Solved Genetics Pedigree Worksheet 1. Use the below pedigree ... - Free Printable

Let’s solve this step by step.

We are given a pedigree chart with symbols:
- Squares = males
- Circles = females
- Filled-in shapes = have the trait (unibrow, based on question A)
- Empty shapes = do not have the trait

Generations are labeled I, II, III. Individuals are numbered left to right within each generation.

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First, let’s answer the questions one by one.

A) How many family members have unibrows?

Look for filled-in shapes in the entire pedigree:

- Generation I: Individual I-1 is filled → 1
- Generation II: None are filled → 0
- Generation III: III-3 and III-4 are filled → 2

Total = 1 + 0 + 2 = 3

✔ Answer: 3

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B) What is the genotype of individual #4?

Individual #4 is in generation II → that’s II-4.

II-4 is an empty circle → does NOT have unibrow.

But look at her children: III-3 and III-4 BOTH have unibrows (filled).

That means she must carry the gene for unibrow — otherwise, how could her kids have it?

Also, her husband is II-3 (empty square), and he also has kids with unibrows → so he must also carry the gene.

So if both parents don’t show the trait but their kids do → the trait must be RECESSIVE.

Therefore, to have a child with a recessive trait, both parents must be carriers → heterozygous.

So II-4 does not have unibrow → so she can’t be homozygous recessive.

She must be heterozygous → carries one copy of the recessive allele.

Genotype: heterozygous

(If we use letters: let’s say “U” = dominant (no unibrow), “u” = recessive (has unibrow). Then II-4 = Uu)

✔ Answer: heterozygous

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C) Are individuals #8 and #9 homozygous or heterozygous?

#8 and #9 are in generation II → II-8 and II-9.

They are both empty squares → no unibrow.

Their parents are I-5 and I-6 → both empty → no unibrow.

Do they have any children with unibrow? Look at their kids: III-6, III-7, III-8 → all empty → no unibrow.

Since none of their children show the trait, and they themselves don’t show it, we cannot be 100% sure if they are homozygous dominant or heterozygous.

BUT — wait! The trait is recessive (we figured that out earlier). So if someone doesn’t have the trait, they could be either UU or Uu.

However, since neither II-8 nor II-9 have affected children, and their parents also aren’t affected, there’s no evidence they carry the recessive allele.

In pedigrees, if there’s no sign of the trait in offspring or ancestors, we usually assume they are homozygous dominant — unless proven otherwise.

But actually — let’s think again.

Is there ANY way they could be heterozygous? Yes — but we have no proof.

The question says: “Are individuals #8 and #9 homozygous or heterozygous?”

Based on available info — we can’t know for sure. But in standard pedigree analysis, if a person doesn’t show the trait and has no affected relatives (especially children), we often label them as homozygous dominant — because if they were heterozygous, there’d be a chance their kids would show it — but none did.

Actually — let’s check: II-8 and II-9 married people outside the family? II-8 married II-7 (who is from another branch). II-7’s parents are I-3 and I-4 — both unaffected. Their kids (III-6,7,8) are all unaffected.

Still no affected individuals.

So — safest assumption: since no one in their line shows the trait, and they don’t show it, they are likely homozygous dominant.

But wait — the question might expect us to say “we can’t tell” — but looking at typical textbook problems, when no affected children exist and parents are unaffected, they’re considered homozygous dominant.

Actually — let’s go back to logic.

The trait is recessive. To express it, you need two copies (uu).

If II-8 or II-9 were heterozygous (Uu), and married someone who is also Uu, then 25% chance of uu child.

But their spouses? II-7 — we don’t know her genotype. She could be UU or Uu.

But since none of their three children are affected, it’s possible they are both UU, or one or both are Uu but got lucky.

In pedagogy, unless there’s evidence of carrier status, we assume homozygous dominant for unaffected individuals with no affected descendants.

So I’ll go with: homozygous dominant

But let me double-check with the next question.

Wait — question C is repeated? In the image, there are two “C)” — probably typo. First C is about #8 and #9, second C is about #2.

Let’s do the second C now.

C) What is the genotype of individual #2?

