Pedigree Analysis worksheet 1 - 91148 Human Genetics and Precision ... - Free Printable

Let’s work through this step by step.

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Step 1: Understand the pedigree

We’re looking at a family tree (pedigree) that shows who has albinism (the “trait”) and who doesn’t.

- Squares = males
- Circles = females
- Filled shapes = have the trait (albinism)
- Empty shapes = do not have the trait

The trait is labeled as “albinism,” which we know from biology is usually inherited in a recessive pattern — but we’ll prove it from the chart.

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Question 1: Is albinism dominant or recessive? Explain.

Look at the parents of Female A (top left part of the chart).

Her parents are both empty symbols → they do NOT have the trait.

But Female A (filled circle) DOES have the trait.

That means: two unaffected parents had an affected child.

This can only happen if the trait is recessive.

Why?

- If the trait were dominant, then at least one parent would have to show the trait to pass it on.
- But here, neither parent shows it — so they must be carriers (heterozygous), and the child got two copies of the recessive allele.

Same thing happens with K and M (in the middle group): their parents are unaffected, but they are affected.

Also, look at Female B’s side: her father has the trait (filled square), mother doesn’t — and Female B does NOT have the trait. That fits with recessive too — she got one normal allele from mom and one mutant from dad, so she’s a carrier but doesn’t show the trait.

✔ So, albinism is recessive.

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Question 2: Genotypes for Females A & B

Let’s use letters:

- Let “A” = normal (dominant) allele
- Let “a” = albino (recessive) allele

Since albinism is recessive, you need two copies (aa) to show the trait.

→ Female A has the trait → she must be aa

→ Female B does NOT have the trait → she could be AA or Aa

But look at her father: he has the trait → he is aa

So he must have passed an “a” allele to Female B.

Therefore, Female B must have at least one “a”.

But she doesn’t have the trait → so she must have one “A” from her mother.

Thus, Female B is Aa

✔ Final genotypes:

- Female A: aa
- Female B: Aa

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Final Answer:
1. The albinism trait is recessive because unaffected parents can have affected children.
2. Female A: aa; Female B: Aa