Genetics Pedigree Worksheet: Analyzing Inheritance Patterns in a Family Tree

Let’s solve this step by step.

First, look at the pedigree chart. We need to figure out if the trait (shown by filled-in shapes) is dominant or recessive.

Look at generation II: individuals II-3 and II-4 are both unaffected (empty shapes), but they have children in generation III who ARE affected — III-3 (female, affected) and III-4 (male, affected).

That’s a big clue! If two parents don’t show the trait, but their child does, that means the trait must be recessive. Why? Because for a recessive trait to show up, the child has to get one copy of the bad gene from each parent. The parents can carry it without showing it — that’s called being “heterozygous.”

So, answer to question 1:
This trait is recessive.
Because two unaffected parents (II-3 and II-4) had affected children (III-3 and III-4). That only happens with recessive traits.

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Now, question 2: How do we know II-3 and II-4 are heterozygous?

Since the trait is recessive, an affected person must have two copies of the recessive allele (let’s call it “a”). So III-3 and III-4 are “aa”.

But their parents (II-3 and II-4) are unaffected — so they must have at least one normal allele (“A”). But since they passed on the “a” allele to their kids, they must also carry one “a”. So their genotype is “Aa” — which is heterozygous.

We know for sure because if either parent was “AA”, they couldn’t pass on an “a” allele — and then none of their kids could be “aa”. But some kids ARE “aa”, so both parents MUST be carriers — heterozygous.

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Now, let’s fill in the genotypes asked:

We’ll use:
- A = dominant (normal)
- a = recessive (affected)

• III-3: She is affected → must be homozygous recessive (aa)

• II-1: Look at her parents. Her father (I-1) is affected → he is “aa”. Her mother (I-2) is unaffected → she must be “A_” (we don’t know yet). But II-1 is unaffected → so she got one “a” from dad, and must have gotten “A” from mom → so she is heterozygous (Aa)

• I-1: He is affected → homozygous recessive (aa)

• II-4: As we just figured out above — she’s unaffected but has affected kids → must be heterozygous (Aa)

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Now, question 3: Brown eyes (dominant) vs blue eyes (recessive).

Let’s assign:
- B = brown (dominant)
- b = blue (recessive)

Woman: brown-eyed, but her father had blue eyes → so her father was “bb”. That means she must have gotten a “b” from him. Since she has brown eyes, she must have a “B” too → so she is Bb

Her mother had brown eyes — we don’t know if mom was BB or Bb, but since the woman is Bb, mom could have given her either B or b — doesn’t matter for now.

Man: brown-eyed, his parents were both brown-eyed. They had a son (this man) who is brown-eyed, but wait — actually, the problem says: “They have a son who is blue-eyed.” Wait — no, reread:

“A brown-eyed woman [...] marries a brown-eyed man whose parents are also brown-eyed. They have a son who is blue-eyed.”

Wait — that’s important. The couple (woman and man) have a son who is blue-eyed → so the son is “bb”.

That means BOTH parents must have given him a “b” allele.

So the woman is already known to be Bb (from her father being bb).

The man is brown-eyed, but he must have given a “b” to his son → so he must be Bb too.

His parents were both brown-eyed — but since he is Bb, that means at least one of his parents must have carried a “b” — but we don’t need to go further unless drawing pedigree.

Now, draw pedigree:

Grandparents (generation I):

Left side (woman’s parents):
- Grandfather: blue eyes → bb (filled square)
- Grandmother: brown eyes → could be BB or Bb (empty circle) — we’re not certain

Right side (man’s parents):
- Both brown-eyed → both could be BB or Bb — but since their son (the man) is Bb, at least one of them must have been carrier. But we don’t know which — so we can’t be certain of their genotypes.

Parents (generation II):
- Woman: Bb (brown eyes, empty circle)
- Man: Bb (brown eyes, empty square)

Child (generation III):
- Son: blue eyes → bb (filled square)

In the pedigree, we indicate:

Certain genotypes:
- Woman’s father: bb (certain)
- Woman: Bb (certain — because she’s brown-eyed but got b from dad)
- Man: Bb (certain — because he’s brown-eyed but passed b to son)
- Son: bb (certain)

Uncertain:
- Woman’s mother: could be BB or Bb
- Man’s parents: each could be BB or Bb — we only know at least one passed b to the man, but don’t know which one or if both did.

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Final Answers:

Genotypes:

• III-3: homozygous recessive
• II-1: heterozygous
• I-1: homozygous recessive
• II-4: heterozygous

1. This trait is recessive. Two unaffected parents (II-3 and II-4) had affected children (III-3 and III-4), which only happens with recessive traits.

2. We know II-3 and II-4 are heterozygous because they are unaffected (so they must have at least one dominant allele), but they passed the recessive allele to their affected children. So they must each carry one recessive allele — making them heterozygous.

3. Pedigree description:

Generation I (grandparents):

- Left: Male (blue eyes, bb) — filled square; Female (brown eyes, ?) — empty circle (genotype uncertain: BB or Bb)
- Right: Male (brown eyes, ?) — empty square; Female (brown eyes, ?) — empty circle (both genotypes uncertain, but at least one must carry b)

Generation II (parents):

- Female (brown eyes, Bb) — empty circle
- Male (brown eyes, Bb) — empty square

Generation III (child):

- Male (blue eyes, bb) — filled square

Certain genotypes:
- Woman’s father (bb), woman (Bb), man (Bb), son (bb)

Uncertain:
- Woman’s mother, man’s parents

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Final Answer:

Genotypes:
- III-3: homozygous recessive
- II-1: heterozygous
- I-1: homozygous recessive
- II-4: heterozygous

1. Recessive — because unaffected parents had affected children.

2. They must be heterozygous because they are unaffected but passed the recessive allele to their affected children.

3. Pedigree:
- Grandparents: woman’s father = bb (affected), woman’s mother = ? (unaffected); man’s parents = both ? (unaffected)
- Parents: woman = Bb, man = Bb
- Child: son = bb (affected)
Certain: woman’s father, woman, man, son
Uncertain: woman’s mother, man’s parents