Individual #2 is in generation I → I-2.

I-2 is an empty circle → no unibrow.

Her partner is I-1 → filled square → HAS unibrow → so genotype must be uu (since trait is recessive).

Their children: II-1 and II-2 → both empty → no unibrow.

So I-1 (uu) passed a “u” to each child.

Children are unaffected → so they must have gotten a “U” from mom (I-2).

Therefore, I-2 must have at least one “U”.

Could she be UU or Uu?

If she were UU, all kids would be Uu → unaffected → matches.

If she were Uu, half kids would be Uu, half uu — but here both kids are unaffected → so possible, but less likely? Wait — only two kids, both unaffected — still possible if she’s Uu.

But — we don’t have more data.

However, since she has no affected children, and her husband is affected, if she were Uu, there’s 50% chance per child to be affected — but neither is.

Still, statistically possible.

But in pedigree analysis, when an unaffected person has an affected spouse and unaffected children, we often conclude the unaffected parent is homozygous dominant — because if they were heterozygous, we’d expect some affected children.

Here, two children, both unaffected — suggests she is likely UU.

But let’s think: if she were Uu, probability both kids are unaffected = (1/2)^2 = 1/4 — possible, but low.

Whereas if she’s UU, probability = 1.

So most reasonable conclusion: homozygous dominant

But wait — actually, in many textbooks, they would say: since she has an affected husband and unaffected children, she must be homozygous dominant — because if she were heterozygous, some children should be affected — but here none are, so she’s probably UU.

Yes.

So I-2: homozygous dominant

Now back to II-8 and II-9.

Same logic: no affected children, no affected parents, so likely homozygous dominant.

So for C (first one): II-8 and II-9 → homozygous dominant

For C (second one): I-2 → homozygous dominant

But let’s confirm the trait type first.

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1. Is this trait dominant or recessive?

We saw that II-3 and II-4 (both unaffected) have children III-3 and III-4 who ARE affected.

That’s classic sign of recessive trait: unaffected parents → affected children → means both parents are carriers (heterozygous), and child got two recessive alleles.

If it were dominant, then affected children would require at least one affected parent — but here parents are unaffected.

So trait is recessive

✔ Answer: recessive

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2. How can you know for sure that individuals II-3 and II-4 are heterozygous?

Because they do not have the trait (so they can’t be homozygous recessive), but they have children who DO have the trait (homozygous recessive).

To produce a homozygous recessive child (uu), each parent must contribute a “u” allele.

Since II-3 and II-4 don’t have the trait, they must have at least one “U” allele.

So their genotype must be Uu — heterozygous.

That’s the only way they can pass “u” to kids without showing the trait themselves.

✔ Answer: Because they are unaffected but have affected children, which means they must each carry one copy of the recessive allele.

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Now, the last part: identify genotypes of specific individuals.

Listed:

- III-3: filled circle → has unibrow → since trait is recessive → genotype = homozygous recessive

- II-1: empty circle → no unibrow. Her father is I-1 (affected, uu), mother is I-2 (unaffected). Since father is uu, he gave her a “u”. She is unaffected → so she must have gotten “U” from mom → so genotype = heterozygous

- I-1: filled square → has unibrow → recessive trait → homozygous recessive

- II-4: empty circle → no unibrow, but has affected children → must be heterozygous (as explained before)

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Now compile all answers.

Final Answers:

A) 3
B) heterozygous
C) [first] homozygous dominant; [second] homozygous dominant
1. recessive
2. Because they are unaffected but have affected children, meaning they must each carry one recessive allele.

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

Wait — the worksheet lists:

• III-3: _______________
• II-1: _______________
• I-1: _______________
• II-4: _______________

So fill those.

And the multiple choice-like questions above.

Let me write final answers clearly.

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Final Answer:
A) 3
B) heterozygous
C) homozygous dominant (for #8 and #9); homozygous dominant (for #2)
1. recessive
2. They are unaffected but have affected children, so they must each carry one copy of the recessive allele.
Genotypes:
- III-3: homozygous recessive
- II-1: heterozygous
- I-1: homozygous recessive
- II-4: heterozygous