What pattern of inheritance does this trait follow?

Let's analyze the pedigree chart to determine the pattern of inheritance for the trait shown.

Step 1: Understand the symbols

- Square = male
- Circle = female
- Red-filled symbol = affected individual
- Unfilled symbol = unaffected individual

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Step 2: Observe the pattern

#### Generation I:
- A male (unaffected) and a female (affected) have children.
- All their children are unaffected (3 daughters, 1 son — all white).

So, even though the mother is affected, none of her children show the trait. This suggests that the trait might not be dominant or that it's recessive and the father is not carrying it.

#### Generation II:
- One of the unaffected daughters from the first couple marries an unaffected male.
- They have three children: two daughters and one son.
- The son is affected (red square), while the other two are unaffected.

This is crucial: an affected male appears in the second generation, but both parents are unaffected.

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Step 3: Consider possible inheritance patterns

Let’s evaluate each possibility:

#### 1. Autosomal Dominant
- Affected individuals usually have at least one affected parent.
- Here, the affected son has two unaffected parents, so this is not consistent with autosomal dominant.

#### 2. Autosomal Recessive
- Can skip generations.
- Both parents must be carriers (heterozygous) to have an affected child.
- In this case, both parents are unaffected, but they have an affected son → possible.
- However, look back at the first generation: the mother is affected and has no affected children. If the trait were recessive, then the mother would need to be homozygous recessive (aa). Then, if the father is AA or Aa, the children could be Aa or aa. But here, all children are unaffected, meaning they are not aa. That means the father must be AA (not a carrier), and all children are Aa (carriers but not affected). So that’s possible.

But wait: if the trait is recessive, the affected mother (aa) can only pass on 'a'. The father (AA) passes 'A'. So all children are Aa → unaffected carriers → matches.

Then, in the next generation: the daughter (Aa) mates with an unaffected man. If he is also Aa, there's a chance of having an affected child (aa). The son is affected → aa → possible.

So far, this fits autosomal recessive.

But let’s check another clue.

#### 3. X-linked Recessive
- Males are more commonly affected.
- Affected males pass the gene to all daughters but not sons.
- Affected females are rare (usually require both parents to contribute).
- Let’s test it.

In generation I:
- Mother is affected (red circle). For X-linked recessive, she must be X^a X^a.
- Father is unaffected → X^A Y.
- Their daughters: get X^a from mom and X^A from dad → X^A X^a → carriers but unaffected.
- Sons: get X^a from mom and Y from dad → X^a Y → affected males.

But in the chart, none of the children are affected — all are unaffected. But according to X-linked recessive, all sons should be affected! Because they inherit X^a from mom and Y from dad → X^a Y → affected.

But here, the son is unaffected → contradiction.

Therefore, cannot be X-linked recessive.

#### 4. X-linked Dominant
- Affected mothers pass the trait to half their children (50% chance).
- Affected fathers pass it to all daughters, but not sons.
- In generation I: mother is affected → X^A X^a or X^A X^A.
- If she is X^A X^a, then:
- Daughters: 50% chance X^A X^a (affected), 50% X^a X^a (unaffected)
- Sons: 50% X^A Y (affected), 50% X^a Y (unaffected)

But in the chart, all children are unaffected → no affected sons or daughters.

So if the mother is affected and X-linked dominant, we'd expect some affected children, especially sons.

But none are affected → contradiction.

So not X-linked dominant.

#### 5. Y-linked
- Only males affected, passed from father to son.
- But here, the mother is affected, and the father is unaffected → cannot be Y-linked.

So ruled out.

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Revisit Autosomal Recessive

Let’s go back.

- Generation I:
- Mother: affected → genotype = aa
- Father: unaffected → must be AA (if he were Aa, some children might be affected, but they aren’t)
- Children: all Aa → unaffected carriers

→ Matches.

- Generation II:
- Daughter: Aa (carrier)
- Marries an unaffected man. He could be AA or Aa.
- Their son is affected → aa.
- So both parents must be carriers → Aa × Aa → 25% chance of aa.
- This is consistent.

Also, the affected individual is male, but that doesn't matter in autosomal traits — both sexes equally affected.

And the trait skips a generation — yes, because the mother was affected, but her children weren't, but grandchildren were.

So this fits autosomal recessive.

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Final Check: Is there any problem?

Wait — in generation I, the mother is affected (aa), father is AA → all children are Aa → unaffected carriers → correct.

Then, one daughter (Aa) marries a man who is likely Aa (since they have an affected child).

They have:
- 2 unaffected daughters → possibly AA or Aa
- 1 affected son → aa

Yes, this is possible.

No evidence of sex bias.

So autosomal recessive is consistent.

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Conclusion:

The trait follows autosomal recessive inheritance.

✔ Answer: Autosomal recessive

Because:
- Affected individuals have unaffected parents (suggests recessive).
- The trait skips generations.
- Both males and females are affected (no sex bias).
- An affected mother has unaffected children (so not X-linked dominant or recessive).
- Not Y-linked (affects females).
- Not dominant (because affected child has two unaffected parents).

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✔ Final Answer: Autosomal recessive inheritance