Pedigree Lab - BIOLOGY JUNCTION - Free Printable

Let’s solve this step by step.

We are looking at a family tree (called a pedigree) that shows how a trait is passed down through generations. The black shapes mean the person has the trait. White shapes mean they don’t. Squares are males, circles are females.

We need to figure out what kind of inheritance pattern this is — for example, is it dominant or recessive? Is it on the X chromosome or not?

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Step 1: Look at Generation I

- On the left: A affected female (black circle) and an unaffected male (white square) have children in Generation II.
- Their children: one unaffected male, one unaffected female, and one affected female.
→ So an affected mother can have unaffected sons and daughters → suggests it’s NOT X-linked dominant (because if mom had X-linked dominant, all her sons would get it).

Also, she passed it to only some kids → could be autosomal dominant or recessive.

But wait — look at the right side of Generation I:

- Unaffected female + affected male → their children in Gen II: one affected male, one unaffected female.
→ Affected father passes trait to son → so it CAN go from father to son → rules out X-linked inheritance (because fathers give Y to sons, not X). So if it were X-linked, dad couldn’t pass it to son unless mom also carried it — but here mom is unaffected.

So since father → son transmission happens → must be AUTOSOMAL (not sex-linked).

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Step 2: Check if it’s dominant or recessive.

Look at Generation II:

Left family: Affected mom + unaffected dad → have unaffected kids → that’s possible with dominant IF mom is heterozygous.

Right family: Unaffected mom + affected dad → have affected son and unaffected daughter → again, consistent with dominant.

Now look at Generation III:

The affected female (from left family) marries the affected male (from right family) → they have an UNaffected daughter.

This is KEY.

If both parents are affected and they have an unaffected child → that means the trait MUST be DOMINANT.

Why? Because if it were recessive, two affected parents would BOTH be homozygous recessive → all kids would be affected. But here, they have an unaffected kid → so the parents must be heterozygous (carrying one normal allele), and the child got two normal alleles → which only works if the disease allele is dominant.

Example: Let’s say “A” = disease allele (dominant), “a” = normal.

Affected parents: both could be Aa.

Child gets aa → unaffected → yes, possible.

If it were recessive: affected = aa. Two aa parents → all kids aa → all affected. But we see unaffected child → contradiction.

Therefore, it’s AUTOSOMAL DOMINANT.

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Final check: Does everything fit?

- Father to son transmission? Yes → autosomal OK.
- Affected parent can have unaffected child? Yes → if heterozygous → fits dominant.
- Two affected parents have unaffected child? Only possible if dominant → confirmed.

✔ All clues point to Autosomal Dominant inheritance.

Final Answer: Autosomal dominant