigree?
✔ 4

3. How many individuals are affected?
✔ 8

4. What is the sex of II-4?
✔ Female

5. How many children did couple I-1 and I-2 have?
✔ 4

6. Why did II-2 marry?
✘ Ambiguous. Possibly a typo.
Possible interpretation: He married to have children.
But better: It is not specified in the pedigree. The reason is unknown.
Or: He married II-1, possibly for companionship or to start a family.
But since the question is poorly worded, assume it's a typo.

7. What is the genotype of individual I-2? II-3? III-1?
- I-2: aa (unaffected, so homozygous recessive)
- II-3: aa (unaffected)
- III-1: Aa (affected, but has unaffected child, so must be heterozygous)

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Add shading to any symbols that need it

But in the original, IV-1 is already unshaded — correct.

All affected individuals are shaded — no missing shadings.

But if the question implies that some individuals should be shaded, but aren’t — but in this case, all affected are shaded.

So no additional shading needed.

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Summary of Answers:

1. 4
2. 4
3. 8
4. Female
5. 4
6. Unclear / likely a typo. Perhaps: "He married to have children."
Or: "He married II-1, who is also affected."
But best: "The reason is not specified."
7.
- I-2: aa
- II-3: aa
- III-1: Aa

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Let me know if you'd like a labeled version of the pedigree or a corrected version of the worksheet.