Genetic pedigree analysis illustrating autosomal dominant inheritance.

Let’s solve this step by step.

We’re looking at a pedigree chart — that’s a family tree showing who has a certain trait (in this case, probably a genetic condition). The key tells us:

- Squares = males
- Circles = females
- Filled-in shapes = affected individuals (have the trait)
- Empty shapes = unaffected

The question asks: Is the disorder autosomal dominant, autosomal recessive, X-linked dominant, or X-linked recessive?

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Step 1: Look for patterns

First, check if the trait skips generations.

→ In generation I: Individual I-2 is affected (filled circle).
→ In generation II: Individuals II-3 and II-5 are affected.
→ In generation III: Individual III-4 is affected.

So it appears in every generation — that suggests dominant inheritance (because dominant traits don’t skip generations usually).

But let’s not jump yet — we need to rule out other possibilities.

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Step 2: Check if it could be X-linked

X-linked means the gene is on the X chromosome.

Important clues for X-linked recessive:
- More males affected than females.
- Affected fathers cannot pass it to sons (because sons get Y from dad).
- Carrier mothers can pass it to sons.

In our pedigree:
- There are both affected males and females → doesn’t strongly suggest X-linked recessive (which usually affects mostly males).
- Look at individual II-3 (affected male) — he has an affected daughter (III-4). That’s possible in X-linked dominant, but NOT in X-linked recessive (because if dad is affected with X-linked recessive, all his daughters would be carriers, but not necessarily affected — unless mom also gives mutant allele).

Wait — look at individual I-2 (affected female). She has sons: II-1, II-2, II-4 — none of them are affected. If it were X-linked dominant, then ALL her sons should be affected (because they get her only X chromosome). But they’re not! So that rules out X-linked dominant.

Also, if it were X-linked recessive, an affected female (I-2) must have two copies of the mutant gene — so she’d pass one to each son → all sons should be affected. But again, her sons are NOT affected. So X-linked recessive is ruled out too.

Therefore, it’s likely autosomal.

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Step 3: Dominant vs Recessive?

If it’s autosomal recessive:
- Two unaffected parents can have an affected child (if both are carriers).
- Often skips generations.

Look at generation II: Individuals II-6 and II-7 are both unaffected, but their child III-4 is affected.

That’s a classic sign of recessive inheritance! Because two unaffected parents had an affected child → they must both be carriers (heterozygous), and passed the recessive allele to the child.

BUT wait — earlier we saw the trait appearing in every generation? Let’s double-check.

Generation I: I-2 affected
Generation II: II-3, II-5 affected
Generation III: III-4 affected

But II-3 and II-5 are children of I-1 (unaffected) and I-2 (affected). If it’s recessive, how can an affected parent (I-2) have affected children when the other parent (I-1) is unaffected? Only if I-1 is also a carrier.

Actually, let’s test both models.

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Test Autosomal Recessive Model:

Assume the disease allele is “a”, normal is “A”.

Affected = aa
Unaffected = AA or Aa

I-2 is affected → aa
I-1 is unaffected → must be A_ (could be AA or Aa)

Their children: II-1, II-2, II-3, II-4, II-5

II-3 and II-5 are affected → aa → so they got “a” from both parents.

That means I-1 must have given them an “a” → so I-1 must be Aa (carrier).

Then II-1, II-2, II-4 are unaffected → they could be AA or Aa.

Now look at II-6 and II-7 — both unaffected, but have affected child III-4 (aa).

So II-6 and II-7 must both be carriers (Aa).

This works!

Now check if autosomal dominant fits.

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Test Autosomal Dominant Model:

Affected = A_ (AA or Aa)
Unaffected = aa

I-2 is affected → A_
I-1 is unaffected → aa

Their children: half should be affected (if I-2 is Aa), which matches: 2 out of 5 affected? Wait, actually 2 out of 5 is less than half, but possible by chance.

But now look at II-6 and II-7 — both unaffected → aa

They have child III-4 who is affected → A_

But if both parents are aa, they can ONLY give “a” alleles → child must be aa → unaffected.

Contradiction!

So autosomal dominant is IMPOSSIBLE here because two unaffected parents (aa x aa) cannot have an affected child (who needs at least one A).

Therefore, it MUST be autosomal recessive.

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Final Check:

- Skips generations? Well, I-2 affected → II-3, II-5 affected → III-4 affected — seems like no skip, BUT II-6 and II-7 are unaffected carriers, and their child is affected — that’s a “skip” in the sense that parents are unaffected. Actually, in recessive, it can appear to skip if carriers are unaffected.

The key evidence is: two unaffected parents (II-6 and II-7) have an affected child (III-4) → only possible in recessive inheritance.

Also, affected females have unaffected sons — which rules out X-linked dominant.

And affected females having unaffected sons rules out X-linked recessive too (as explained before).

So yes — autosomal recessive.

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Final Answer:
Autosomal recessive